Mass Measurement

Mass Measurement
If the amount of gas exiting a column or split vent is measured in units of mass, the flow rate is mass based. Unlike volumetric flow rate, this measurement is not affected by atmospheric temperature or pressure changes. Also, no compensation for water vapor effect is required. The devices used for mass flow measurements in GC are usually thermal flowmeters, which are commonly referred to as mass flowmeters. The operating principle is that the gas flow transfers heat between two sensors in proportion to its mass and velocity. The resulting heat imbalance produces an electrical signal in the flow sensor, which is used to calculate mass flow in mass/unit time. This measurement is then converted to a volumetric value using constant temperature and pressure values, as well as the density of the gas. Fluctuations due to ambient temperature are minimized by the heat. Because gases have differing thermal conductivities and densities, each mass flowmeter must be periodically calibrated for the specific gas to be measured. In contrast, a volumetric flowmeter is non-specific (it can be used for any gas).

Benefits of a mass flowmeter over a volumetric flowmeter include:
  • Not affected by atmospheric temperature or pressure changes
  • No need to compensate for water vapor effect
  • Can be used for measurements outside the laboratory

The main drawback is the need for periodic calibration for the gas to be measured.