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Appendix A - Back-Pressure and Barometric
Pressure - Effect on Accuracy
Even though thermal flowmeters are inherently mass flowmeters (flow
is independent of gas density), there is a pressure component related
to Mach number which cannot be accounted for without knowing the
gas pressure. The 4000/4100 High Performance Linear OEM Mass
Flowmeters accept an external pressure signal that the flowmeter can
use to perform pressure compensation. The flow rates measured by
these devices assume a flow tube pressure of 101.3 kPa (14.69 psia).
Deviation from this pressure may require a correction for optimum
accuracy. Common causes of pressure deviation include normal
system backpressure and changes in barometric pressure due to
weather and altitude.
If the typical deviation from this “standard pressure” is minor, you may
not need to make any correction. See the following chart to determine
the additional uncertainty due to changing pressure.
If your application requires improved accuracy, the correction for
pressure can be done in two ways:
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