CFM-88L 07/20/0925
13.0 METER ACCURACY FIELD TESTING
13.1 METER ZERO FUNCTION
Take several air flow readings with no air passing through the FlowHood unit. The meter should display zero readings, but
may occasionally show a low reading such as 25 cfm. An occasional positive or negative reading is random electronic
"noise" and may be disregarded.
If the zero test readings tend to be either positive or negative and greater than 25 cfm, the zero bias indicated will affect the
accuracy of low air flow measurements. Check for any obstruction of the meter connections and clean if needed.
Be sure that there is no moisture or condensation in the meter or meter connections. If the meter has been exposed to water
or high humidity, followed by rapid cooling, water may have condensed inside the meter. In this event, the meter should be
placed in a warm dry atmosphere (between 80
/F and 130/F) for 24 hours. Afterward, the batteries will need to be recharged
for 10 hours at a temperature less than 113
/F. If a zero bias or other problem persists, call the factory for advice.
13.2 ABSOLUTE PRESSURE FUNCTION
You may confirm the accuracy of the absolute pressure correction by taking a reading with the meter ports open to the
atmosphere and comparing the reading with the actual barometric pressure. The reading should be within ± 2%
(approximately 0.5 in Hg) of the actual barometric pressure to maintain the specified accuracy for air flow density correction.
NOTE: Weather service or airport reports of barometric pressure have usually been adjusted for altitude, so the pressure
can be used for altimeter adjustment. The barometric pressure announced on television or radio stations is generally
obtained from weather service reports. This altitude corrected barometric pressure must not
be used in density correction
equations for comparison with a FlowHood. An estimation of the actual barometric pressure may be obtained by deducting
approximately 1.0 in Hg for each 1000 feet above sea level.
CAUTION: Testing of absolute pressures greater or less than local barometric pressure must be performed with the
reference pressure applied to the positive (+) and the negative (-) ports at the same time. This precaution avoids excessive
pressure input to the differential pressure transducer.
13.3 AIR FLOW ACCURACY
Air flow accuracy is confirmed by comparing a very careful pitot tube traverse with the results obtained using the FlowHood.
An accurate inclined manometer, micromanometer, or AirData Multimeter may be used for the duct traverse. An inclined
manometer used for the duct traverse should have minor scale divisions of 0.005 in wc or less, and should have direct
velocity markings down to at least 400 fpm. The accuracy of the comparison tests will depend on both the accuracy of the
velocity traverse, and the accuracy of the air flow calculations.
A multipoint pitot tube traverse is performed on a supply duct which serves a single supply diffuser. The pitot tube should
be a type approved by the NPL (National Physics Laboratory, U.K.) or AMCA (Air Moving and Conditioning Association,
U.S.). The duct velocity should be at least 800 fpm; the duct should be properly sealed and taped; and the connection to
the diffuser should be airtight. The duct traverse location should have straight duct for six to eight duct diameters upstream,
and three to four diameters downstream. For example, a one foot diameter duct requires eight feet of straight duct upstream
and four feet downstream.
13.4 DUCT TRAVERSE COMPARISONS - INCLINED MANOMETER OR MICROMANOMETER
An inclined manometer or standard micromanometer does not correct for density effects due to barometric pressure or
temperature.
The density correction necessary for duct traverse readings which are to be compared with FlowHood readings taken with
the TemProbe in place is as follows:
Where: P
b
= local barometric pressure (in Hg)
/F = temperature of the measured airstream
If the TemProbe is not attached during the flow measurement, the FlowHood meter will assume standard 70
/F (or 21.1/C)
conditions (as do the inclined manometer and the standard micromanometer). In this case, it is not necessary to correct the
duct traverse velocity for the density effect due to temperature. However, since the FlowHood does automatically correct
for the density effect of barometric pressure, the duct traverse readings must be corrected as follows:
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