Xylem YSI H-3401SDI User Manual
H-3401SDI
SMART SD I-12 TIPPING BUCKET RAIN GAUGE
USER MANUAL
D61-R3
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2
Table of Contents
CHAPTER 1 .................................................................................................................................................................... 4 – 12
1.1 Introduction ........................................................................................................................................................................... 4
1.2 Theory of Operation............................................................................................................................................................. 4
1.3 Smart Bucket Theory ......................................................................................................................................................... 5-6
1.4 Making Connections to the H–3401SD I ............................................................................................................................ 7
1.5 Installation ............................................................................................................................................................................. 8
1.6 Programming Your Data Recorder ..................................................................................................................................... 8
1.7 Programming the H–3401SD I ............................................................................................................................................. 9
1.8 Factory Calibration ..........................................................................................................................................................9-10
1.9 User Calibration .................................................................................................................................................................. 11
1.10 Adjusting the Tipping Bucket Mechanism .................................................................................................................... 12
CHAPTER 2 .................................................................................................................................................................. 13 – 30
2.1 SD I–12 Command and Response Protocol ..................................................................................................................... 13
2.2 Master SD I–12 Command List ........................................................................................................................................... 14
2.3 Measure Command ...........................................................................................................................................................15
2.4 Uncompensated Measure Command .............................................................................................................................16
2.5 Concurrent Measurement Command ..............................................................................................................................17
2.6 Uncompensated Concurrent Measurement Command ................................................................................................ 17
2.7 Send Data Command ........................................................................................................................................................18
2.8 Continuous Measurements ............................................................................................................................................... 19
2.9 Measurements with CRC ................................................................................................................................................... 19
2.10 Initiate Verify Command .................................................................................................................................................. 20
2.11 Send Acknowledge Command ...................................................................................................................................... 20
2.12 Send Identification Command .......................................................................................................................................21
2.13 Change Sensor Address..................................................................................................................................................22
2.14 Extended “Read Slope” Command ...............................................................................................................................22
2.15 Extended “Write Slope” Command ............................................................................................................................... 23
2.16 Extend “Read Offset” Command ....................................................................................................................................23
2.17 Extended “Write Offset” Command ..............................................................................................................................24
2.18 Extended “Get Time” Command....................................................................................................................................25
2.19 Extended “Set Time” Command ....................................................................................................................................25
2.20 Extended “Get Daily Reset Time” Command ...............................................................................................................26
2.21 Extended “Set Daily Reset Time” Command ................................................................................................................ 27
2.22 Extended “XTEST” Command ........................................................................................................................................28
2.23 Extended “XHELP” Command ........................................................................................................................................ 28
Appendix A | Specifications ..................................................................................................................................................... 29
Appendix B | Ordering Guide ................................................................................................................................................. 30
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Introduction
The WaterLOG® H-3401SDI is a Tipping Bucket Rain Gauge with built-in SDI-12 electronics. The SDI-12 feature converts
a standard tipping bucket rain gauge into a “smart” SDI-12 sensor with additional capabilities. The H-3401SDI is easy to use
and works with any data recorder/logger with a SDI-12 interface. The “Serial-Digital Interface” is ideal for data logging
applications with the following requirements:
• Battery powered operation with minimal current drain
• Low system cost ! Up to 200 feet of cable between a sensor and the data recorder
The H-3401SDI has the following features:
• Connects directly to any data recorder with a SDI-12 port.
• Scales the bucket tips into units of inches, meters etc
• Provides built-in accumulation of daily and total rainfall
• Provides mathematical correction for bucket volume errors due to varying rainfall rate.
Theory of Operation
The H-3401SDI has a built-in microprocessor which monitors the tipping bucket sensor. Whenever a bucket tip occurs the
microprocessor wakes up from its low power sleep mode and adds an appropriate rainfall increment to the rainfall accumulators.
The electronics has a filter circuit which prevents contact bounce in the bucket tip reed switch from causing false counts. In
addition, the time between bucket tips is measured, bucket tips which occur less than 500mS apart are ignored. This helps
protect from false counts of a poorly adjusted bucket mechanism.
The H-3401SDI protects its rainfall accumulations with a software flag or “signature.” If the power is interrupted momentarily
the H-3401SDI will resume operation using the accumulation values in memory. If the power is lost long enough to destroy
the signature, when the power is restored the H-3401SDI resets the rainfall accumulators to 0000. To intentionally reset the
H-3401SDI’s accumulators, disconnect the power for 5 to 10 seconds or use the extended “aXRA” command.
The H-3401SDI provides daily rainfall accumulation values which are useful for data loggers which do not have special provisions
for computing daily rainfall. The H-3401SDI has a built-in real-time-clock which triggers a “daily reset time” event. When this event
occurs the H-3401SDI automatically updates its “yesterday’s total accumulation” value using “today’s total accumulation”, then
zeros the “today’s total accumulation” value.
During normal operation, the data recorder sends an address together with a command to the H-3401SDI sensor. The H-3401SDI
wakes up from its low power sleep mode and stores the requested data in its data buffer. Once the data is ready, the data recorder
collects the data from the H-3401SDI’s data buffer.
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Smart Bucket Theory
0.01 inch
Tipping bucket rain gauges have a reputation for being inaccurate. Inaccuracies can be caused by algae in the bucket,
evaporation and poor calibration. The greatest source of inaccuracy however, is the volume at which the tipping bucket
mechanism tips is sensitive to rainfall rate. This occurs because the water droplets cause surface waves in the bucket
and high rainfall rates cause superposition or “pile up” of extra water in the bucket before it actually tips over. The following
graph shows the relationship between bucket volume and rainfall rate of a typical tipping bucket. In this example the bucket
volume/tip changes from .01 in/tip to .013 in/tip (a 30% change) as the rainfall changes from 0 to 25 in/hour. To overcome
this problem the microprocessor in the H-3401SDI uses an electronic timer to precisely measure the time between bucket
tips. This time value corresponds to the current rainfall rate. On a bucket-by-bucket basis the microprocessor uses a built-in
polynomial to correct for the intensity sensitivity of the bucket mechanism. For example, bucket tips which are 50 seconds
apart (1 in/hr) are computed with a 0.01in/tip accumulation. Bucket tips 2 seconds apart (25 in/hr) produce a .012in/tip
accumulation.
Correlation Between Rate of Rain Versus Inches per Tip
0.013
0.0125
0.012
0.015
Inches per Tip
0.011
0.0105
0.01
0 5 10 15 20 25 30
Inches of Rain per Hour
Figure 1: 0.01” Bucket Tip Mechanism Sensitivity
5
0.2mm
The following graph shows the relationship between bucket volume and rainfall rate of a typical tipping bucket. In this example
the bucket volume/tip changes from 0.2mm/tip to 0.26mm/tip (a 30% change) as the rainfall changes from 0 to 500 mm/hour.
Correlation Between Rate of Rain Versus Millimeters per Tip
0.27
0.26
0.25
0.24
0.23
Millimeters per Tip
0.22
0.21
0.2
0 50 100 150 200 250 300 350 400 450 500
Millimeters of Rain per Hour
Figure 2: 0.02mm Bucket Tip Mechanism Sensitivity
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Making Connections to the H-3401SDI
The H-3401SDI has a 6-pin wiring terminal block for connecting the data logger and power. The terminal block has screw
clamp terminals which work with both large and small wires and provide reliable long-term connections. To open the
terminal ports to insert or remove a wire, insert a small flat screwdriver blade in the circular opening and tighten or
loosen the screw to open or close the terminal port.
Words of Caution
Keep the lead wires as short as possible.
•
Use shielded cables in noisy environments.
•
Figure 3: Internal View of H–3401SDI Rain Gauge
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Installation
Install the H-3401SDI in an open location where it will not be in the rain shadow of trees or buildings. A rooftop location helps
prevent vandalism and the accumulation of leaves and debris. Bolt or screw the 3-mounting feet to the structure to prevent
upset in high winds. Adjust the three self-leveling wingnuts on the raingage board until the “bulls eye” level is centered.
Programming Your Data Recorder
You must prepare your data recorder to receive and record the H-3401SDI data. Since data recorders differ widely, refer to your
recorder manufacturer’s directions. In general, program the data recorder to input five values via the SDI-12 port. Usually only
one or two of the parameters is actually recorded. Your data recorder must issue an “aM!” command then collect the data with
a “aD0” command, as explained in Chapter 2. The H-340SDI places five data values in its data buffer:
+AA.AAA+BBB+CC.CCC+DD.DDD+EE.EEE
Where:
+AA.AAA = Rainfall accumulation since last measurement (mm/in of rain)
+BB = Number of raw bucket tips since last measurement (counts)
+CC.CCC = Total rainfall accumulation since reset. (mm/in of rain)
+DD.DDD = Total rainfall accumulation today (mm/in of rain)
+EE.EEE = Total rainfall accumulation yesterday (mm/in of rain)
All five of these parameters are automatically reset at power-up, or manually with the extended SDI-12 “aXRA” Reset
Accumulators command.
NOTE: There are also some additional measurement commands, M1, M1C, C1 and C1C that will return the following data.
+AA.AAA+BBB+CC.CCC
or
+AA.AAA+BBB+CC.CCC+KKKK
Where:
+AA.AAA = Uncompensated rainfall accumulation since last measurement (mm/in of rain)
+BB = Number of raw bucket tips since last measurement (counts)
+CC.CCC = Uncompensated total rainfall accumulation since reset. (mm/in of rain)
+KKKK = CRC
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Programming the H-3401SDI
The H-3401SDI comes from the factory with the following settings:
SDI Address: 0
Slope: 1.0
Daily Reset Time: 00:00:00
Time of day: 00:00:00
With these values the data will be in units of inches of rainfall. The setup is stored in EEPROM within the H-3401SDI and will not be
lost if the power is disconnected. The extended commands for changing the slope and sensor address are described in detail in
Chapter 2. If more than one sensor is to be connected to the SDI-12 bus, make certain each sensor has a unique sensor address.
In some cases the user may wish to preset the H-340’s Total Accumulation value to match the current rainfall of the water year.
The H-340 has an offset value stored in memory which is automatically added to the Total Accumulation value. The offset value
can be written or read with two extended SDI-12 commands (“aXWO” and “aXRO”), see Chapter 2 for details. Please note both
the Total Accumulation and Offset values are reset at power-up, or with the extended SDI-12 Reset Accumulators (“aXRA”)
command. To change the Total Accumulation to a new value you must first reset the accumulators with the “aXRA” command,
then set the offset with the “aXWO” command.
Factory Calibration
The H-3401SDI is calibrated in the factory with an apparatus which provides a constant 4in/hour flow rate. The water exiting
from the bottom of the rain gauge is captured in a precision graduated cylinder. After 100 bucket tips or more have occurred,
the flow is stopped and the volume in the graduated cylinder is measured. The procedure is as follows:
Issue an extended “aXRA!” (reset accumulator) SDI-12 command.Empty the graduated cylinder. Start the water flow into the funnel.
Wait until at least 100 bucket tips have occurred (1% resolution).Stop the water flow.Observe and record the water volume in the
graduated cylinder.Use the formula below to compute the actual rainfall accumulation. Issue a SDI-12 “aM!” measure command and
record the “smart bucket” accumulation.Compare the difference between the smart bucket rainfall reading he rainfall volume in the
graduated cylinder. Adjust the counterweight as needed and repeat until the two values are identical or within an acceptable
difference.
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The rainfall “volume” equation below converts the volume in the graduated cylinder (mL) into the equivalent rainfall (in inches)
captured by an 8 inch diameter funnel.
3
in
NmL x L x 61.025
Rainfall in =
1000mL
π(4.0 in)
2
L
= NmL x 1.21405 κ10
3
m
-3
in
NmL x L x 0.001
Rainfall mm =
1000mL
π(4.0 in)
2
L
= NmL x 30.83633 κ10
-3
mm
Other related equations of interest:
Rate in =
hr
Rainfall in =
tip
Rate mm =
tip
Rainfall in
N min
Rainfall in
N tips
Rainfall mm
N tips
min
x 60
hr
10
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