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
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
- 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
- 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
It is for all these reasons that inadequate flow rates
should be avoided and Michell recommends the standards (in US gals/hour):
||(4 & 0.4 mm pipe
||(1mm inner bore)