Flow meters work by measuring the amount of gas, liquid, steam, or other material that passes through or around their sensors. While they all serve the same function, there are many ways to do it – hence the many flow meter types to choose from.
This article will help you choose the flow meter that will deliver accurate and repeatable measurements for specific applications, whether for general research, process control, or monitoring. You will learn how to specify the flow meter you need, and how to buy the right one for your application.
Which Flow Meter Is For You?
There are a number of applications where a flow meter can work, and they call for different devices depending on the measured material, the amount of flow, and other conditions:
- High or Low Budget
- Closed, full pipe, or partially filled or open channel flow?
- Very large flows or only small flows?
- Gas, liquid or steam?
- Viscosity issues?
- High or low temperature or pressure conditions?
- Simple or complicated pipe layouts?
- Costly products that need precise measurement?
The answers to each of these questions will point to the correct flow meter for your application.
Budget High or Low
The budget you have for your flow meter application will point you to some specific types of meter. Electromagnetic (“magmeters”), ultrasonic, and Coriolis meters are fairly expensive, while turbine meters and paddlewheel meters are much less expensive. Depending on what the application parameters are, you may find it necessary to adjust your budget upward to get the flow meter you need.
Closed, full pipe, or partially filled or open channel flow?
You need to know if the application is a closed, full pipe or partially filled pipe or an open channel flow. Nearly all flow meters will work for closed pipes that are full of liquid. Some will only work for clear liquids. Others will work for pipes that are only partially filled. Others will work for open channel flow measurement. Magmeters will work for any liquid flow in a closed full pipe which has a conductivity higher than about 10-30 uSiemens. Turbine and positive displacement flow meters will work on clean fluids, even viscous ones. Still unsure or have questions ask one of our engineers : Shop@visayasolutions.com
Very Large Flows or Only Small Flows?
You need to know what the minimum and maximum flow you want to measure is. You also need to have a good idea of the accuracy and repeatability you need in your application. If you have a very high flow rate, you will need a larger flow meter. Some flow meters, like magmeters, turbine meters, and ultrasonic meters are available in line sizes from very small, fractional inch/centimeter sizes to very large sizes (up to 2 meters diameter). Some flow meters, like positive displacement flow meters and Coriolis Mass Flow Meters, are not available in very large pipe sizes. Calculating the minimum, maximum, and operating flow range can help to pinpoint the right type and size meter for your application. In some cases, the meter can be smaller than the pipe, but we must know the pressure drop limit of these processes.
Type of fluid (Gas, Liquid or Steam? Viscosity issues?)
It matters whether you are measuring a liquid, or a gas, or steam. Whether the liquid is a clear fluid, like water, or is viscous, like oil or paint, the type of flow meter you need may depend on what the fluid is. It also matters whether the fluid is corrosive or abrasive.
As you can see, we have to know the characteristics of the liquid or gas you are measuring. In many cases, providing it’s name will help the vendor identify the required needs. If it is water, or, for example, SAE30 motor oil, just saying that will identify the fluid. However, we should still know the product’s conductivity, density, and viscosity in case the vendor doesn’t have those details.
The minimum, maximum, and operating temperatures of the process will eliminate some flow meters from our list. The ways they affect the product can also influence our selection process.
The pressure can limit the model and material of the meter. Again, we should know the minimum, maximum, and nominal pressure here as well. High-pressure processes sometimes need a special version of a certain flow meter too.
Measuring principle recommendations
- Electromagnetic Flow Meters: For water or any conductive fluid the “magmeter” is the easy choice. However, they are not inexpensive.
- Thermal mass flow meters: These flow meters can precisely measure mass flow of gases from low to high flows.
- Vortex flow meters: These flow meters are ideal for measuring liquid and steam flows at high temperatures and pressures.
- Ultrasonic flow meters: These flow meters work well in liquids when we want to avoid pipe cutting or shutdown
- Older technologies such as differential pressure, positive displacement, and turbine meters also have their places.
Volumetric vs mass flow meters
Flow meters measure either mass or volume. In a volumetric flow meter, the flow (Q) equals the cross section of the pipe (A) times the velocity of the fluid (v): Q = A * v
For things like chemical reactions, combustion, or buying and selling gases, you need the mass flow rate. In a mass flow meter, the mass flow (ṁ) equals the volumetric flow rate (Q) times the fluid density (ρ): ṁ = Q ∗ ρ
And for things like chemical reactions, combustion, or buying and selling gases, we need the mass flow rate. So in a mass flow meter, the mass flow (ṁ) equals the volumetric flow rate (Q) times the fluid density (ρ): ṁ = Q ∗ ρ
Here are the most commonly used types of flow meter:
Electromagnetic flow meter
Electromagnetic flow meters require the measured liquid to be a water-based or conductive. This makes the electromagnetic flow meter an excellent choice for wastewater or process water or other kinds of dirty water. The Electromagnetic Flowmeter, or magmeter” is the most widely applicable type of flow meter, because it only requires a minimum conductivity in the fluid, and a full pipe. Magmeters cost about $500 per inch diameter, which is on the expensive side. You can see a video that explains how a magnetic flow meter works here:
Coriolis Mass Flow Meter
Coriolis Mass Flow meters use the principle of the Coriolis force acting on a tube through which the liquid or gas is flowing. The higher the mass flowing through the tube, the higher the vibrating frequency of the tube becomes. This is a direct measurement of mass flow, and both volumetric flow and density can be derived from it.
Here’s a video that describes the Coriolis Mass Flow measurement principle:
Turbine Flow Meter
Turbine Flow Meters were the very first flow meter other than a bucket. Invented in the 1790s by Reinhard Woltmann, they position a helical rotor at right angles to the flow. The velocity of the flow turns the rotor, which is either electronically coupled or mechanically coupled. This coupling drives a meter readout which provides a flow rate and a flow total. They operate in water, and in many other liquids with relatively low viscosities.
The paddlewheel flow meter, often furnished as an insertion device, is a specific type of turbine meter.
Here’s a video showing how a Woltmann style turbine meter works:
Vortex shedding flow meter
Vortex shedding flow meters are also immersed in the flow of material, but they use vortices and a computed fluid density to measure both volumetric and mass flow. The principle of a vortex shedding flow meter can easily be visualized by looking at a flag waving behind a flagpole. The flag pole acts as a “bluff body” and the vortices shed by the bluff body are what causes the flag to wave. In a flow meter, the vortices shed by a bluff body in the flow stream are proportional to the velocity of the fluid in the pipe. These vortices are sensed by a sensor embedded in the bluff body, or just behind it, that counts the pressure pulses made by the vortexes as they are shed. Watch the video:
Transit-time ultrasonic flow meter
Transit-time ultrasonic flow meters can measure flow speed from outside the pipe using ultrasound. Consequently, they don’t need to be immersed in the measured material. Transit time flow meters measure the difference in transit time of sound caused by the velocity of the fluid as it flows past the sensors. Transit time flow meters can be extremely accurate. Transit time flow meters require clean homogenous fluids in order to function. The other type of ultrasonic flow meter, the Doppler meter, is very application sensitive and isn’t likely to be very accurate. But it can work in fluids that carry large amounts of solids or entrained air or gas. Here’s a video on transit time flow meters:
Capillary thermal mass flow meter
A capillary thermal mass flow meter relies on heat transfer between gas in a very small tube (the capillary) and a set of sensors that indicate the gas mass flow. Each of these sensors is an RTD temperature sensor. The temperatures of both sensors, at zero flow, are the same. When the flow increases, the heat gets transferred to the downstream sensor. The difference in temperature between the upstream sensor and the downstream sensor is proportional to the mass flow through the flow meter. Here is a short video that will let you see how a capillary thermal mass flow meter works:
Immersible thermal mass flow meter
An immersible thermal mass flow meter also relies on heat transfer, but in a different way. The heat is transferred from a heated sensor immersed in the flow. These are also known as thermal dispersion mass flow meters. In this type of flow meter, the heat is transferred to the boundary layer of the fluid flowing over the heated surface. These flow meters have no moving parts, and can be quite accurate (up to 0.5% of indicated rate). To see how one works, watch the video:
To know more about flow meters or for advice on choosing the right device for your application, please ask our engineers! They can help you purchase the right flow meter and get it right the first time.