Xylem H-340 - Manual (Page 7)

Chapter 1
H-340 Operation
H-340
H-340 User Manual 1-1
1.1 Introduction
The W
ATER LOG ® H-340 is a rugged, durable, Tipping Bucket Rain Gauge.
The H-340 has the following features:
Switch closure output connects directly to a data logger
Patented bucket tip mechanism is easy to adjust
Removable stainless steel funnel screen
Rustproof, powder painted aluminum enclosure and cast base
Side windows allow inspection of mechanism without removing cover
No plastic parts
All internal parts anodized aluminum or stainless steel
1.2 Theory of Operation
The H-340 has an anodized aluminum tipping bucket mounted on precision stainless steel
bearings. The patented bucket tip mechanism allows easy calibration. The gauge is factory
calibrated to provide one tip per 0.01 inches of rainfall.
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 small 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 as the rainfall
changes from 0 to 25 in/hour.
The user should be aware of this inerrant inaccuracy in all simple tipping bucket rain gauges.
Realizing the non linearity of the mechanism, it is important for the user to calibrate the rain
gauge for the rainfall rates most common in the geographical area to be monitored.