Volumetric flowmeters, positive displacement or are the only type of flow meter that indicates the flow rate. There are models of all shapes and sizes. As a family, are together the best devices for measuring flow rate. However, only can be used with clean fluids and have limitations in size and flow rate compared to other types of flowmeters. They are known worldwide simply as "counters PD" (positive displace-ment Flowmeters).

Vista marks a volumetric flow meter SIKA

Measuring principle

Devices that operate on this principle have movable cameras that divide the fluid known precisely fixed volumes as it passes through the flowmeter (see figure below). The count of individual volumes of fluid gives the total value of the flow. The internal measuring chambers (sets of gears, pistons or other elements) are moved by the pressure of the pipe itself. The number of turns is an indicator of the volume that has passed through the flowmeter.

Fig: Operating principle of volumetric flowmeters (here an example of oval gear flow meter).

Volumetric flowmeters are only measuring devices and flow volume can only record the flow in one direction (are unidirectional). The cyclical filling and emptying chamber imposes a degree of pulsation to the fluid. The intensity of the pulse at the output depends on the flow velocity and flow model. It is therefore necessary to accommodate analog signals before using these pulses as control functions.

The internal motion of the measuring chambers can be transmitted directly to a mechanical flowmeter. Or, alternatively, volumetric flowmeters can be equipped with inductive coils or other types of signal generators. Unlike other types of meters, volumetric flowmeters can operate without external power supply.

These flowmeters have in common a wide range of measured values, which exceeds the ratio 40:1 (can reach up to 400:1) without changing its systematic measurement error. The care in handling and fitting of the interstices between the measuring element and the flowmeter body help it. PD counters are one of the few types of flow meters that measurement accuracy increases with increasing fluid viscosity. In addition, its performance is not affected by turbulence, vortices or other distorting velocity profiles. However, the presence of a second phase (air or solids in the liquid) can significantly disturb execution.

Ill.: Measurement error E (solid line) and pressure drop Ap (dashed line) with PD meters.

The figure above shows the typical performance characteristic of a PD meter. For low fluid velocities, the frictional resistance to the flow is large compared with the driving force. Part of the fluid can be drained without registering at the interstices between the mechanical parts. These losses are called "slides fluid" and are also common in some types of turbine meters. As the fluid velocity increases, the measurement error increases to a maximum and then decreases again. A high fluid velocities, the error also increases the inertial effects that occur inside the body of the flowmeter. Prolonged use at high speeds causes component wear and thus greater slippage of fluid through the flowmeter. We summarize the features of this meter upo:

- The pressure drop increases with fluid velocity depends on material and workmanship. Consequently, even the smallest of the uncertainties affecting production within the measurement chambers can cause noticeable leakage during operation of the flowmeter.

- The readings are generally lower than they should when the flow rate is either very low or very high.

The following figure shows a typical family of curves of execution. For the same flow, decreases with increasing viscosity of fluid leakage and improves measurement uncertainty, while the pressure drop increases. Due to the effects of viscosity, PD meters should be calibrated for conditions close to the process and work with the fluid to be used (for example, measuring the economic value of certain substances, such as some hydrocarbons).

Figure: Overall effect of viscosity on the measurement error E (continuous line, in% of Q) and pressure drop Ap (dashed line) for PD-type counters. a = Petrol, B = water, c = light oil, d = 20 cP oil medium, and heavy oil = 100 cP, / = 300 cP oil

Some types of PD meters offer superior accuracy to ± 0.1%, and in some cases even provide values ​​close to those achieved by a calibration system. At the opposite end of the scale, thousands of low-cost models are used less accurate worldwide for routine flow of liquids and gases.

All these devices are easily affected by the intrusion of solid bodies, which tend to clog the interstices of the flowmeter. If foreign matter intrusion is a recurring problem (as is the case when performing installation work often), it is advisable to install a fine mesh filter upstream of the flowmeter.