OBS-5+ System
Revision: 11/13
Copyright © 2008-2013
Campbell Scientific, Inc.
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Table of Contents
PDF viewers: These page numbers refer to the printed version of this document. Use the
PDF reader bookmarks tab for links to specific sections.
1. Introduction.................................................................1
2. Cautionary Statements............................................... 1
3. Initial Inspection .........................................................1
3.1 Ships With............................................................................................1
4. Overview......................................................................2
4.1 Optics ...................................................................................................2
4.2 SSC-Measurement Principle ................................................................3
5. Specifications .............................................................5
5.1 Measurement Range.............................................................................5
5.2 Accuracy ..............................................................................................6
5.3 OBS-5+ Sensor ....................................................................................6
5.4 Other Data............................................................................................6
5.5 Dimensions...........................................................................................6
6. Operation.....................................................................7
6.1 Instrument Setup ..................................................................................7
6.1.1 Mounting Suggestions...................................................................7
6.1.2 Surveys..........................................................................................7
6.1.3 Logging Data ................................................................................8
6.1.4 Battery Installation........................................................................8
6.2 OBS-5+Utility Software.......................................................................9
6.2.1 Software Installation .....................................................................9
6.2.2 Running the OBS-5+ Utility .......................................................10
6.2.3 Pull-Down Menus .......................................................................11
6.2.4 Communication Settings .............................................................11
6.2.5 Testing Sensors ...........................................................................12
6.2.6 Monitoring Turbidity (NTU).......................................................13
6.2.7 Water Density and Barometric Corrections ................................13
6.2.8 Sample Statistics .........................................................................13
6.2.9 Sampling Modes and Terms........................................................14
6.2.10 Surveying ....................................................................................15
6.2.11 Cyclic Sampling ..........................................................................17
6.2.12 Data Retrieval .............................................................................18
6.2.13 Shutdown ....................................................................................18
6.2.14 Graphing and Printing.................................................................18
6.2.15 Excel Spreadsheets......................................................................19
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Table of Contents
7. Calibration .................................................................20
7.1 Sediment and NTU Calibration......................................................... 20
7.1.1 General Guidance....................................................................... 21
7.1.2 Equipment and Materials............................................................ 24
7.1.3 Procedure for Sediment.............................................................. 24
7.2 Turbidity (NTU) Calibration ............................................................. 28
7.2.1 Equipment and Materials............................................................ 28
7.2.2 Procedure for Turbidity .............................................................. 28
8. Troubleshooting........................................................29
9. Maintenance ..............................................................32
9.1 OBS-5+ Sensor.................................................................................. 32
9.2 Pressure Sensor.................................................................................. 32
9.3 Batteries............................................................................................. 33
9.4 Pressure Housing............................................................................... 33
9.5 User-Serviceable Parts....................................................................... 33
10. Interfering Factors ....................................................34
10.1 Particle Size ....................................................................................... 34
10.2 NIR Reflectivity ................................................................................ 35
10.3 Particle Shape, Flocculation, and Disaggregation ............................. 36
11. References.................................................................38
Appendix
A.
Turbidity Standards ................................................A-1
Figures
4-1. Dimensions (top), sensor endcap with copper antifoulant (Cu a.f.)
collars (left) and connector endcap (right)....................................... 2
4-2. Schematic of optical system................................................................ 3
4-3. Sample calibration curves (fine and bold lines), lookup tables I, II,
and III (bold curves), and sediment concentrations pf ND (open
arrow) and FD (solid arrow) peaks. ................................................. 4
4-4. Calibration curves for four different sediments and SSC values for
near-detector peaks (colored arrowheads). ...................................... 5
6-1. Electrical connections.......................................................................... 8
6-2. Battery installation: A) endcap removal, B) wiping, C) cable
disconnect, and D) battery contact................................................... 9
6-3. New data log prompt......................................................................... 10
6-4. Designating your own file name and destination............................... 10
7-1. Manual (left) and automatic (right) sediment suspenders.................. 21
7-2. Lookup tables and table limits (a, b, and c)....................................... 22
7-3. OBS-5+ in big black tub of clean water ............................................ 25
7-4. OBS-5+ in suspender tub................................................................... 26
7-5. OBS-5+ in 100 mm cup..................................................................... 29
ii
Tables
Table of Contents
8-1. Internal components...........................................................................31
10-1. Effects of sediment size .....................................................................35
10-2. Near-infrared reflectivity of minerals.................................................36
10-3. Effects of disaggregation methods.....................................................37
6-1. Working and Maximum Depths...........................................................7
7-1. Schedule of Concentrations for Sediment Calibrations......................23
7-2. SSC-Calculation Spreadsheet.............................................................23
9-1. Battery Life in Hours with 100% Power............................................33
10-1. Relative magnitude of the effects of sediment characteristics on
OBS-5+ sensitivity .........................................................................34
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Table of Contents
iv
OBS-5+ System
1. Introduction
The manual describes the features of the OBS-5+®, as well as its use for
surveys and battery-powered, internal-storage operations. Using backscatter
from a 780 nm laser diode and a patented dual-detection system (U.S. Patent
Number 5,796,481), a calibrated OBS-5+ measures suspended sediment
concentrations, suspended solids concentration (SSC), as large as 200 g L
which is about 10 times higher than standard OBS technology.
Before installing the OBS-5+, please study:
• Section 2, Cautionary Statements
• Section 3, Initial Inspection
2. Cautionary Statements
• Although the OBS-5+ is rugged, it should be handled as a precision
scientific instrument.
–1
,
• Maximum depth for the OBS-5+ is limited by the installed pressure
sensor. If the maximum depths are exceeded, the pressure sensor will
rupture and the housing will flood. See Section 6.1, Instrument Setup, for
more information.
• Always orient the unit so that the OBS-5+ sensor “looks” into water clear
of reflective surfaces.
• Pad the end caps that contact metal with electrical tape, neoprene, or
expanded polyethylene tubes.
• Never mount the instrument by its end caps or attach anything to them.
This could a cause a leak.
• Always put the OBS-5+ in sleep mode when it will not be used for a while
to conserve battery capacity (see Section 6.2.13, Shutdown).
3. Initial Inspection
• Upon receipt of the OBS-5+, inspect the packaging and contents for
damage. File damage claims with the shipping company.
• Check this information against the shipping documents to ensure the
correct product is received (see Section 3.1, Ships With).
3.1 Ships With
21304 Accessory Kit
20919 Software Support CD
ResourceDVD
1
OBS-5+ System
4. Overview
FIGURE 4-1 shows the dimensions of the OBS-5+, the sensors in the sensor
endcap, and underwater connection. Detailed specifications are provided in
Section 5, Specifications. The OBS-5+ can be operated in Survey or Cyclic
Modes. In Survey Mode, the unit sends data via RS-232 or RS-485 to a PC at
two hertz, and in Cyclic Mode, it logs as many as 200,000 scans of time, date,
depth, and g L
sampling continuously, the unit will run about 125 hours on three C-size
alkaline batteries in 20ºC water (about two weeks of eight-hour survey days).
When using the instrument for surveys, the data are captured by a PC running
the OBS-5+ software. Pressure is measured with a silicon strain-gauge
pressure sensor and the depth of the instrument is calculated from water density
and barometric pressure entered by the operator.
–1
in flash memory (one sample per hour for 23 years). When
2
FIGURE 4-1. Dimensions (top), sensor endcap with copper antifoulant
(Cu a.f.) collars (left) and connector endcap (right)
4.1 Optics
The heart of the OBS-5+ is an optical system comprised of a near infrared
(NIR) laser diode and a photodetector positioned 10 mm from the laser, called
the near detector (ND), and one mounted 25 mm from the laser, called the far
detector (FD), see FIGURE 4-2. The detector acceptance-cone angle is 55º,
which means that photons must enter the detector sapphire windows at angles
less than 27.5º to be detected. The laser light is collimated to a 3 mm by 1 mm
elliptical beam with convergence < 2.5 mrad. The angle between the NIR
beam and detector acceptance-cone axes is 45º. The OBS-5+ can detect light
scattered by particles illuminated by the NIR beam at angles between 105º to
165º. With its automatic-power-control circuit, the laser diode provides stable
optical power over time and the 0 to 30ºC operating temperature range.
FIGURE 4-2. Schematic of optical system
OBS-5+ System
4.2 SSC-Measurement Principle
In FIGURE 4-2, suspended particles scatter light from the NIR beam onto the
near and far detectors, and suspended solids concentration (SSC) is estimated
with signals counts from these detectors by a microcontroller, using a set of
logic rules and lookup tables derived from calibration data. Counts are the
digital equivalents of the ND and FD signals and are proportional to
backscatter intensity. Sample calibration curves relating ND and FD counts to
sediment concentration in g L
concentration increases from 0, the ND and FD signals rise to maxima at peak
concentrations, shown by the red (open) and blue (solid) arrowheads above the
x axis. Beyond the peak sediment concentrations, light attenuation is the
dominant factor controlling the light level at the detectors, and so the ND and
FD counts decline. The scattered NIR must travel farther to be detected by the
FD than by the ND, and therefore the peak of the FD curve occurs at a lower
sediment concentration than the peak of the ND curve. The instrument exploits
these peak offsets to estimate sediment concentration. Using either the ND or
FD counts, whichever is indicated by the logic rules, the microcontroller does a
spline interpolation between calibration values to derive an SSC value. It then
combines this value with time and pressure data, and sends the results to a PC.
–1
are shown on FIGURE 4-3. As particle
3
OBS-5+ System
FIGURE 4-3. Sample calibration curves (fine and bold lines), lookup
tables I, II, and III (bold curves), and sediment concentrations pf ND
(open arrow) and FD (solid arrow) peaks.
The concentrations associated with the ND and FD peaks depend on sediment
characteristics as shown by the variety of response curves and the colored
arrowheads on FIGURE 4-4. Other sediment characteristics being equal, such
as shape and NIR reflectivity (see Section 10, Interfering Factors), larger
particles produce higher peak concentrations and greater OBS-5+ measurement
ranges than smaller particles.
4
OBS-5+ System
FIGURE 4-4. Calibration curves for four different sediments and SSC
5. Specifications
Features:
5.1 Measurement Range
Mud (D50=20μm): 0 to 50,000 mg/l
values for near-detector peaks (colored arrowheads).
• Connects directly to a PC—no datalogger needed
• Operates up to six months on three C-cell batteries
• Monitors high sediment concentrations (up to 200g/L)
• Logs depth, wave height, wave period, temperature, and salinity
• Records 200,000 scans of data in the OBS-5+ flash memory
Sand (D
Pressure
Turbidity: 0.4 to 1,000 NTU
=250μm): 0 to 200,000 mg/l
50
1
: 0 to 10, 20, 50, 100, or 200 m
5
OBS-5+ System
5.2 Accuracy
5.3 OBS-5+ Sensor
5.4 Other Data
Mud: 2.0% of reading
Sand: 4.0% of reading
Pressure: 0.5% of full scale
Turbidity: 1.5% of full scale
Laser wavelength: 780 nm
Scattering angles (clean water): 105º to 165º
Drift over time: <30 ppm per month
Drift over temperature: <200 ppm per ºC
Maximum sampling rate: 25 Hz
Maximum data rate: 2 Hz
Data capacity: 8 MB/200,000 lines
Battery capacity: 8 A h
Maximum battery life
External supply voltage: 6 to 18 Vdc
External supply current: 55 mA
Serial-data protocols: RS-232 & RS-485
Maximum housing depth: 300 m (984 ft)
Operating temperature range: 0º to 40ºC
Storage temperature range: –20º to 70ºC
5.5 Dimensions
Length / diameter: 380 mm (15 in) / 60 mm (2.4 in)
2
: 3,000 hrs
6
Weight: 2.04 kg (4.5 lb)
Weight (submerged): 1.02 kg (2.3 lb)
1
Range depends on pressure sensor option chosen.
2
Sampling interval is two hours and duration is two minutes.
6. Operation
6.1 Instrument Setup
6.1.1 Mounting Suggestions
OBS-5+ System
CAUTION
Maximum depth for the OBS-5+ is limited by the installed
pressure sensor. If the maximum depths are exceeded,
the pressure sensor will rupture and the housing will flood.
The depth limits are listed in TABLE 6-1.
TABLE 6-1. Working and Maximum Depths
Pressure Sensor Working Depth Maximum Depth
5 Bar 0 to 50 meters 75 meters
10 Bar 0 to 100 meters 150 meters
20 Bar 0 to 200 meters 300 meters
(1 Bar = 10 dBar ≅ 10 meters of fresh water)
The following precautions should be followed to ensure the unit can function
properly and is not lost or damaged.
• Always orient the unit so that the OBS-5+ sensor “looks” into water clear
of reflective surfaces.
• Pad the endcaps that contact metal with electrical tape, neoprene, or
expanded polyethylene tubes.
6.1.2 Surveys
• Never mount the instrument by its endcaps or attach anything to them.
This could a cause a leak.
The OBS-5+ will usually be towed with a cable harness for surveys. The serial
cable can tow the OBS-5+ without a depressor weight or vane as long as the
connector is strain relieved. Strain relief can be provided by attaching the
cable to the stainless steel housing (FIGURE 4-1) with a cable grip and a
length of 3 mm (1/8 in) wire rope. Install a cable clamp with a 0.5 m wire rope
to the serial cable, and clamp the wire rope to the pressure housing with two
stainless steel hose clamps, providing a small loop of slack cable to absorb
towing forces. The unit can be powered with an external battery, as shown on
FIGURE 6-1, and the serial output can be transmitted by either RS-232 or
RS-485 protocols. The latter protocol is recommended for cable lengths
greater than 25 m. An RS-485/232 serial converter is provided with each unit.
An RS-232 to USB converter is an available option.
7
OBS-5+ System
FIGURE 6-1. Electrical connections
6.1.3 Logging Data
In applications where a survey cable is impractical or when the OBS-5+ must
be attached to an instrument frame, it can be powered by the internal batteries
and the data can be logged by the data flash memory
Cyclic Mode sampling in Section 6.2.11, Cyclic Sampling.
6.1.4 Battery Installation
Remove the set screws from the endcap with the handle and connector. If the
unit was submerged during the previous day, turn the sensor end up, so water
around the O-rings can drain out when the endcap is removed, FIGURE 6-2
(A). Pull endcap out and disconnect the inline connector (B). Wipe water
from the inside wall of the housing tube with a paper towel (C). Turn the
connector end up and push the ridge on the battery sliding contact until the
spent batteries pop out (D). Insert new batteries with the positive terminal (+)
toward the sliding contact. Push the batteries down and slide the contact over
the top of them and against the housing wall. Inspect the O-ring in the cap,
clean and grease or replace it if necessary, and replace the cap and set screws.
8
6
7
5
4
3
2
6
6 – 18 V d.c. (Red)
Power GND (Black)
DB-9
3
8
4, 1, 5, 6, 9
DB-9
2
3
5
(RS485)
(A)
(B)
(GND)
(RS232)
(RD)
(TD)
(GND)
. See instructions for
8
OBS-5+ System
For extended deployment time, lithium batteries are a good alternative to
alkaline batteries. Campbell Scientific sells a C-cell-sized battery spacer
(pn 21905) that allows lithium C-cell batteries to be used with the OBS-5+.
Lithium C-cell batteries have a higher voltage than their alkaline counterparts,
necessitating the spacer. Campbell Scientific does not sell lithium C-cell
batteries.
FIGURE 6-2. Battery installation: A) endcap removal, B) wiping,
C) cable disconnect, and D) battery contact
6.2 OBS-5+Utility Software
6.2.1 Software Installation
Insert the CD and select “Install OBS-5+ Utility”. Follow the installation
wizard to install the software. The OBS-5+ Utility is the GUI interface with
the OBS-5+. As part of the installation, there are several additional files
included.
This section explains how to program and operate the unit with the OBS-5+
Utility. It covers: 1) turning the OBS-5+ ON and OFF and testing the sensors,
2) selecting sensors and data statistics, 3) scheduling data logging, 4) recording
9
OBS-5+ System
6.2.2 Running the OBS-5+ Utility
data with a PC or uploading data from the OBS-5+, 5) importing data into a
spreadsheet, and 6) plotting data with OBS-5+ Utility.
1) Set PC to the time standard for your project.
2) Select the OBS-5+ program to start the OBS-5+ Utility and open the Data
Window and Toolbar with the View pull-down menu.
3) The OBS-5+ Utility will create a new data log file and prompt you to
accept the name (see FIGURE 6-3). Files are automatically named with
Greenwich Date and Time as follows: OBS5+_20010808_172433.log. Or
you can create your own file name and destination by choosing N
FIGURE 6-4). Data received from the OBS-5+ while it is connected to the
PC will be stored in this file.
o (see
FIGURE 6-3. New data log prompt
FIGURE 6-4. Designating your own file name and destination
4) Connect the OBS-5+ to a PC with the test cable (FIGURE 6-1).
5) Click
OBS-5+ clock with your PC by clicking
Connect/Disconnect to get a green light and synchronize the
.
10
6.2.3 Pull-Down Menus
The OBS-5+ Utility has four pull-down menus for File, OBS-5+, View, and
Help.
OBS-5+ System
The File menu allows you to select the location and formatting for OBS-5+
files. Files can be opened as plots or ASCII text that can be brought into
spreadsheet programs or text editors. Plot files display OBS-5+ data
graphically in the main GUI window.
The OBS-5+ menu is used to: 1) put the instrument into a low-power Sleep, 2)
wake it up if it is sleeping (Wakeup), 3) make a Barometric Correction (see
Section 6.2.7, Water Density and Barometric Corrections), 4) view a list of
calibration tables, 5) set detector gains (Set Gains), 6) Retrieve an Active
Table, 7) Retrieve detector Gains, or 8) switch to RS-485 serial
communication.
The View menu controls the display on your PC. Switches are provided for:
• Toolbar toggles the icons ON and OFF.
• Status Bar toggles the status bar at the bottom of the screen ON or OFF.
• Data Window pops the data window into view.
6.2.4 Communication Settings
The Plot and Port Settings button has a serial port tab for configuring the
communication settings. The default settings are: 115 kB, 8 data bits, no
parity, no flow control. These settings will work for most applications and
with most PCs. In order to pick a slower baud rate and avoid data-transfer
errors, select the desired rate from the dialog box and click Apply. The rate
11
OBS-5+ System
adjustment takes two seconds. If your PC is set to the wrong rate for some
reason, use the check box to select ONLY change host computer port. Then
click Apply and the
If you get the OBS-5+ information box, the baud rate of the unit is
synchronized with your PC. If you don’t get an information box, repeat the
above procedure until communication is established.
6.2.5 Testing Sensors
1) Before daily operations and deployments, verify that the instrument works
by clicking
sensors and click Start Survey. An example plot of data is shown below.
OBS Settings button.
Open Plot, and then clicking Survey. Select all
12
OBS-5+ System
2) Wave your hand in front of the OBS-5+ sensor; the turbidity and SSC
levels on the top plot will fluctuate as data scrolls across the plot.
3) Blow into the pressure sensor or press your thumb on it to compress air on
the diaphragm (FIGURE 4-1). A small elevation in the pressure signal
will occur (bottom plot).
4) Click
depth corrections, and software versions.
Stop and then OBS Settings to view time, serial numbers,
6.2.6 Monitoring Turbidity (NTU)
The OBS-5+ was calibrated and factory-certified using AMCO Clear, U.S.
EPA-approved turbidity standards (www.gfschemicals.com). In order to
measure turbidity, the electronic gain of the near detector is set to the
calibration value and the active lookup table is overridden. Consequently, the
unit cannot simultaneously measure SSC (g L
NTU is selected. You must choose one or the other.
–1
) and NTU when either g L–1 or
6.2.7 Water Density and Barometric Corrections
Instrument depth is estimated from pressure and it is important to set the water
temperature and salinity so the OBS-5+ can correct for water density and
calculate depth correctly. The sensor measures absolute pressure so the
instrument must also correct for barometric pressure. Be sure to do this while
the OBS-5+ is at the surface. Depending on the magnitude of barometric
pressure fluctuations at the site and the desired accuracy, you may want to
correct data for atmospheric effects using barometric pressure simultaneously
recorded at a nearby site.
6.2.8 Sample Statistics
The individual measurements are not recorded in the data flash memory and
you must select the sample statistics that will be recorded. Two types of
statistics can be selected for OBS-5+ measurements.
13
OBS-5+ System
6.2.9 Sampling Modes and Terms
1) Measures of central tendency, including the mean and median.
2) Measures of variation or spread in sample values, including the standard
deviation (σ) and cumulative percentages, such as X
25 and X75 (where X is
the depth, SSC, or NTU values).
The mean is the arithmetic average of the values (∑ x / n), where ∑ x is the
sum of the sample values (x) and n is the number of values (sample size). The
median (X50) is the value that exceeds 50% of the sample values and is the
best measure of central tendency when a sample has outliers. The percentages
X25, X50, X75, etc., exceed 25, 50, and 75% of the sample values.
The following terms concern OBS-5+ sampling schedules.
Interval: The time in seconds between the start of one sample and the
beginning of the next. The interval must be longer than the duration to allow
for statistical computations and data storage. The computer will prompt you if
you select an interval that is too short.
Duration: This is the length of time in seconds that the OBS-5+ will measure
its sensors. The duration must always be less than the interval. The minimum
duration is five seconds and the maximum is 2,048 seconds (0.57 hour).
Rate: Rate is the frequency of sampling for the duration of measurements. All
sensors are sampled at the same rate, typically 2, 5, 10, or 25 times per second
(Hz).
Power: This indicates the percentage of time over the duration of a sample that
sensors are ON. Higher power levels mean larger samples and better statistics,
but shorter battery life. Lower levels spare the batteries but result in more
random noise in sample statistics (lower signal-to-noise ratios, SNR)
Sample Size: The number of measurements made by a sensor in each interval;
sample size equals rate times duration.
The main factors to consider when setting up OBS-5+ Cyclic sampling
schedules include:
• Sampling interval needed to characterize the processes of interest (for
example, water level fluctuations, flood and transport duration, tidal
conditions, dredge operations, etc.).
• Maximum sediment concentration.
• Statistical requirements, such as sample size and sampling rates.
• Battery capacity.
14
The goal is to pick a sampling scheme that gets essential information about the
process of interest without taking too many samples or sampling too often.
Inefficient sampling produces excessive battery consumption and a data
avalanche with unnecessary processing. Sampling schedules are set with the
Interval, Duration, and Rate parameters. Interval sets how often data are
OBS-5+ System
recorded. Select the longest interval that will show the changes in turbidity and
water depth that you wish to investigate. Rate sets measurement frequency.
The quicker turbidity and depth change, the higher the sampling rate should be
to get a stable average value for a sample. Finally, Duration sets how long
sensor outputs will be averaged. For example, with an interval of 30 seconds
and a duration of five seconds, the OBS-5+ will make measurements for five
seconds starting every 30 s. The sample size would be 5 x 25 = 125
measurements for a rate of 25 Hz. Always select duration and rate to give a
sample size of at least 30, and to reduce random sampling noise below 50% of
its maximum value, select them to give a size greater than 200.
Survey: Select the survey mode when operating the unit with a cable
connection to a PC and when high data rates are desired. Data can be logged
with a PC at rates up to 120 lines per minute (2 Hz).
Cyclic Sampling: Use cyclic sampling to record data internally in the 8
MB, flash memory at regular intervals; for example, every 1, 5, 15, or 30
minutes. Depending on the number of sensors measured and the statistics
selected, the OBS-5+ can log as many as 200,000 lines of data, one per hour
for 23 years, including time, date, depth, NTUs, and SSC.
6.2.10 Surveying
CAUTION
Click the OBS-5+ menu and select Barometric Correction.
Do not do this while the OBS-5+ is submerged.
The OBS-5+ takes about five seconds to measure the surface pressure and
compute a barometric correction.
1) Connect the OBS-5+ to PC with survey cable and start OBS-5+ Utility.
2) Open the Data Window with the View pull-down menu.
15
OBS-5+ System
3) Click the
lookup table icon and select a calibration table for your
survey. The last active table will be used otherwise.
4) Click
Survey to select: sensors, lines per minute, and depth units
(Meters or Feet). Set temperature and salinity for the survey area.
16
5) Click Start Survey and check the data flow in the Data Window.
6) A file for logging data was created when you started the OBS-5+ Utility.
You can review data at any time with
Open and import the log file
directly into an Excel spreadsheet for post-survey processing and plotting
(see Section 6.2.15, Excel Spreadsheets).
6.2.11 Cyclic Sampling
This mode is for logging data at regular intervals such as 1, 10, 15, 30, etc.
minutes, for example.
1) Request Barometric Correction from the OBS-5+ menu. Be sure to do
this while the OBS-5+ is at the surface (see Section 6.2.10, Surveying ).
2) Open the Data Window with the View pull-down menu.
3) Activate the lookup table for your survey area with and
activate buttons (Step 3, Section 6.2.10, Surveying).
OBS-5+ System
4) Click Cyclic Sampling and select sensors, statistics, depth units (meters or
feet), water temperature, and salinity.
5) Select Interval, Duration, Rate, and Power level; see recommendations
in Section 6.2.9, Sampling Modes and Terms.
6) Click Start Sampling to begin logging data and watch a few lines as they
are displayed in the data window to be sure the schedule is what you want.
Unplug test cable; install dummy plug and locking sleeve. The instrument
is ready for deployment.
17
OBS-5+ System
6.2.12 Data Retrieval
1) Remove dummy plug and connect OBS-5+ to PC with test cable (FIGURE
6-1).
2) Run OBS-5+ Utility.
3) Open the Data Window to verify that the instrument is transmitting data.
6.2.13 Shutdown
4) Click
a file.
5) Highlight the data with the start and end times you want.
to end data collection and use Offload Data to save data in
6) Click Browse, select a destination file and click OK.
7) Wait for the progress bar to disappear and examine data as a plot or text
file (see Section 6.2.5, Testing Sensors).
From the OBS-5+ menu, select Sleep.
18
6.2.14 Graphing and Printing
1) Use File menu to select how the data file will be opened.
OBS-5+ System
2) Click
when Open As Plot is selected. To print a text file, select Open As Text,
and use the Word Pad file print functions. For spreadsheet operations, see
the next section. The
your plot looks.
3) Use the Min and Max and Sample Range (End and Start) values to
bracket the data you need on the graph. Plot Width allows the graph to be
sized to fit a PC screen. On the depth plot, select Max = 0 and Min = the
maximum depth to display depth increasing downward.
Open and select a file to view. Print will print a graph
Plot and Port Settings is used for setting up
6.2.15 Excel Spreadsheets
To make an Excel spreadsheet from OBS-5+ data, start Excel and set file type
to All. Open a data file and select Delimited in Step 1 of 3 of the Text Import
Wizard. Click Next > and select the delimiter Space; check Treat consecutive
delimiters as one; and {none} for Text qualifier. In Step 3 of 3, select the
General Column data format and click Finish.
19
OBS-5+ System
7. Calibration
7.1 Sediment and NTU Calibration
In addition to the concentration, the size, shape, and reflectivity of suspended
sediment particles vary from one location to another and will influence OBS5+ measurements. When these sediment characteristics change, they will
produce apparent changes in SSC by themselves unless sediment calibrations
are performed. All sediments produce a unique set of calibration curves like
the examples shown in FIGURE 4-4. The sediment calibration procedure is
complicated, and for a modest fee, we will calibrate an OBS-5+ sensor with
your sediment. Call for a quotation to perform this service.
20
FIGURE 7-1. Manual (left) and automatic (right) sediment suspenders
7.1.1 General Guidance
The OBS-5+ uses response curves from the near and far detectors to create
three lookup tables like those shown on FIGURE 4-3 and FIGURE 7-2. The
objective of sediment calibration is to create lookup tables from the sediment
you will monitor. The OBS-5+ can store lookup tables for as many as 14
sediments. The fifteenth table is reserved for auto-saved archives.
OBS-5+ System
To view a calibration table, click
button (step 3 in Section 6.2.10,
Surveying), highlight a table number and click on the Activate button; then
click View Active. The calibration data table contains all the information
needed by the OBS-5+ to interpolate SSC values from the detector signals.
From left to right, the column lists: 1) SSC values (g L
derivatives for ND curve, 4) FD counts, and 5) 2
–1
nd
), 2) ND counts, 3) 2nd
derivatives for FD curve.
During a calibration, the first table, highlighted in red, is created from the
rising limb of the near detector response between zero counts and point “a”
(FIGURE 7-2). The second table, bold blue curve, is used for mid-range SSC
values. It is derived from the descending limb of the far detector curve
between points “a” and “c”. High SSC values are estimated from a third
lookup table, indicated by the bold green line and point “b”, and are derived
21
OBS-5+ System
from the descending limb of the near detector curve. A calibration consists of
a set of 15 to 30 calibration points that each includes an SSC value, a near
detector count, and a far detector count.
FIGURE 7-2. Lookup tables and table limits (a, b, and c)
Six to ten calibration points are needed to define each table. For the lookup
tables to function properly, the peaks in the ND and FD curves must be within
± 2,500 counts of one another, and to maximize resolution, the peak heights
should be between 62,000 and 64,500 counts. Start with the schedule shown in
TABLE 7-1 and adapt it to your sediment as required during the calibration
procedure.
Sediment preparation is a critical factor in calibration quality. Use dry material
whenever possible because it can be accurately weighed. Keep in mind that
mixing, grinding, and sieving can produce smaller sediment than you will
measure in the field and that the OBS-5+ is very sensitive to particle size (see
Section 10, Interfering Factors). This means that disaggregation can produce
measurement errors. Vigorous disaggregation with a sonic probe, for example,
can produce smaller particles that result in more ND and FD counts per unit of
SSC than less aggressive methods. An OBS-5+ calibrated with the former
material will underestimate SSC in the field.
22
OBS-5+ System
TABLE 7-1. Schedule of Concentrations for Sediment Calibrations
MUD (D60 < 62 µm) Sand (D60 > 62 µm)
Low
SSC
(0-5 g/l)
Mid-
range
(5-20 g/l)
High
SSC
(> 20 g/l)
Low
SSC
(0-10 g/l)
Mid-
range
(10-40 g/l)
High
SSC
(> 49 g/l)
0.0 5 25 0.0 12 50
1.0 6 30 2.0 14 60
1.5 7 40 3.0 15 70
2.0 8 50 4.0 20 80
2.5 9 60 5.0 25 90
3.0 10 6.0 30 100
3.5 15 8.0 35 140
4.0 20 10.0 40 160
The operator can control the electronic gain of the detector circuits to optimize
the instrument for his sediment. This can be done with the ND Gain and FD
Gain boxes in the OBS-5+ program during the calibration using the + and –
buttons to toggle gain up or down. The unit automatically scales all calibration
counts to match the selected gain. Set the initial ND and FD gain values to 16,
starting with the third SSC value in TABLE 7-2, adjust the gain to keep the ND
and FD counts in the range of 50,000 to 60,000.
CAUTION
While calibration points can be deleted at any time, never
start a new SSC value until you are satisfied with the
current values and gain setting.
The FD counts will peak first at around 2 to 7 g L–1 and the ND will peak
between 5 to 20 g L
calibration in cells D1 and E1 (default volume and density are 3.0 l and
2,650 g L
–1
). Every time sediment is added to the suspender, enter the grams
–1
. Set the water volume and sediment density for your
added to column A and read the current SSC value in column C.
TABLE 7-2. SSC-Calculation Spreadsheet
A
Grams
Added
0.00 0.00 0.00
1
1.00 1.00 0.33
2
2.00 3.00 1.00
2
3.00 6.00 2.00
3
4.00 10.00 3.33
4
B
Total
Grams
C
Cs (g/l)
(g/l)
D
Water
Volume (l)
E
Sediment
Density (g/l)
3.0 2650
23
OBS-5+ System
=
Sediment concentrations can also be calculated manually with the following
equations:
M
=
l/g
s
⎡
M
+
V
i
⎢
ρ
⎣
7.1.2 Equipment and Materials
• Dry, completely disaggregated bottom sediment or suspended matter from
the monitoring site
• Large black, neoprene or polyethylene tub
• 1-gallon (4 l) brown Nalgene polypropylene bottle with top cut off
• 1-liter volumetric flask
• Hand-drill motor (manual suspender)
• Mixing propeller (manual suspender)
• Scale with 10 mg accuracy
• Automatic suspender (optional)
s
=
⎤
s
⎥
s
⎦
i
=
ρ
s
(g) mass SedimentM
(liters) volume nitialIV
density edimentS
g/l) 103 x 2.65(usually
• Tea cup with round bottom and teaspoon
7.1.3 Procedure for Sediment
1) Put batteries in the OBS-5+ and connect it to a PC with test cable using the
RS-232 plug (FIGURE 6-1).
2) Start OBS-5+ Utility program; wake the OBS-5+; and click the
Settings button to verify its response.
OBS
24
OBS-5+ System
3) Click the
Select an EMPTY Table number for the sediment calibration.
4) Start the calibration with the button and secure the unit in a big black tub
filled with clean tap water (FIGURE 7-3). The sediment-calibration dialog
will appear (the initial display will not show the red and green symbols).
button to view the list of lookup tables stored in the unit.
FIGURE 7-3. OBS-5+ in big black tub of clean water
25
OBS-5+ System
5) Enter 0.001 in the value box and click the Record button to log the clear-
water data point. The unit will take 1,200 measurements in 60 seconds.
When the process is complete, the data appear in the data table and the ND
(red) and FD (green) points will be plotted on the calibration graph. If you
are satisfied with the data, mount the unit in the suspender.
6) Position the OBS-5+ so that it produces the minimum FD signal in clear
water (FIGURE 7-4).
FIGURE 7-4. OBS-5+ in suspender tub
26
7) Weigh the first increment of sediment with the electronic balance (see
TABLE 7-1) and transfer it to a tea cup with a rounded bottom.
OBS-5+ System
8) Withdraw about 10 ml of water from the suspender and add it to the tea
cup containing the dry sediment. Stir the water-sediment mixture into a
homogeneous slurry, breaking up clumps of sediment as you go.
9) Pour the sediment slurry into the suspender and rinse the cup with
suspender water until it is clean. Make sure all the sediment gets from the
cup to the suspender.
–1
10) Compute the SSC value in g L
for the current calibration point with
SSCcalculator spreadsheet (TABLE 7-2), or use the SSC formula, Section
7.1.1, General Guidance, and enter the SSC value in the Value box.
11) Click the View Sensor button to see the ND and FD counts before they are
recorded. Adjust the gain if necessary, then click the Record button.
12) The second calibration pair of points will appear on the graph and the data
will be listed in the table. If the data for the current point is unacceptable
for any reason, too much or too little gain for example, highlight the data
line number (#) and click the Delete button. Adjust the gain and record it
again with the same SSC value.
CAUTION
Do not proceed to the next SSC value until you are
satisfied with the current data. Once you add sediment to
the suspender, you cannot remove it.
13) Repeat Steps 7 through 12 for the remaining SSC values following the
guidelines provided above. When the calibration data is complete, the data
table and plot will look like the ones shown below.
14) To compute the lookup tables, click the Calculate Fit button and supply
the requested information. Referring to FIGURE 7-2, pick point ‘a’ by
counting the number of data points on the ND curve from the origin (0, 0)
to the first point beyond where the ND and FD curves cross, 6 in FIGURE
7-2. Select ‘b’ at the first point on the falling limb beyond the ND peak
where it becomes linear, 10 on FIGURE 7-2. Point ‘c’ is the first point in
27
OBS-5+ System
7.2 Turbidity (NTU) Calibration
7.2.1 Equipment and Materials
7.2.2 Procedure for Turbidity
the FD curve where it starts decay exponentially, also 10 on FIGURE 7-2
9. Points ‘b’ and ‘c’ will usually have the same numerical value.
15) Save the table in the EMPTY Table number selected at Step 3.
• Large black, neoprene or polyethylene tub
• 100 mm test cylinder (www.deslinc.com, No. TC4)
• AMCO Clear turbidity standards (GFS No. 8429, 8430, and 8431,
www.gfschemicals.com)
1) Put batteries in the OBS-5+ and connect it to a PC with test cable using the
RS-232 plug (FIGURE 6-1).
2) Start the OBS-5+ Utility software; wake the OBS-5+; and click the OBS
Settings button to verify its response.
3) Use the OBS-5+ pull-down menu, select Gain and set ND and FD gain to
2. This is the setting used for the factory calibration.
4) Start the calibration with the button and secure the unit in a big black tub
filled with clean tap water (FIGURE 7-3). The NTU-calibration dialog
will appear.
28
5) Enter 0.3 in the value box and click the record button to log the clear-water
data point. The unit will take 1,200 measurements in 60 seconds. When
the process is complete, the data appear in the data table and the point will
be plotted on the calibration graph. If you are satisfied with the data,
mount the unit in a 100 mm calibration cup as shown on FIGURE 7-5.
OBS-5+ System
FIGURE 7-5. OBS-5+ in 100 mm cup
6) Add enough 250-NTU standard to cover the sensor end (FIGURE 4-1) and
swipe bubbles off the sapphire windows with your finger. Click the
Record button.
7) Repeat Step 6 for the 500 and 1,000 NTU standards.
8) Review the data table and graph, and if they look satisfactory, click the
Calculate Fit button.
9) Verify that the fit curve passes through the calibration points and that the
residuals are less than 10 NTU. Then click the Done button.
8. Troubleshooting
This section will help isolate problems that can be easily fixed, such as cablecontinuity, processor reset, and battery replacement, or more serious ones, such
as sensor, computer and electronic malfunctions, and damaged mechanical
parts that will require assistance. The problem symptoms are shown in bold
text.
Power failed because of contact corrosion or a broken power wire.
Check for a broken red wire connecting the battery tube and circuit board.
Green powder or tarnish on the battery contact parts indicates salt-water
corrosion. Remove the electronics by removing the set screws from the sensor
endcap and sliding it out of the pressure housing. Pull battery-contact-retainer
pin out with needle-nose pliers and slide the contact from its track. Clean the
29
OBS-5+ System
corroded surfaces of the contact and track with a scouring pad and reassemble
unit.
Unit does not communicate with PC.
There are several possible causes for this symptom.
1) The batteries are dead.
2) The OBS-5+ will not wake up.
3) The test/umbilical cable is damaged or improperly connected
4) The OBS-5+ and PC are set to different baud rates or communication
protocols (for example, RS-232 versus RS-485).
• Click
port tab. The default baud rate is 115.2 kbps. If the PC is not set to
this speed, follow the steps in Section 6.2.4, Communication Settings,
to set it.
• If the OBS-5+ still fails to respond, try changing PC speeds and
clicking
example, 57.6, 38.4, 19.6, 9.6 kbps, etc.). If this fails, switch the PC
back to 115.2 kbps and do the following steps.
• Reconnect the cable and click
• Replace the batteries and click
• If you have a survey cable, connect instrument to external power and
click
• Remove the unit from the pressure housing and press and release the
RESET button. Click
Plot and Port Settings and check port settings on the serial
OBS Settings until communication is established (for
.
.
.
.
30
OBS-5+ System
FIGURE 8-1. Internal components
OBS-5+ or pressure sensor malfunction.
• Open unit and inspect for: 1) broken sensor wires, and 2) loose pressure
sensor connector (FIGURE 8-1).
• Check sensor power by clicking
green LEDs should illuminate. If they do not, the sensor power circuit
may not be working.
• If the depth sensor reads high and does not change, it may need to be
cleaned (see Section 9.2, Pressure Sensor).
• If the sensors appear to be in working order, the digitizer or
microcontroller may be damaged. Such problems require factory service.
Survey and selecting all sensors; the
31
OBS-5+ System
Bright sun near the surface (< 2 m) or black-colored sediments cause
erroneous OBS readings.
Do not survey in shallow water between 10:00 and 14:00 local time and avoid
areas with suspended black mud; see Section 10.2, NIR Reflectivity.
OBS-5+ indicates different NTU values in the field than other
turbidimeters.
Not all turbidity meters read the same! OBS-5+ sensors are checked with U.S.
EPA-approved AMCO Clear turbidity standards before leaving our factory (see
Appendix A). Other turbidimeters will read different NTU values on natural
water samples.
OBS-5+ indicates different suspended sediment levels in the field than in
the laboratory.
This results from a change in sediment size or color (see Section 10, Interfering
Factors). You may have to perform a field calibration with water samples.
9. Maintenance
9.1 OBS-5+ Sensor
The sapphire windows over the laser diode and the detectors must be kept
clean to make accurate SSC measurements (FIGURE 4-1 and FIGURE 4-2). A
gradual signal decline over a period of days to weeks indicates fouling with
mud, oil, or biological material. Regular cleaning with a water jet, mild
detergent and warm water, or a scouring pad will remove most contaminants
encountered in the field. A cloth with solvent or mineral spirits can be used to
remove oil and grease. However, do not use MEK, benzene, toluene, acetone,
TCE, or electronic cleaners as they could damage the epoxy bond between the
sapphire and the optic bushings. At the conclusion of each survey or
deployment, clean the OBS-5+ sensor. If thick bio-fouling has developed,
scrape the material off the window with a flexible knife then swipe it with a
scouring pad.
9.2 Pressure Sensor
The silicon strain-gauge pressure sensor is located under a perforated disk and
spring-clip that protects the Hastelloy diaphragm isolating it from water
(FIGURE 4-1). Do not touch the diaphragm with tools or pointed objects, as
the instrument will leak if it is pierced. Clean the sensor with a water jet
directed at the disk after each survey or deployment to flush sediment from
between the disk and the sensor. Do not allow sediment to dry on the sensor
diaphragm because dry sediment will reduce accuracy and is difficult to wash
off. To clean the diaphragm, remove the spring clip with tru-arc pliers and the
disk with plastic tweezers, then gently wipe sediment off the diaphragm with a
wet cotton-tipped swab. Replace the disk and spring clip and flush with a water
jet.
32
9.3 Batteries
OBS-5+ System
The unit runs on three, C-size, alkaline batteries. Buy the expensive ones with
the longest expiration date (“use before May 20XX”). While operating
continuously, the OBS-5+ will run 125 hours (15, eight-hour surveys) in
Survey Mode and for as long as 3,000 hours in the Cyclic Mode.
CAUTION
Always put OBS-5+ to sleep for storage to conserve
battery capacity (see Section 6.2.13, Shutdown).
Refer to FIGURE 6-2 for battery installation. Battery life will depend on the
percentage of time the unit is sampling. TABLE 9-1 shows battery life as a
function of sample duration and interval to assist with planning your sampling
schedule (see Power in Section 6.2.9, Sampling Modes and Terms).
TABLE 9-1. Battery Life in Hours with 100% Power
Interval
(s)
60
600
900
1800
3600
> 3000 2050 780 1180 400
> 3000 > 3000 1100 2040 600
> 3000 > 3000 1930 > 3000 1100
> 3000 > 3000 > 3000 > 3000 1930
9.4 Pressure Housing
10
100%
480 NO NO NO NO
60
50%
60
100%
120
50%
120
100%
The pressure housing and O-ring seals require little maintenance other than
careful inspection every six months and service before moored deployments.
1) Disassemble O-ring seals and inspect mating surfaces for pits and
scratches.
2) Inspect O-rings for cuts and nicks; replace if necessary.
3) Clean O-rings and mating surfaces with a cotton swab and alcohol.
Remove fibers from groove and mating surfaces then grease O-rings
Molykote
®
Compound 55 and reassemble.
9.5 User-Serviceable Parts
Alkaline C cells and the components of the 21304 Accessory Kit can be
purchased as replacement parts. Campbell Scientific manufacturing part
numbers and product descriptions follow:
pn 20993 End-cap O-ring, Parker 2-136
pn 20989 Optic Bushings O-ring, ARP5 7.5 X 1.2 mm OBN
pn 21141 S.S. End-cap Screws, 5/16” X 3/8”
pn 21120 Dummy Plug, Subconn
pn 21122 Plug Locking Sleeve, Subconn
®
MCDC8M
®
MCDLSF
33
OBS-5+ System
pn 4576 Alkaline C-Cells Batteries
pn 20806 OBS-5+ Test Cable, 2 m (6.5 ft)
pn 21381 7-piece Allen Wrench Set, 5/64 to 3/16 Ball End
pn 21139 SS Hex Socket Screw, #2-56 x .187
10. Interfering Factors
Changes in sediment concentration (SSC) are the primary cause for OBS-5+
output fluctuations in the environment. In some monitoring areas, however,
factors other than SSC, will cause the OBS-5+ to indicate SSC variation that
are invalid and which the user does not wish to measure. These factors are
called interferences because they cause apparent shifts in SSC that are not real.
Interferences include particle size, shape, reflectivity, flocculation, and
disaggregation. This section summarizes some of the important ones that you
might encounter while using an OBS-5+.
Sensitivity is the change in light scattering intensity, indicated by ND and FD
counts, per unit change in SSC or an interfering factor. It is therefore a good
measure of relative interference. An interference that reduces sensitivity will
cause SSC to appear to decrease and one that makes an OBS-5+ more sensitive
will cause the opposite effect. Interfering factors are ranked by the ratio of the
sensitivity change that is caused when the factor changed over its full range in
the environment. For example, the size of suspended sediment particles in the
environment ranges from about 0.5 to 125 μm. This range causes relative OBS
sensitivity to change from 2 to 0.008, (see next section), giving a factor of 250.
Interfering factors ranked in this way are summarized in TABLE 10-1. The
ranking shows, for instance, that the size of suspended particles can affect
OBS-5+ measurements more than particle shape or NIR reflectivity by a factor
of 25. Interferences can be tolerated so long as the resulting errors fall within
acceptable limits.
Relative Magnitude <250 10 10 2
10.1 Particle Size
The trend of the line on FIGURE 10-1 shows that relative sensitivity declines
at a rate inversely proportional to the particle diameter. The graph provides a
useful method for estimating the relative effect of grain size on OBS-5+
sensitivity. Using this method, for example, one gram of silt with a grain size
of 10 microns, suspended in a liter of water, might produce an OBS signal of
55,000 counts, whereas a gram of sand per liter with a grain size of 100
microns would produce only 5,500 counts, with other factors such as shape and
reflectivity being the same.
TABLE 10-1. Relative magnitude of the effects of sediment
characteristics on OBS-5+ sensitivity
Interfering Factor
Size Shape Reflectivity Flocculation
34
OBS-5+ System
FIGURE 10-1. Effects of sediment size
10.2 NIR Reflectivity
The output of an OBS-5+ will increase with the NIR reflectivity of suspended
sediment independent of SSC. This can degrade accuracy when unknown
reflectivity changes occur during a monitoring campaign. For instance, when a
dredge cuts through a layer of oxidized, light-brown, reflective mud into an
underlying layer of black anoxic mud, the OBS-5+ will indicate that SSC of
mud stirred up by the cutter has dropped even when it has not. The relative
signal level per unit of SSC varies from a low of about one for dark, nonreflective sediment to a high of about ten for white, reflective sediment
(FIGURE 10-2). In areas where sediment color changes with time, more than
one calibration curve may be required to measure SSC with an OBS-5+.
35
OBS-5+ System
Reactive Sensitivity
FIGURE 10-2. Near-infrared reflectivity of minerals
10.3 Particle Shape, Flocculation, and Disaggregation
Particle shape can be an interfering factor. The sensitivity of an OBS-5+
sensor to plate-shaped particles is about ten times higher than it is to spherical
particles. Disaggregation of dry sediment by grinding can cause the sediment
to become finer grained than it was in the environment and this will bias a
sediment calibration. A good example of how much OBS-5+ readings can
change as a result of the disaggregation is shown in FIGURE 10-3. The slope
of each line on the graph indicates the sensitivity of an OBS sensor when
calibrated with sediment disaggregated in a particular way. The more
mechanically aggressive a disaggregation method is, the more sensitive the
sensor will be. Sediments susceptible to disaggregation effects include: 1)
organic-rich estuarine mud, 2) cohesive and flocculated suspended matter, and
3) clay-rich sediment.
36
OBS-5+ System
FIGURE 10-3. Effects of disaggregation methods
Finally, flocculation of clay particles in estuaries can affect sensitivity by
causing small particles to clump together into larger ones to which the OBS-5+
is less sensitive. For example, when a dredge works into a zone of saline water
where flocculation occurs, an OBS-5+ can indicate less than the actual level of
SSC.
A summary of interference effects on OBS-5+ measurements follows.
1) Any action that makes sediment particles smaller, such as disaggregation,
or larger, aggregation, during the calibration process than they will be in
the environment will cause SSC to appear to increase or decrease when
there has been no actual change.
2) Processes in the environment that make sediment particles larger than they
were during a calibration will produce an apparent decline in SSC.
Conversely, an environmental process that makes particles smaller, such as
deflocculation or disaggregation, will cause SSC to appear to increase.
3) When particles become more or less reflective than they were during
calibration, or their shape changes, SSC will appear to decrease or increase
independent of an actual SSC change.
37
OBS-5+ System
11. References
Black, K.P., M.A. Rosenberg. 1994. Suspended Sand Measurements in a
Turbulent Environment: Field Comparison of Optical and Pump Sampling.
Coastal Engineering, 24, pp. 137-150.
Conner, C.S. and A.M. De Visser. 1992. A Laboratory Investigation of Particle
Size Effects on an Optical Backscatterance Sensor. Marine Geology, 108, pp.
151-159.
Downing, John. 2006. 25 Years with OBS Sensors: The Good, the Bad, and the
Ugly. Continental Shelf Research.
Downing, John. 2005. Turbidity Monitoring. In: Environmental
Instrumentation and Analysis Handbook. John Wiley & Sons, New York, pp.
511-546.
Downing, John. 1998. Suspended Particle Concentration Monitor. U.S. Patent
Number 5,796,481.
Downing, John and Reginald A. Beach. 1989, Laboratory Apparatus for
Calibrating Optical Suspended-solids Sensors. Marine Geology, 86, pp. 243-
249.
Gibbs, R.J. 1978. Light Scattering from Particles of Different Shapes. Journal
of Geophysical Research, 83, pp. 501-502.
Gibbs, R.J., E. Wolanski. 1992. The Effects of Flocs on Optical Backscattering
Measurements of Suspended Material Concentration. Marine Geology, 107, pp.
289-291.
Sutherland, T.F., P.M. Lane, C.L. Amos, and John Downing. 2000. The
Calibration of Optical Backscatter Sensors for Suspended Sediment of Varying
Darkness Level. Marine Geology, 162, pp. 587-597.
38
Appendix A. Turbidity Standards
AMCO Clear, supplied by GFS Chemicals (www.gfschemicals.com), is an
approved calibration standard and is the one we use to certify our instruments.
It is made from styrene divinylbenzene (SDVB) microspheres. SDVB spheres
have a median size and standard deviation of 0.28μm (~1/5 that of formazin
particles) and 0.10 μm respectively and a refractive index of 1.56. As shown
on the SEM image, they are dimensionally uniform. SDVB standards are
formulated especially for OBS meters and cannot be used with different
meters. Superior physical consistency of AMCO Clear results in a more
precise calibration standard, giving standard errors less than 1% compared to
2.1% for formazin and better linearity, 0.15 NTU compared to 0.32 for
formazin.
(Photos courtesy of GFS Chemicals)
The key benefits of SDVB standards are: 1) < 1% lot-to-lot variation in
turbidity; 2) consistent optical properties from 10 to 30°C; 3) guaranteed oneyear stability; 4) mixing and dilution are not required; and 5) they are not toxic.
Two drawbacks are that SDVB standards can only be used with the instruments
for which they are made and they are more expensive than formazin. For
example, one liter of 4000-NTU standard costs about twice as much as an
equivalent amount of 4000-NTU formazin. Our instruction manuals explain
how to use turbidity standards and the instructions provided by the suppliers
tell how they should be handled.
A-1
Appendix A. Turbidity Standards
In the USA, formazin is a primary standard for the calibration of
turbidimeters. The median particle size of formazin is 1.5 μm; the standard
deviation of size is 0.6 μm (see size distribution graph); and as shown by the
SEM images below, formazin particles have many different shapes. The
preparation, storage, and handling of formazin will affect its accuracy and
stability. Recommended formazin storage times are listed in the accompanying
table. Working standards are prepared by volumetric dilution of 4000-NTU
stock formazin with distilled water. For example, a 2000 NTU calibration
standard is made by mixing equal volumes of stock formazin and distilled
water.
Turbidity
(NTU)
Maximum
Storage Time
1 – 10 1 day
2 – 20 1 day
10 – 40 1 day
20 – 400 1 month
> 400 1 year
Besides being the primary standard, formazin has two other advantages. It is
available from several chemical and scientific suppliers (www.vwrsp.com,
www.ColePalmer.com, www.riccachemical.com, and www.labchem.net) and it
is the least-expensive, commercially available standard. Formazin also has a
couple of disadvantages: 1) it has a MSDS health-hazard rating of 2;
2) turbidity can vary by ± 2% from the lot to lot; 3) the size, shape, and
aggregation of formazin particles change with temperature, time, and
concentration; 4) it settles in storage and must be mixed immediately prior to
use; and 5) dilute formazin standards have a storage life as short as one hour.
A-2
Appendix A. Turbidity Standards
We must emphasize that unlike SSC, which has physical units, turbidity values
(NTUs, FTUs, etc.) do not. Therefore, if you measure water turbidity to be 100
NTU, you cannot directly infer any physical quantities from it. Turbidity
values do not represent particular SSC values, indicate light levels at the
bottom of a stream, or quantify biological process’. Moreover, it is often
assumed that turbidity standards behave optically like sediment. This is
possible when the size, NIR reflectivity, refractive index, and shape of the
sediment and the turbidity standard are similar; this is an extremely rare
occurrence. For example, even the median diameters of the two approved
calibration standards differ by a factor of more that five and the shape of
SDVB and formazin particles also differ; see NTU-SSC relationships.
Reference:
John Downing. 2005. Turbidity Monitoring. Chapter 24 in: Environmental
Instrumentation and Analysis Handbook. John Wiley & Sons, Pages: 511-
546. 2005.
Sadar, M. 1998. Turbidity Standards. Hach Company Technical Information
Series – Booklet No. 12. 18 pages.
Papacosta, K. and Martin Katz. 1990. The Rationale for the Establishment of
a Certified Reference Standard for Nephelometric Instruments. In:
Proceedings, American Waterworks Assoc. Water Quality Technical
Conference. Paper Number ST6-4, pp. 1299-1333.
Zaneveld, J.R.V., R.W. Spinrad, and R. Bartz. 1979. Optical Properties of
Turbidity Standards. SPIE Volume 208 Ocean Optics VI. Bellingham,
Washington. pp. 159-158.
A-3
Appendix A. Turbidity Standards
A-4
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