A Wet Gas (or Drum) meter consists of a horizontally disposed drum divided into sections as shown below. The drum is free to rotor about its axis and a fluid, usual water, is filled to just over the centre line.
The gas to be measured enters the drum at the centre and, as it fills a section, it displaces the fluid allowing the drum to rotate. When the compartment is filled the inlet will be sealed by the fluid. The inlet port to the next section then opens allowing the drum to continue to rotate. As it rotates the fluid enters the first section and the trapped gas is expelled through the outlet. Given this principle, once the drum has been calibrated one revolution of the drum is a known volume of gas.
Image courtesy of TÜV NEL
The gas passing into the meter is processed through a wetting agent; this reduces an error in measurement due to humidity variations. Where the fluid selected is oil based, this prevents evaporation or high humidity issues. However the oils should be low viscosity and very high vapour pressure.
With this type of technology the pressure and temperature of the measured gas are usually close to ambient. Under controlled conditions accuracies of +/-0.2% of reading can be achieved.