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INSTRUCTION MANUAL
Oxygen Meter
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LIMITED ONE YEAR WARRANTY
Manufacturer warranties all instruments (excluding batteries, damage
caused by batteries, probes, standards, buffers) against defects in
materials and workmanship for one year from date of original purchase.
During this warranty period, the manufacturer will repair or at their
option, replace at no charge a product which proves to be defective,
provided the product is returned, shipping prepaid to the manufacturer's
service center.
This warranty does not apply to damage caused by accident or misuse
or as a result of service or modification by other than an authorized
service center. No other express warranty is given. Repair or replacement of product is your exclusive remedy. In no event shall the manufacturer be liable for consequential damages.
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If any of these problems exist, change the membrane and solution,
also check batteries.
TABLE OF CONTENTS
9.3 No response or readings are inconsistent.
The batteries should provide about 100 hours of continuous operation.
An analog meter will have no response when the batteries need to be
replaced. A digital meter will display "BAT" when the batteries are low.
Inspect the meter and batteries for corrosion.
9.3.1 To replace batteries in bench models:
9.3.1.1 Turn the meter off.
9.3.1.2 Remove screws, one in each foot on bottom.
9.3.1.3 Gently lift the shroud off.
9.3.1.4 Remove batteries and replace, noting polarities.
9.3.1.5 Reassemble meter.
9.3.2 To replace batteries in field units, follow sections 9.3.1.1
and 9.3.1.4.
9.3.3 On Unit with rechargable battiries return to dealer for repair.
If the problem continues, call your distributor/dealer.
1.0 Introduction......................................................................2
2.0 Theory of Operation............................................................2
3.0 Specifications....................................................................3
4.0 Instrument Familiarity...........................................................4
5.0 Probe Familiarity..........................................................5
6.0 Operation...........................................................................6
7.0 Calibration......................................................................9
8.0 Measurement Guidelines ....................................................14
9.0 Troubleshooting Guide......................................................15
Warranty .............................................................Inside Back Cover
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8.2 Membrane Life
Depending on usage and environment, membranes can last from
a few hours to as long as several months. The membrane should
be replaced when readings become erratic or the calibration is
unstable. If the filling solution evaporates or a large bubble forms
in the reservoir, the probe should be refilled.
8.3 Stirring Effects
Since oxygen is reduced as it reaches the electrode, it is necessary to insure a continuous supply of fresh sample in order not to
deplete the oxygen surrounding the probe. A flow of at least one
foot per second is ideal, do not exceed five feet per second.
When there is no flow, stirring can be accomplished by raising and
lowering the probe, in the sample. In the lab, a magnetic stirrer
works well. Adjust the stirring speed until increasing the speed
has no affect on the oxygen reading. Keep in mind that the heat
from the stirrer will affect the sample, the sample should therefore
be insulated.
8.4 Measurement Errors
The membrane is a gas permeable teflon fluorocarbon resin. In
addition to oxygen, the following gases will affect the meter
reading; sulfites, sulfates, carbon monoxide, carbon dioxide,
hydrogen, nitrogen, chlorine, nitrous oxide, and nitric oxide. If an
erroneous reading is suspected, the interfering gas should be
removed by chemical means.
In some applications (i.e.. .sea water or muddy water tests), buildups may form on the membrane and reduce permeability. Frequent cleaning or membrane replacement may be required. Avoid
submersing the probe into chemicals that may attack the
probe. Consult a Chemical Resistance chart if you are not sure of
the affect on the probe.
9.0 TROUBLESHOOTING
9.1 First isolate the problem to the meter or the probe.
9.2 If the sensitivity of the oxygen sensor decreases:
9.2.1 Inspect the probe membrane for air bubbles, wrinkles
or tears in the membrane, and adequate reservoir solution.
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EXAMPLE: IF TEMPERATURE = 20°C
TABLE III PRESSURE = 760 mmHg
CHLORINITY = 20 g/L
TABLE III OXYGEN = 7.3 mg/L
ACTUAL PRESSURE = 775 mmHg
CORRECTION FACTOR = 1.02
THEN OXYGEN = (7.3 mg/L x 1.02)
= 7.45 mg/L
TABLE IV ALTITUDE CORRECTION FACTOR
5
Atmospheric Equivalent Correction Atmospheric Equivalent Correction
Pressure Altitude Factor Pressure Altitude Factor
(mmHg) or (ft) = (mmHg) or (ft) =
775 -540 1.02 638 4,756 .84
760 0 1.00 623 5,403 .82
745 542 .98 608 6,065 .80
730 1,094 .96 593 6,744 .78
714 1,688 .94 578 7,440 .76
699 2,274 .92 562 8,204 .74
684 2,864 .90 547 8,939 .72
669 3,466 .88 532 9,694 .70
654 4,082 .86
8.0 MEASUREMENT GUIDELINES
8.1 Membrane Function
In a polarographic probe, the membrane is one of the most important
elements. The thin, permeable membrane must be stretched uniformly
over the sensor. This isolates the internal sensor elements and filling
solution from the environment while still allowing oxygen to enter. When
polarizing voltage is applied to the sensor, the oxygen which passes
through the membrane reacts, at the cathode, causing a current flow.
Oxygen passes through the membrane at a rate proportional to the pressure difference across it. The oxygen pressure inside is zero, due to its
rapid consumption at the cathode. Therefore, the oxygen diffusion through
the membrane is proportional only to the absolute oxygen pressure outside
the membrane.
pressure = O2 diffusion = CURRENT
O
2
Any large air bubbles trapped under the membrane will cause the membrane to deform as the pressure changes. It is, therefore, desirable to
eliminate the bubbles. Tapping the probe gently will eliminate the bubbles.
5
Derived from the CRC Handbook of Chemistry and Physics, 69th Ed., 1988 - 1989.
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1.0 INTRODUCTION
Your new dissolved oxygen meter is a compact, battery operated,
field or lab instrument designed to measure the dissolved oxygen
level and temperature of water and aqueous solutions. This
meter is rugged and weather resistant. It is not, however, waterproof. Do not drop the meter into a lake, pond or other body of
water, nor allow it to become overly wet in the rain. The heart of
the meter is the oxygen probe, a polarographic sensor designed
for fast response and low maintenance. It includes a thermistor
for temperature measurement and to compensate for changes in
oxygen sensor response due to temperature.
Some models have analog (continuous needle movement) displays. Others have digital (numeric) displays. All models are
powered with 8 AA batteries. Bench models will also operate
from an optional 9 VDC 200 mA wall plug adaptor. The battery
pack disconnects when the adaptor is plugged into the meter.
2.0 THEORY OF OPERATION
There are two basic methods used to measure dissolved oxygen;
the chemical methods, such as the Winkler or iodometric method,
and the instrument method, which uses a membrane electrode.
The test method used depends on the interferences present, the
accuracy desired, convenience, and expedience.
The chemical methods are the methods of choice when interferences are minimal, the required number of tests are small, and
high precision and reliability are required. Measuring dissolved
oxygen with an instrument using a membrane electrode is preferred for field testing, continuous monitoring, in situation determinations, or anytime a quantity of samples need to be read in a
short time period. For nondestructive testing, instrumental methods are the only methods available.
The membrane electrode is composed of a cathode and anode in
contact with an electrolyte solution, separated from the sample by
an oxygen permeable plastic membrane, usually made of polyethylene or fluorocarbon, which is permeable to oxygen and acts
as a diffusion barrier against
impurities.
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The measurement of oxygen is accomplished by applying a voltage
across the sensor, reducing the oxygen which has passed through
the membrane. The resulting diffusion current is linearly proportional to the concentration units displayed on the meter display.
Determine the chlorinity and temperature of the sample, then the
air saturated oxygen can be found, using the table. The chlorinity
is found by using this formula:
Chlorinity = salinity : 1.80655
A change in temperature will have a significant effect on the dissolved oxygen measurement. Many oxygen sensors include
thermistors, in the electrode circuit, to automatically compensate
for temperature changes. If the oxygen sensor does not have a
thermistor or if very high temperatures are present, then a calibrated nomographic chart should be used to correct for the
temperature effect.
3.0 SPECIFICATIONS
Readout: 6"Analog Meter 3 Digital LCD
Range: 0 - 15 ppm 0 - 19.99 ppm
Accuracy: 0.1ppm 0.1ppm
Saturation Range: 0 - 120% NA
Saturation Accuracy: 1% NA
Temperature Compensation: Automatic
Temperature Readout: 0 - 50 °C
Electrode: Polarographic
Size
Bench Models: 5"H x 8"W x 5"D
Field Models: 4"H x 12"W x 8"D
Weight
Bench Models: 2.7 lbs (1.2 Kg)
Field Models: 4 lbs (1.8 Kg)
Power
Bench Models: 8 - 1.5 V AA Batteries or
Optional Wall Plug Adaptor
Field Models: 8 - 1.5 V AA Batteries or 8
AA Nicads
Battery Life: approx. 200 hrs.
EXAMPLE: IF TEMPERATURE = 20°C
PRESSURE = 760 mmHg
SALINITY = 36.1 g/L
CHLORINITY = 20 g/L
THEN OXYGEN = 7.3 ppm (mg/L)
TABLE III SOLUBILITY OF OXYGEN IN WATER
AT ATMOSPHERIC PRESSURE (760 mmHg)
Chlorinity g/L
0 5 10 15 20
4
Temp °C Dissolved Oxygen mg/L
0 14.6 13.7 12.9 12.1 11.4
2 13.8 13.0 12.2 11.5 10.8
4 13.1 12.3 11.6 10.9 10.3
6 12.5 11.7 11.0 10.4 9.8
8 11.8 11.2 10.5 9.9 9.4
10 11.3 10.7 10.0 9.5 9.0
12 10.8 10.2 9.6 9.1 8.6
14 10.3 9.7 9.2 8.7 8.2
16 9.9 9.3 8.8 8.4 7.9
18 9.5 9.0 8.5 8.0 7.6
20 9.1 8.6 8.2 7.7 7.3
22 8.7 8.3 7.9 7.5 7.1
24 8.4 8.0 7.6 7.2 6.8
26 8.1 7.7 7.3 7.0 6.6
28 7.8 7.4 7.1 6.7 6.4
30 7.6 7.2 6.8 6.5 6.2
7.2.4 Atmospheric Pressure Compensation
Table III gives dissolved oxygen concentrations at various
chlorinity (salinity) concentrations at an atmospheric pressure of
760 mmHg. To determine the dissolved oxygen concentration at
other atmospheric pressures, multiply the values obtained from
Table III by the correction factor given in Table IV.
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4
Standard Methods for the Examination of Water & Wastewater, 17th Ed., pg 4 - 154, 1989.
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TABLE II DISSOLVED OXYGEN CALIBRATION VALUES
(ppm)
TEMP. PRESSURE mmHg
°C °F 775 760 750 725 700 675
0 32 14.9 14.6 14.4 13.9 13.4 12.9
2 36 14.1 13.8 13.6 13.2 12.7 12.2
4 39 13.3 13.1 12.9 12.5 12.0 11.6
6 43 12.7 12.4 12.3 11.8 11.4 11.0
8 46 12.0 11.8 11.7 11.3 10.9 10.5
10 50 11.5 11.3 11.1 10.7 10.4 10.0
12 54 11.0 10.7 10.6 10.2 9.9 9.5
14 57 10.5 10.3 10.1 9.8 9.5 9.1
16 61 10.0 9.8 9.7 9.4 9.0 8.7
18 64 9.6 9.4 9.3 9.0 8.7 8.4
20 68 9.2 9.1 8.9 8.6 8.3 8.0
22 72 8.9 8.7 8.6 8.3 8.0 7.7
24 75 8.6 8.4 8.3 8.0 7.7 7.4
26 79 8.3 8.1 8.0 7.7 7.4 7.2
28 82 8.0 7.8 7.7 7.4 7.2 6.9
30 86 7.7 7.6 7.4 7.2 6.9 6.7
32 90 7.4 7.3 7.2 7.0 6.7 6.4
34 93 7.2 7.1 7.0 6.7 6.5 6.2
36 97 7.0 6.8 6.7 6.5 6.3 6.0
38 100 6.8 6.6 6.5 6.3 6.1 5.8
40 104 6.6 6.4 6.2 6.1 5.9 5.7
42 108 6.4 6.2 6.1 5.9 5.7 5.5
44 111 6.2 6.0 5.9 5.7 5.5 5.3
46 115 6.0 5.8 5.8 5.5 5.3 5.1
48 118 5.8 5.7 5.6 5.4 5.2 5.0
50 122 5.6 5.5 5.4 5.2 5.0 4.8
3
4.0 INSTRUMENT FAMILIARITY
4.1
<
<
<
4.1 Readout
4.1.1 Analog Meter - displays oxygen in ppm, percent
saturation, and temperature in degrees centigrade.
4.1.2 Digital LCD - displays oxygen in mg/L (ppm) and
temperature in degrees centigrade.
4.2 Function Selector
4.2.1 OFF - power is removed from circuit.
4.2.2 STBY - power is applied to circuit, no input signal is
available.
4.2.3 O
4.2.4 TEMP - meter displays temperature of the sample.
4.3 O
CAL - calibration adjustment when measuring
2
dissolved oxygen.
- meter displays dissolved oxygen level of sample.
2
4.3
4.2
7.2.3 Salinity Compensation
The following table shows the dissolved oxygen concentration in
air saturated water at an atmospheric pressure of 760 mmHg and
at various chlorinity (salinity) concentrations.
3
Michael L. Hitchman, Measurement of Dissolved Oxygen, John Wiley & Sons, New York, NY,
1978.
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4.4 REAR PANEL
4.4.1 PROBE INPUT - 5 pin DIN jack connector for
detachable oxygen probe.
4.4.2 POWER - input for optional AC wall plug adapter.
Batteries disconnect when plug is inserted into jack.
(Available for bench models only.)
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5.0 PROBE FAMILIARITY The barometric pressure, as reported by radio and television
stations, is not the actual atmospheric pressure. For standardization, the values reported are corrected to sea level. To convert
these reported values to the actual atmospheric pressure, your
altitude above sea level must be known. A local airport should be
able to provide this information.
To convert from a reported, standard barometric pressure to an
actual atmospheric pressure, in Torr (mm of mercury), the values
from the following table may be used.
5.4
5.3
5.1 5.2
5.1 Membrane - Membrane Cartridge- a special telfon material
with a metal cartridge in the membrane cover. This allows gases
to pass through, but not liquids.
5.2 Cathode - platinum tip of probe.
5.3 Anode - silver coil inside probe chamber.
5.4 Thermistor - responsible for temperature readout and
temperature compensation in the O
5.5 Membrane Applicator - this unique device serves as both a
membrane applicator and an O ring applicator.
mode.
2
TABLE I ALTITUDE MULTIPLIER
2
Altitude Multiplier Altitude Multiplier
-500 ft 1.018 3500 ft 0.880
sea level 1.000 4000 ft 0.864
500 ft 0.982 5000 ft 0.832
1000 ft 0.964 6000 ft 0.801
1500 ft 0.947 7000 ft 0.772
2000 ft 0.930 8000 ft 0.743
2500 ft 0.913 9000 ft 0.715
3000 ft 0.896 10000 ft 0.688
Take the multiplier from the table corresponding to your altitude
and multiply the reported barometric pressure by it. This is your
actual atmospheric pressure.
7.2.2 Dissolved Oxygen Calibration
Obtain the temperature of the sample, with the meter in TEMP
mode, and the atmospheric pressure as described in section
7.2.1. Use the following table (Table II) to determine the calibration value for dissolved oxygen in air saturated water.
EXAMPLE: IF TEMPERATURE = 20°C
PRESSURE = 750 mmHg
THEN OXYGEN = 8.9 mg/L
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2
Derived from : "U.S. Standard Atmosphere (1976)", CRC Handbook of Chemistry & Physics,
69th Ed., 1988-1989, pg.F-147.
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7.0 CALIBRATION
6.0 OPERATION
The dissolved oxygen probe requires periodic calibration. Organic
matter can foul the membrane pores, reducing its effectiveness.
Since the probes response is dependent upon the mass transport characteristics of the membrane, as well as the condition of
the silver electrode, it needs to be calibrated whenever the
membrane is replaced.
7.1 Air Calibration
7.1.1 For maximum accuracy, calibration should be done within
5°C of measurement temperature.
7.1.2 The calibration point, for the oxygen sensor, is water
saturated air (100% humidity). This value is determined using the
calibration tables in section 7.2.
7.1.3 Wrap the probe in a wet cloth. DO NOT TOUCH THE
MEMBRANE.
7.1.4 The membrane should remain covered with a thin layer of
water, while wrapped in the cloth.
7.1.5 Allow the probe to stabilize (approx. 2 min.).
7.1.6 Adjust the CAL control for the value determined from the
calibration tables in section 7.2.
7.2 Calibration Tables
The amount of oxygen dissolved in water is dependent upon
several parameters. Temperature, salinity, and atmospheric
pressure each have an effect on the measurement of dissolved
oxygen. The following tables provide the necessary information
to correctly compensate for your specific environment.
6.1 Probe Preparation
6.2 Membrane Replacement
A B
C D
E
A. Removing membrane cover
B. Membrane removal
C. Membrane insertion
D. Probe filling solution (note: when filling with solution to the mem
brane fill only to white teflon inside the cartridge)
E. Membrane Applicator 5.5
7.2.1 Altitude Pressure Compensation
Prior to calibrating the oxygen meter for ppm or mg/L, the correct
atmospheric pressure must be determined. This is due to the
solubility of oxygen in water being dependent upon the partial
pressure of oxygen in the atmosphere which is dependent on
your altitude or elevation.
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6.3 Probe Polarization
6.3.1 Whenever a new membrane or filling solution is used, the
oxygen probe must be polarized prior to use.
6.3.2 Polarization requires placing the prepared probe on the
meter and turning the function selector to the O
position.
2
6.3.3 Run the probe for 20 to 30 minutes.
6.3.4 Wrap the probe in a moist cloth or paper towel to allow for
quick calibration once the probe polarizes.
6.4.4 Using the CAL knob, set the display to read the value
determined from Table II.
6.4.5 Your instrument is now calibrated.
6.5 Dissolved Oxygen Measurement
Use the following steps to determine your dissolved oxygen
concentration.
6.5.1 Connect prepared probe to the DIN jack on instrument.
6.3.5 After 30 minutes, calibrate the instrument.
6.4 Calibration
The oxygen probe requires periodic calibration. How often depends on it's use. The following is a simple and quick calibration
method for general use. For additional calibration information and
concerns (high elevations, high or low temperatures, etc.), see
section 7.0.
6.4.1 Wrap the probe in a moist cloth or paper towel. The pur-
pose is to create a water saturated environment (100% humidity.)
6.4.2 From Table II determine the atmospheric oxygen level for
your air temperature and normal air pressure. If you are at sea
level and the air temperature is 72°F, your atmospheric oxygen
level would be 8.7 ppm (mg/L).
6.4.3 Allow the instrument 20 to 30 minutes to polarize if it hasn't
already been done.
6.5.2 Turn instrument to the O
position.
2
6.5.3 Calibrate the instrument following the procedure in
section 6.4 or 7.0.
6.5.4 Place probe in test sample and gently stir. If the sample
temperature is different from calibration temperature, allow 3
minutes for reading to stabilize.
6.5.5 Read the dissolved oxygen in ppm (mg/L).
6.5.6 Turn the instrument to TEMP position and read sample
temperature.
6.5.7 Rinse probe with deionized water in-between sample
readings and upon completion of meter use.
6.6 Dissolved Oxygen Units
Dissolved oxygen units are usually reported in units of parts per
million (ppm) or milligrams/liter (mg/L). To convert from mg/L to
ppm or ppm to mg/L, the specific gravity of the sample must be
known. This instrument will read either ppm or mg/L depending
on what it is calibrated to.
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The formula for conversion is:
ppm = mg/L or mg/L = ppm x specific gravity
specific gravity
For 0 to 30°C temperature ranges, the difference between ppm
and mg/L is less than 0.5% in fresh water and less than 5% for
most sea water.
1CRC "Handbook of Chemistry & Physics", 69th ED., pgs D-250 & F-10, 1988 - 1989.
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