A Displacement Volumetric Meter is a device which measures total volume. It can operate on the principle of the fluid sweeping out a fixed calibrated volume, which produces a rotation of a calibrated gauge. an analogy can be drawn like this.suppose water is falling in a basin and a boy is putting small glass under the tap.after the glass is filled then it is replaced by another glass which is again filled.so total number of glass multiplied by volume of each glass divided by total time to carry out the task gives the displacement volumetric flow measurement.By maintaining constant conditions and taking readings over a prescribed period the flow rate can be calculated for the specific operating conditions. The instrument essentially measures total volume. Some water meters work on the principal to measure fixed volumes of water for billing. These are commonly referred to as "positive displacement meter".