Some of the various types of volumetric meters are named after the element used for measurement. There are four basic models:

1. Offset (measuring element type sprocket, screw or vane)

2. Alternative (measuring element reciprocating plunger type)

3. Oscillating disk (measuring element rotary action)

4. Nutation (element disk-type measuring nutation)

Ill.: Exterior view of a volumetric flowmeter

Reel Counters:

The counters of the first group are the flowmeters of this type more accurate. Its design is shown below in Fig. A winch fitted to the measured cavity volume displaces an amount fixed to sweep a quarter circle, as shown in the figure. These counters are usually used for transporting oil.

Figure: Flow Vane

Counters oval gear wheels:

A gear wheel meter oval consists of a head that houses two interlocking oval gears. The positive closing action effectively prevents leakage between the two gears. The torque produced by the action of the flow varies with the relative positions of the two oval gear wheels. This moment causes the rotation of each gear around the other and passes through the meter portion of fluid of known volume. This principle is characteristic of many designs for volumetric meters, even those where no gears are oval shaped (Fig. below).

The materials used for moving cameras, the oval gear wheels, cams and pivots may vary according to different process conditions. Typical materials used for the oval gear wheels including cast iron, steel Cr-Ni-Mo cast steel and bronze. These materials are commonly used in many other designs.

Ill.: Counter gear. Between the teeth of the sprockets spend lots exact volume of fluid.

Rotary piston meters:

The most common models of the third group are the rotary piston meters, used to monitor water consumption in buildings, apartments and industries. The piston oscillates around a central axis. The volume of fluid sealing occurs between the partition plate and the outer edge of the measuring element in contact with the body of the flowmeter.

Figure: Cross section of a rotary piston meter, flowmeter body a = b = c = Measuring chamber partition sheet

The other two types of counters listed (reciprocating piston and diaphragm or oscillating disc / wobble) base their measurement principle that moves-ers act as a fixed volume of fluid per revolution.

In all these models, the central shaft transmits the rotary motion to a toothed wheel counters, or simply to external sensors, which count revolutions. The figure below shows the basic configuration of a helical screw counter.

Ill.: Counter volumetric screw two screws.

Gas Meters:

The gas meters typically use a low resistance membrane is governed by a valve. Two bellows (B, C) are filled and emptied alternately gas (Fig. below). A valve governed by the compartments A and D controls the filling and emptying.

Figure: Principle of measurement of gas meters in apartment buildings

A mechanical link connecting the reciprocating movement of the valve with the output shaft. Each movement produces a single rotation axis. Encoders can be used to generate high resolution output high frequency pulse signal. They have made millions of flowmeters of this type for use as gas meters in homes, shops and offices.

Advantages and disadvantages

The list of advantages and disadvantages offered by this measuring principle will inevitably be given in general terms because of the many different types and models available on the market. The following summary, therefore, provides no more than usual. For detailed information about the boundary conditions specific to each type of counter will always be necessary to contact the manufacturer.

Advantages:

- Measurement method in use for over 100 years. Widely accepted and reliable.

- High accuracy (up 0.1% or higher) and high repeatability, particularly true for accountants and reel-type plunger.

- Can be used with conductive liquids and gases and non-conductive.

- Particularly suitable (except gas meters) for measuring the flow of highly viscous fluids.

- The measurement is independent of viscosity in a wide range of measurement values.

- Some models do not require power.

- A wide range of materials available.

- The conditions for entry and exit do not influence the measurement.

Disadvantages:

- Not suitable for fluids contaminated or two fluid phases.

- Because of its inherent design, some models produce pulsating flow in the pipe.

- Only allow to measure the flow in one direction.

- Nominal diameters should be large compared with other methods.

- Restrictions on temperature and pressure to avoid the effects of interstitial leak in the meter body.

- Pressure drop in filters upstream, increasing the viscosity and / or higher speeds.

- A bad usage can eventually block the pipe system.

- The mechanical friction imposes an additional burden if the counter is used to measure flow of aggressive chemicals.

- Risk of freezing (ice formation) if used outdoors.

- Possibility of mechanical damage by air or steam inlet in the fluid.

- Not especially cheap compared to other equivalent technologies for measuring fluid flow rates.