15VORTEK Doc.# VT5 OM 050499, Rel. 2.1
For best results, a traverse should be done with the airflow at a fixed CFM. This can be
done by: Fixing the output to the damper/valve which throttles the air in that duct, or by
removing the output to the valve/damper, and keeping all doors closed in the area that
is affected by this airflow.
The CFM reading can then be compared to the transmitter output. A milliamp meter
should be connected in series with the output signal wires. (refer to figure 4 on page 11).
The reading will fall between 4 and 20 mA. Use the formula below to convert the mA
value to the equivalent CFM. The transmitter Full Scale CFM can be found on the
transmitter enclosure.
Current Transmitter CFM Value = [Full Scale CFM/16]*(milliamp reading - 4)
If the transmitter CFM reads differently than the traverse, then an adjustment can be
made. If there is a front end system receiving the control signal. A multiplication con-
stant should be added to the scaling formula to agree with a balancer reading. If this is
not possible, the transmitter itself can be adjusted. First calculate the corresponding
milliamp value which matches the traversed CFM with this formula:
New desired mA value = [(traversed CFM/Transmitter Full Scale CFM)* 16]+4
Turn the span potentiometer (see figure 3) clockwise to increase the mA signal, counter
clockwise to decrease the signal, until the desired mA signal is output.
4.2 Bench Calibration
Full transmitter calibration is normally calibrated at a bench, although it is possible to
bring the bench calibration instruments to the transmitter in the field. Connect a digital
voltmeter to the transmitter as shown in figure 4. A frequency generator is required and
can be used in conjunction with a Multi-meter. The full procedure is as follows.
Set Zero- Set switches 2, 3, and 4 to “OFF”. Adjust the “ZERO” potentiometer until 4.00
mA is read on the digital voltmeter.
Set Full Scale- (see figure 5, next page) Set switches 2, 3, and 4 off. Connect the
negative lead of the frequency generator to TP 1 on the mother board (next to the wire
connectors). Connect the positive lead to TP 5. Set the generator to the full scale Hertz
indicated on the label fixed to the transmitter enclosure. Set the frequency generator to
sinusoidal
wave and the output voltage to the minimum level. Adjust the span potenti-
ometer until the output reads 20.0 mA. Turn the span potentiometer clockwise to in-
crease the mA signal, counter clockwise to decrease the signal.
Set Optional Display- With the above connections and settings in place from the
previous step, adjust the “DVM” potentiometer to read the appropriate scale display
value. As the maximum display is 1999, a multiplier of x10 or x100 may have to be
utilized to get the total CFM display.
Threshold Adjustment- With the above connections and settings in place from the
previous step, set the frequency generator to approximately 60 Hz. Turn the “THRES”
potentiometer clockwise until the transmitter output switches to 4 ma. Then, slowly turn
the “THRES” adjustment counter clockwise until the output switches back to a reading
other than 4 mA. Threshold is now set.
Return To Service- Return switches 2 and 3 to the “ON” position prior to returning the
unit to service.
Chapter 4 Calibration and Maintenance
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