Xylem H-340SDI - Manual (Page 7)

Chapter 1
H-340SDI Operation
H-340SDI
H-340SDI Operation 1-1
1.1 Introduction
The W
ATER LOG ® H-340SDI 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-340SDI 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-340SDI 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.
1.2 Theory of Operation
The H-340SDI 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-340SDI protects its rainfall accumulations with a software flag or “signature.” If the
power is interrupted momentarily the H-340SDI 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-340SDI resets the rainfall accumulators to 0000. To intentionally reset the
H-340SDI's accumulators, disconnect the power for 5 to 10 seconds or use the extended “aXRA”
command.
The H-340SDI provides daily rainfall accumulation values which are useful for data loggers
which do not have special provisions for computing daily rainfall. The H-340SDI has a built-in
real-time-clock which triggers a “daily reset time” event. When this event occurs the H-340SDI
automatically updates its “yesterday’s total accumulation” value using “today’s total
accumulation”, then zeros the “today’s total accumulation” value.