14VORTEK Doc.# VT5 OM 050499, Rel. 2.1
3.5 Quick Calibration Check
Transmitter calibration can be checked quickly by setting DIP switches 2, 3, and 4 as
follows, and then removing the probe connectors from the bottom of the transmitter;
DIP 1 On or Off
DIP 2 Off
DIP 3 Off
DIP 4 On
The proper output of the transmitter can be established by making the following calcula-
tion. Full scale Hz data can be obtained from the transmitter label on the front door;
Expected mA= [16*(122/Full Scale Frequency Hz)]+4
If the value observed does not agree with the value expected, ±0.5% full scale, then the
transmitter may require recalibration. The use of this internal frequency standard for
calibration of full scale values is possible but will not be as accurate as calibration using
a frequency generator set at full scale Hz.
3.6 Threshold Adjustment
The threshold adjustment is normally made on a calibration bench. However, the thresh-
old can be set empirically in the field by following the procedure in section 4.2. The
threshold adjustment serves to limit the lower operating limit of the transmitter to in-
crease sensitivity at low flow conditions.
Threshold is factory set at a point equivalent to 350 FPM. Below this level the threshold
circuit forces the output to go to 4.0 ma. Lowering the threshold below 350 increases
chances that the transmitter may show flow when none is present if a high ambient
noise condition is present.
The threshold should only be adjusted if the transmitter indicates flow when in fact there
is none present. Before adjusting, check to see that zero is properly adjusted. Then,
with no flow in the duct, turn the threshold adjustment clockwise until the transmitter
indicates 4 ma.
4.0 CALIBRATION & MAINTENANCE
4.1 Field Calibration
The VorTek transmitter comes calibrated for the CFM or FPM specified by the customer.
The calibration factor is established as a function of the type of VorTek sensor utilized in
the construction of the probe bars. These calibration factors are established by test and
represent flow conditions expected when normal Minimum Installation Requirements are
observed. (See Appendix A, page A-1)
When these conditions are not present and duct turbulence is a problem, corrections
may be required to match flow transmitter output to readings observed by traverse. In
these cases transmitter span can be adjusted to read the desired output. Adjustment of
transmitter span will not effect transmitter zero.
Chapter 4 Calibration and Maintenance
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