WMOD00583

• Contract/Permit #: 1-07-81-V0175
• Title: Some Laboratory Tests of the Accuracy of the Belfort Universal Recording Raingage - Technical Report
• Date: 3/1/1985
• Weather Modification - Doc Type: Report

I. Introduction

Evaluation of winter orographic wE~ather modification programs
often requires accurate measurements of small amounts of
precipitation. Most winter storms in the Rocky Mountain region,
for example, have precipi ta tion totals in the range of a few
hundredths to a few tenths of an inch water equivalent.
Increases due to seeding may only fractionally affect those small
amounts, leading to difficulties in detection of seeding
effects. The most commonly used instrument for these
measurements is the Belfort Universal Recording Raingage for
which the manufacturer's specification.s quote an accuracy of 0.5
percent of the full scale travel {0.06 inch for a
l2-inch-capacity dual tranverse model} and a sensitivity of 0.01
inch precipitation. A limited review of the literature and
consultation with the manufacturer disclosed no test data to
substantiate the validity of these specifications. Therefore, a
laboratory test program was undertaken to provide this
information.

II. Nature and Purpose of Study

The purpose of the experiment was to determine the accuracy
of the Belfort Universal Recording Raingage (model 5-780
series). As used here, accuracy is defined as the agreement of
the measurement with the "true" value of the quantity measured.
Precipitation rates representative of winter snowfall
characteristics were investigated. Flow rates in the approximate
range 0.25 to 2.5 mmlh {0.01 to 0.1 in/h} were established for
periods of one to several hours to simulate snowfall occurrences.

The flow rate during each test period was estimated by
intercepting the flow for known time periods just before, and
just after, each test per iod. These briO samples were weighed,
and the flow rates were calculated and averaged to estimate the
flow rate during the test period. During the test period the
flow entered a beaker set in the middle of the gage catch
bucket. The beaker was weighed after leach test to establ ish the
actual total flow into the gage.

Flow data were plotted and compared with gage chart readings,
because the accuracy of the instrument" s chart record was of
primary interest. In addition, the temporal response of the gage
could be evaluated by comparison of thE~ chart trace to the known
rate of test flow input.

I I I. Ma tel' i al s

Three Belfort Universal Recording Raingages {model 5-780
series} with dual traverse measuring capability were used. The
housing shroud and collector funnel were removed from each gage
assembly to allow convenient operating access. The gages were
not equipped with potentiometers or other peripheral equipment
that might create operational drag on the system. All

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