ADM-870 05/23/0840
14.0 METER ACCURACY FIELD TESTING
14.1 METER ZERO FUNCTION
Take several readings with the meter set for air flow or velocity, 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 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.
14.2 DIFFERENTIAL PRESSURE FUNCTION
Check the DIF PRES function zero accuracy by taking a reading with both meter ports open to the atmosphere.
Be sure the ports are not exposed to a draft. The reading should be 0.0000 in wc ± 0.0001 in wc.
Check the differential pressure reading accuracy by using the AirData Multimeter to measure a static pressure
source at approximately the following pressures: 0.1250 in wc; 2.000 in wc; 30.00 in wc. Confirm the pressure
readings by comparison with an appropriately accurate inclined manometer at the 0.1250 and 2.000 in wc
differential. The 30.00 in wc differential may be checked using a U-tube and distilled water.
Since the AirData Multimeter passes a very small sample of the air through the meter during each
measurement (see SPECIFICATIONS), it should be connected to the pressure source independently of any
other meter, manometer, or U-tube. To avoid pulsation or "cross talk" through the pressure source tubing, use
shutoff valves to isolate the AirData Multimeter and the reference pressure gage. The pressure source must
be self-replenishing, such as a pressure drop across a needle valve, orifice, or orifice plate.
14.3 ABSOLUTE PRESSURE FUNCTION
Confirm the accuracy of the absolute pressure correction by taking a reading with the meter ports open to the
atmosphere and compare the reading with the actual barometric pressure. The reading should be within ± 2%
(approximately .5 in Hg) of the actual barometric pressure to maintain the specified accuracy of velocity and
air flow density correction.
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 (-) parts at the same time. This precaution
avoids excessive pressure input to the differential pressure transducer.
14.4 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 .005 in
wc or less, and should have direct velocity markings down to at least 400 fpm. The accuracy of the comparison
test will depend on both the accuracy of the duct velocity traverse, and of the air flow calculated from the
traverse velocity results.
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.
14.5 DUCT TRAVERSE COMPARISONS, INCLINED MANOMETER, 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:
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