Shortridge Instruments ADM-860C, Operating Instructions Manual

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ADM-860C 04/14/08 52
14.0 METER ACCURACY FIELD TESTING
14.1 METER ZERO FUNCTION
Disconnect all tubing from the positive (+) and negative (-) ports of the meter. Perform several readings with the
meter set for the flow or velocity mode, with
no air passing across the meter ports. The meter should display
zero readings, but may occasionally show a low reading such as 25 cfm or fpm. 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 or velocity 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
o
F) for 24 hours.
Afterward, the batteries will need to be recharged for 10 hours at a temperature less than 113
o
F. If a zero bias
or other problem persists, call the factory for advice.
14.2 DIFFERENTIAL PRESSURE FUNCTION
Check the differential pressure 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 measuring a constant 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 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
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 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 (-) ports 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 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.