Gas test gauges rely on a gas acting against a fluid. When the gauge is connected to the gas appliance, the pressure of the gas inside in the appliance or pipework forces the fluid in the u-tube to travel up the scale.
The fluid indicates the difference between the atmospheric pressure at one end of the tube and the pressure of the natural gas or LPG being tested at the other.
The pressure of the gas is equal to the value shown on the scale of the gauge. This value is read from the gauge and recorded. On this gauge, the pressure shown is 20 mbar.
