Volumetric Measurement

Volumetric Measurement
If the amount of gas exiting a column or split vent is measured in units of volume, the flow rate is volumetric based. For example, measuring the volume of gas in milliliters (mL) per unit time in minutes will result in a volumetric flow rate in mL/min. The most common device for measuring a volumetric flow rate is a bubble flowmeter. These devices are used to determine flow by measuring the time required for a gas stream to move a soap bubble through a specific volume. There are several important considerations when using a volumetric flowmeter:
  • The flow measurement is based on volume, and can be affected by atmospheric pressure and temperature conditions.
  • If water vapor is present, it can result in an elevated flow rate measurement. The gas being measured can diffuse rapidly through the soap bubble resulting in an erroneously low flow rate measurement. This is especially a consideration for helium and hydrogen.

Usually, flow rates are measured at ambient temperature and pressure. If it is necessary to compare flow rates taken under different temperature and/or pressure conditions, a correction factor relative to a set standard temperature and pressure should be applied:

  • Fref = Famb (Pamb/Pref) (/TrefTamb)

Where:
  • Fref = flow corrected to reference conditions
  • Famb = flow measured at ambient conditions
  • Pamb = atmospheric pressure at ambient conditions
  • Pref = pressure at reference conditions (1 atm commonly used)
  • Tref = temperature at reference conditions in Kelvin (K) (298 K commonly used)
  • Tamb = temperature at ambient conditions in K

In the case of water vapor, a correction factor can be applied to the flow measurement to compensate:

  • Corrected flow = measured flow x (1-Pw/Pamb)

Where:
  • Pw = vapor pressure of water at ambient temperature
  • Pamb = ambient pressure

To minimize the error introduced by diffusion of air, one can purge the flowmeter tube with several volumes of the gas being measured prior to taking the flow reading.