Flowmeter Selection Basics

For a thermal-mass flowmeter, it is important to understand its principle of operation and calibration requirements. As is true for most instrumentation, application conditions must be completely and accurately known to avoid costly mistakes that can delay a start-up or even damage the device. Information required to properly specify a mass flowmeter includes gas type, estimated flow rate, pressure range, temperature range and the allowable pressure drop for the system.

 Once flow parameters have been defined, the proper flow-body size can be determined by following these steps: 

  1. Consult manufacturer product specifications for the acceptable flow range of each flow body.
  2. Convert the estimated flow rate to the corresponding units listed in the product specifications.
  3. If the gas to be measured is not the manufacturer’s specified standard gas (typically nitrogen), convert the flow rate to the equivalent nitrogen flow rate. This is done with a correlation factor, K, defined as the ratio of the actual gas flow rate to the equivalent nitrogen flow rate. To obtain the equivalent nitrogen flow rate, divide the actual gas flow rate by the K factor.
  4. Select the flow-body size that will accommodate the estimated flow rate.
  5. Ensure that the differential pressure falls within the acceptable range for the selected flow-body size and flow rate. (Note: As mentioned previously, primary calibration with the actual gas or a gas of similar molecular characteristics is the only way to confirm proper accuracy.)

Flow variations with correspondingly large pressure drops require a limitation on turndown to maintain accuracy. Sizing, turndown and accuracy must therefore be considered when selecting a thermal-mass flowmeter.

Remember, too, that mass flowmeters are not sensitive to inlet pressure and temperature. In other words, they “auto compensate” for changes in these variables.

All flowmeters, regardless of type, should be checked periodically for calibration and accuracy before flow-measurement errors start affecting your process. Thermal-mass flowmeters use electronic measurement. Therefore, sensor failure modes are not always known. A failed sensor may give readings that are either too high or too low. Users can visually inspect most variable-area flowmeters by following the manufacturer’s instructions.

References can be found in Part 2.