Abstract: The invention relates to improved methods of proving the calibration of meters, such as oil and water meters used in measuring fluid output of a producing oil well prior to the fluid reaching a stock tank. The method generally involves connecting a prover vessel, having an accurate volume reading sight glass, downstream of the meter to be tested, introducing a first volume of fluid into the prover vessel, degassing the fluid and pressurizing the vessel so as to establish a baseline reading. A preselected volume of fluid as registered by the meter is then introduced into the vessel, degassing and pressurizing are repeated and a second volume reading is obtained. The volume represented by the difference between the second and first volumes read at the prover vessel is divided by the volume registered by the meter to obtain a factor by which a meter reading may be multiplied to derive the true volume of fluid passing thereby. The volumes recorded may be temperature corrected to 15.degree. C.