Flow Meter: What They Are & Types of Flow Meters

What is a Flow Meter?

A flow meter is a piece of equipment used to measure the flow rate of solids, liquids or gases. Flow meters can do this linearly, non-linearly, volumetrically or on a weight basis. They are also known as flow gauges, flow indicators or liquid meters.

Types of Flow Meters

The main types of flow meters include:

1. Mechanical Flow Meters
2. Optical Flow Meters
3. Open Channel Flow Meters

Mechanical Flow Meters

Positive Displacement Flow Meters

These meters measure flow rate by capturing the fluid in a chamber and measuring its volume. This resembles filling a bucket with water to a specific level and then letting it flow out.

These flow meters can measure intermittent flows or meager flow rates and suit any liquid irrespective of its viscosity or density. Positive displacement flowmeters can be considered robust because turbulence in the pipe does not affect them.

Nutating disc meter, reciprocating piston meter, oscillatory or rotary piston meter, bi-rotor type meters such as gear meter, oval gear meter (Figure 1) and helical gear meter fall under this category.

Mass Flow Meters

These meters estimate flow rate by measuring the mass of the substance flowing through them. They are typically used in chemical industries where weight-based measurements matter more than volume.

Thermal meters (Figure 2a) and Coriolis flowmeters (Figure 2b) fall under this category. In thermal meters, the flowing fluid cools a probe pre-heated to a set degree; sensing that heat loss yields the rate at which the fluid flows. Coriolis meters work on the Coriolis principle: fluid flowing through a vibrating tube changes the tube’s frequency, phase or amplitude, which gives a measure of its flow rate.

Differential Pressure Flow Meters

Differential pressure flow meters measure flow by observing the pressure drop as fluid passes an obstruction in its path. As the flow increases, the pressure drop across the obstruction increases too, and the meters record it. The flow rate is proportional to the square root of this pressure drop, following Bernoulli’s equation.

Orifice plate meter, flow nozzle meter, flow tube meter, Pitot tube meter, elbow tap meter, target meter, Dall tube meter, cone meter, Venturi tube meter, laminar flow element and variable area meter (rotameter) are a few examples of differential pressure flow meters.

Velocity Flow Meters

Velocity flow meters estimate flow rate by measuring the fluid’s velocity, which bears a direct proportionality to the flow rate. These meters measure velocity using various methods, including turbines.

Depending on the method used to find the velocity, we have various kinds of velocity flow meters such as turbine flow meter, vortex shedding flow meter, Pitot tube flow meter, propeller flow meter, paddle or Pelton wheel flow meter, single jet flow meter and multiple jet flow meter.

Flow-rate measurement of fluids in hazardous environments, including mining, demands non-intrusive flow meters. SONAR flow meters, a type of velocity flow meter, serve these requirements. Besides, ultrasonic flow meters as well as electromagnetic flow meters also form part of the velocity-type family.

Optical Flow Meters

Optical flow meters use light to measure flow rate. They typically employ a laser beam and photodetectors: gas particles scatter the laser beam and create pulses that the receiver detects, and the timing between those signals reveals the speed of the gas.

Because these meters measure the actual speed of the particles making up the gas, thermal conditions and variations in gas flow leave them unaffected. Hence they provide highly accurate flow data even in very unfavourable surroundings, for example high temperature and pressure or high humidity.

Open Channel Flow Meters

Open channel flow meters measure the flow rate of a fluid whose flow path involves a free surface. Weir meters and flume meters (Figure 6) are open channel flow meters that use secondary devices such as bubblers or floats to gauge the depth of the fluid at a particular point; from that depth, the flow rate follows.

In dye-testing based open channel measurement, a predetermined amount of dye or salt changes the concentration of the flowing stream, and the resulting dilution gives a measure of the flow rate. It should also be noted that the precision required of a flow meter depends on its application: monitoring garden-water flow tolerates a lower precision than monitoring an alkali in a chemical process. Another point worth noting is that flow meters, when used together with flow valves, can perform controlling actions successfully.

What is a Water Meter?

A water meter is a type of flow meter used to monitor the flow rate of water through a pipe. Two common approaches to water-flow measurement exist: displacement and velocity. Common displacement designs include oscillating piston and nutating disc meters; velocity-based designs include single-jet and multi-jet meters and turbine meters.

Types of Water Meters

Water meters can be classified into various types depending on the mechanism by which they measure the flow of water.

Gear Type Water Flow Meter

Generally, all residential water meters are of the positive-displacement type. These can in turn be gear-meter type (Figure 1), oscillating piston type or nutating disc type. Here, water enters a chamber and leaves only once the chamber fills.

By counting these chamberfuls, one can estimate the rate of water flow. Such meters suit moderately low flow rates.

Single Jet Water Meter

Velocity water meters form another category of water flow meters. In these meters, the rate of flow is determined by monitoring the speed at which the water moves. The subcategories under this type are jet (single- and multi-jet) and turbine flow meters.

In a single-jet meter, one water jet impinges on the impeller, while a multi-jet meter drives it with several jets. In either case the rotational speed of the impeller gives a measure of the flow rate. Turbine-type water meters instead use a turbine wheel whose rotation speed determines the flow rate.

Note that jet-type water meters suit low flow rates while turbine-type meters suit high flows. When both high and low flow rates must be measured, a compound-type water meter combining both categories in one device is the better choice.

Electromagnetic Water Meter

Water meters can also measure flow rate using Faraday’s law of induction. Such meters are referred to as electromagnetic water meters (Figure 2) and usually serve where unclean, untreated or waste water must be measured.

Here, the water flowing through a non-magnetic, non-electrical pipe induces a voltage in the meter’s magnetic field. The magnitude of this voltage is proportional to the magnetic flux density and hence to the velocity of the water in the pipe, from which the flow rate follows.

Transit Time Type Water Meter

Water meters can also be of the ultrasonic type, measuring the water flow with SONAR techniques. Sound waves sent through the flowing water reveal its velocity; once the velocity is known, the associated flow rate follows because the cross-sectional area of the meter body is known in advance. These kinds of meters are either of the Doppler type or of the transit-time type.

Applications of Water Meter

The applications of water meters include:

1. Water supply departments are the primary users of water meters. They install these kinds of meters in every building to track consumption so customers can be billed accordingly.
2. Large infrastructures employ water meters to ensure proper water flow through each sub-structure, free from leakage and breaks.
3. Industries whose processes include cooling use water meters to monitor the rate at which the cooling water flows.
4. Agricultural industries and laboratories employ water meters to analyse different properties of water such as salinity, pH level and acidity.
5. Hydroelectric plants that generate power from water employ water meters to maintain a controlled flow through them.
6. Water meters, such as turbine types, are used in fire-protection systems.

Flow Measurement

Flow measurement is a method of measuring how much material flows through a particular area of interest (Figure 1). Generally, industrial processes demand measurement of three types of ‘materials’, namely solids, liquids and gases.

Measurement of Solids

Solids are usually measured on a weight basis: the flow rate of solids is expressed as the mass of solid per unit time or the weight of solid per unit time. The associated units include kilograms per second, grams per second, kilograms per hour and tonnes per hour.

Measurement of Liquids

Liquids can be measured in terms of weight or volume, so their flow rate appears as the weight of liquid per unit time or the volume of liquid per unit time. Accordingly the rate may be quoted in units such as tonnes per hour, kilograms per second, liters-per-hour, m³-per-minute and so on.

Measurement of Gases

Gases are usually measured in terms of volume, meaning the flow rate of gas is expressed as the volume of gas flow per unit time. Thus the units in which the flow of gases is measured are m³-per-hour, Nm³-per-hour, and the like. One more factor matters when measuring gas flow: gases are compressible fluids (unlike liquids), so their volume varies with pressure and temperature. To account for this, values measured at actual conditions are usually converted to normal temperature and pressure (NTP) or standard temperature and pressure (STP) conditions.

Flow Meters

Flow measurement is accomplished through flow meters (Figure 2), discussed in detail earlier in this article.

These flow meters come in many types depending on the quantity being measured and their operating principle. Differential pressure meters, variable area meters, turbine flow meters, Coriolis meters, gear meters, Woltman meters, single jet meters, multiple jet meters, paddle wheel meters, current meters, Venturi meters, cone meters, linear resistance meters, vortex flow meters, magnetic flow meters and non-contact electromagnetic flow meters are only a few among the numerous types in use.

Flow Sensors

Sensors are devices used to monitor the change in a particular parameter. Accordingly, flow sensors monitor changes in the flow rate of a particular state of matter. They are expected to record even slight changes in flow rate and therefore need high precision.

The existence of numerous kinds of flow meters and flow sensors is a direct consequence of the importance attributed to flow measurement across industry, including the design of automatic control systems.