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Gas flow rates and Michell Hygrometry Products.

In theory, flow rate should have no direct effect on the measured moisture level in a gas system, but in practice, the flow rate of a gas can affect the system's accuracy and influence the systems' response time.

Excessive flow rates in piping systems can introduce pressure gradients. Care should be taken to ensure that the sampling system accommodate the required flow rate for the measurement instrument.

An inadequate flow rate can result in errors caused by several side effects:

  • Ineffective purging of the sampling system: In a complex sampling system, inadequate flow can allow pockets undisturbed wet gas to remain in the sampling system or sensor, which will gradually be released into the sample flow.
  • Low flow rate: This condition will accentuate adsorption and desorption effects in the volume of gas passing through the sampling system. This will become more significant as flow rates are lowered. Furthermore, there is a chance of contamination from back diffusion: Ambient air can flow from the exhaust back into the system and causes a sensor response to lag.
  • If an application requires a large flow rate, there is a real chance of reducing response speed, as it will introduce measurement errors by introducing back-pressure and the sensor may take longer to stabilize.
  • For a cold mirror, a high flow rate over the condensation surface will also cause an excessive thermal load on the mirror. The resulting reduction in the mirror's cooling capability will adversely influence the lowest dew point that can be measured. This is particularly problematical when measuring dew points in gases with high thermal conductivity, such as helium or hydrogen. A compromise must be struck between response time, which favors a high flow, and available dew point depression.

It is for all these reasons that inadequate flow rates should be avoided and Michell recommends the standards (in US gals/hour):

US gals/hour Ceramic Sensors
  Easidew Portable Cermax Portable
  (4 & 0.4 mm pipe fittings) (1mm inner bore)
High 320 47
Optimum 80 16
Low 8 4.7