Xylem H-340SDI - Manual (Page 8)

1-2 H-340SDI Introduction
H-340SDI
Figure 1 Bucket tip mechanism sensitivity
During normal operation, the data recorder sends an address together with a command to the
H-340SDI sensor. The H-340SDI 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-340SDI's data buffer.
1.3 “Smart Bucket” Theory
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 .009 in/tip to .013 in/tip (a 70% change) as
the rainfall changes from 0 to 25 in/hour. To overcome this problem the microprocessor in the
H-340SDI 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 0.09in/tip
accumulation. Bucket tips 2 seconds apart (25 in/hr) produce a .012in/tip accumulation.