H-340 sdi – Xylem H-340SDI User Manual
Page 11
H-340SDI
H-340SDI Introduction 1-5
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.
1.8 Factory Calibration
The H-340SDI 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:
1. Issue an extended “aXRA!” (reset accumulator) SDI-12 command.
2. Empty the graduated cylinder.
3. Start the water flow into the funnel.
4. Wait until at least 100 bucket tips have occurred (1% resolution).
5. Stop the water flow.
6. Observe and record the water volume in the graduated cylinder.
7. Use the formula below to compute the actual rainfall accumulation.
8. Issue a SDI-12 “aM!” measure command and record the “smart bucket” accumulation.
9. Compare the difference between the smart bucket rainfall reading the rainfall volume in the
graduated cylinder.
10. Adjust the counterweight as needed and repeat until the two values are identical or within an
acceptable difference.
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.
RAINFALL
NmL
L
mL
in
L
in
NmL
x
inches
inches
=
Ч
Ч
=
Ч
−
1000
61025
4 0
121405 10
3
2
3
.
( .
)
.
π