Gas Flow Measurement – Different Types of Flow Meters

Flow sensors can be found across various industrial, medical and aerospace applications. Flow is defined as the mass or volume of a fluid that passes per unit of time. In practice, flow sensors (or flow meters) are essential in every operation that requires measuring the mass or volume of fluids or gases that are dispensed, distributed, or consumed per unit time.

What is a flow meter and how do we measure gas flow?

A flow meter (alternatively called flow sensor) is an instrument that has been manufactured to measure with precision the rate of flow in a pipe. The accurate gas or liquid flow measurement ensures a safe, efficient, and environmentally compliant operation in many applications.

But how do we measure flow?

Measuring flow can vary from a very simple principle to a very complicated process.

The way of measurement really depends on the technology used. In this article, we are going to look at the eight most commonly used technologies in gas or liquid flow measurement. These flow meter types are:

1. Electromagnetic
2. Vortex Time
3. Paddle wheel
4. Thermal dispersion
5. Floating Element
6. Ultrasonic Differential pressure
7. Coriolis

Challenges in Gas Flow Measurement

For each application that demands gas flow measurement, different challenges might arise that require careful attention and consideration. Some of them include:

• Capability to measure low and high flows: Required to measure the high and lower level of gas flows with precision.
• Compatibility to the size: Special consideration should be taken as to the suitability of each flow-meter component to the implemented place, small or large.
• Durability to environment and hazards: Environmental conditions that a flow sensor must reliably operate in.
• Exact calibration to actual process conditions: It is essential for undisturbed operations.

Gas Flow Meter Types – Technology Comparison

Of course, there is no single, all-in-one technology that can be implemented for all operational requirements, performance, and conditions.

There are at least 8 common gas flow measurement technologies being used today, all with their strengths and limitations. By understanding the advantages and disadvantages of each, costly mistakes can be prevented.

1. Electromagnetic Flow Meters

Electromagnetic flow meters detect flow by using Faraday’s Law of induction. Inside an electromagnetic flow meter, there is an electromagnetic coil that generates a magnetic field, and electrodes that capture electromotive force (voltage). As the fluid flows through the pipe, the electromagnetic field changes due to the forces generated by induction. These changes are then translated to flow rate.

Pros:

• Unaffected by the temperature, pressure, density, or viscosity of the liquid.
• Able to detect liquids that include contaminants (solids, air bubbles).
• There is no pressure loss.
• No moving parts (improves reliability).

Cons:

• Electromagnetic flow meters cannot detect gases and liquids without electrical conductivity.
• A short section of straight pipe is required.

Best applied to: Electromagnetic flow meters are primarily used in food industries, chemical applications, natural gas supplies, and power utilities as they are largely unaffected by changes in pressure, density, and temperature.

2. Vortex Time Flow Meter

Vortex flowmeters make use of a principle called the von Kármán effect. According to this principle, flow will alternately generate vortices when passing by a bluff body. A bluff body has a broad, flat front. In a vortex meter, the bluff body is a piece of material with a broad, flat front that extends vertically into the flowstream.

Flow velocity is proportional to the frequency of the vortices. Flowrate is calculated by multiplying the area of the pipe times the velocity of the flow. In some cases, vortex meters require the use of straightening vanes or straight upstream piping to eliminate distorted flow patterns and swirl. Low flowrates present a problem for vortex meters because they generate vortices irregularly under low flow conditions.

The accuracy of vortex meters is from medium to high, depending on the model and manufacturer. In addition to liquid and gas flow measurement, vortex flowmeters are widely used to measure steam flow.

Pros:

• The vortex flowmeter has no moving parts, and the measuring component has a simple structure, reliable performance and long service life.
• The volumetric flow rate of the vortex flowmeter is not affected by thermal parameters such as temperature, pressure, density, or viscosity of the fluid being measured.
• It measures the flow of liquids, gases, or vapors, has very wide applications.
• It causes little pressure loss.

Cons:

• It has poor anti-vibration performance. External vibrations can cause measurement errors in the vortex flowmeter.
• The high flow velocity shock of the fluid causes vibrations in the vortex body, which reduces the measurement accuracy.
• Can only measure clean media.
• Straight pipe requirements for mounting.
• Not suitable for low Reynolds number fluids measurements.
• Not suitable for the pulsating flow.

Best applied to: Vortex flow-meters are more commonly used in power generation and heat-supply systems such as compressed air, saturated steam, superheated steam etc.

3. Paddle Wheel Flow Meter

This is classified as a turbine flow meter. Paddle wheel flow meters are generally divided into two mechanical classes

1. Tangential-flow flow meters, with a water wheel structure.
2. Axis-flow paddle wheel flow meters, with a windmill structure.

The flow and the revolutions of the paddle wheel are proportional to each other. Thus, by spinning the paddle wheel with the force from the flowing fluid, it becomes possible to measure the rate of this flow from the number of revolutions. By embedding a magnet in the rotation axis and on the edge of the paddle, pulses can be extracted as signals, converting the number of revolutions into the flow rate.

Pros:

• Reliable performance.
• Low cost.
• Can measure flow in either direction.

Cons:

• Moving parts
• Require clean fluids. Particulates can prevent the paddle from spinning properly.
• Require a turbulent flow profile to guarantee the most accurate results.

Best applied to: Paddle wheel flow meters can be used in fume scrubbers, reverse osmosis and in various other fields.

4. Thermal Dispersion Flow Sensor

Thermal dispersion flow meters use heat to measure the flow rate of a fluid. The usual structure is that there is a heating element in the middle and two temperature sensors on either side of the heating element. As gas flows, the heat is transferred towards the direction of the flow and the temperature sensor upstream is “getting colder” while the downstream temperature sensor is “getting hotter”. The flow rate can be calculated by measuring the difference between the temperature sensors.

Pros:

• No moving parts.
• Reliable performance.
• Very accurate measurement.
• Low total error band.
• Can measure flow in either direction.

Cons:

• Not suitable for liquid flow measurement.
• Not ideal for measuring gases at high temperatures (>50oC).

Best applied to: Some of the typical applications can be found in the medical and industrial fields such as respiratory devices, anesthesia equipment, CPAP devices and central gas monitoring systems.

ES Systems has developed two distinct product series of MEMS thermal sensors, ESRF-ESF and ESRF-HF.

• ESRF-ESF: Based on the hot-film anemometer principle for mass gas flow measurements, this sensor with bidirectional gas flow sensing of up to ±300 ln/min ideal for medical, process & pharmaceutical equipment.
• ESRF-HF: The ESRF-HF is a family of mass flow transmitters that enable fast and accurate measurements of gas flow over a wide dynamic range. Its compact size, combined with the ruggedized stainless-steel housing, makes it ideal for use in industrial applications within confined spaces.

5. Floating Element Flow Sensor

This is one of the simplest flow measurement technologies. The method usually involves float in a tapered pipe. When the fluid is forced in between the tapered pipe and the float, a differential pressure is generated which causes the float to be moved accordingly. You can measure the flow rate by reading the visual scale of the meter.

Pros:

• Cost.
• Easy to use.

Cons:

• Manual measurement.
• Not suited for high flow rate measurements.

Best applied to: They are widely used for numerous applications including chemicals, compressed air, and other gases.

6. Ultrasonic Flow Meter

Type Ultrasonic flow sensors measure the volumetric flow rates of a wide variety of fluids relying on ultrasound and the Doppler Effect.

This technology is very accurate and is independent of the pressure, temperature, and viscosity of the medium. In idle operation, the transmitter sends ultrasonic waves that are bounced in the pipe and perceived from the ultrasonic sensor. Since there is no fluid movement, the frequency of the received signal is the same as the transmitter. Once the flow starts, the frequency of the received waves is either higher or lower (depending on the direction of the flow) than the one transmitted.

This frequency difference can be translated to flow rate.

Pros:

• No moving parts.
• Low maintenance cost.
• High accuracy.

Cons:

• High cost.
• Cannot measure fluids that do not reflect ultrasonic frequency.

Best applied to: Ultrasonic flow sensors have many applications, spanning from process flow to custody flow.

7. Differential Pressure Flow Meter

Differential pressure sensors measure flow through capacitive pressure sensors using Bernoulli’s equation. Differential pressure flow meters use laminar plates, an orifice, nozzle, or Venturi tube to create an artificial constriction then measure the pressure loss of fluids as they pass that constriction. The higher the pressure drop, the higher the flow rate. These rugged, accurate meters are ideal for a wide range of clean liquids and gases.

Pros:

• No moving parts.
• Accurate measurements.
• Reliable operation.

Cons:

• Not suited for liquid flow measurement.
• Requires induced pressure drop for operation that could be avoided using other techniques.

Best applied to: Due to their compatibility, there are used in many industries, such as power supply, food and beverage, medical, aerospace, and HVAC.

ES Systems have designed the ESCP-BMS1 differential pressure sensor which has:

• Silicon capacitive technology that provides exceptional accuracy.
• Long-term stability.
• High overpressure tolerance.

8. Coriolis Mass Flow Meter

The main working principle of coriolis flow meters is the use of a vibrating tube where the flow of gas can cause changes in frequency, or phase shift proportional to the mass flow rate. At an idle state, the tube vibrates at a predefined frequency. As the fluid flow begins, the vibration of the tube alters proportional to the flow rate of the medium. This change in vibration is measured by sensors across the tube and then translated to flow rate.

Pros:

• True mass flow measurement.
• Unaffected by pressure, temperature and viscosity.
• No inlet and outlet sections required.

Cons:

• Moving parts.
• Environmental vibrations cause inaccuracies in measurement.
• High cost.

Best applied to: They are often applied in many different industrial sectors that need a measurement of sanitary and corrosive but relative clean gases.

Installation and Maintenance

Before making your choice about a gas flow meter type, you should consider the location of the manufacturer’s installation requirements. It is possible that a stable gas-flow profile upstream and downstream from the point of meter installation might be required. Moreover, the degree of maintenance needed should be considered as some technologies differ from each other.

With long experience in designing innovative flow sensors, ES Systems provide high – end products with cutting edge technology, high-performance capabilities, and other unique features. Choose the one that covers the needs of your business and industry.