During your research and discovery process of searching for flow measurement products, have you taken the time to consider whether your organization requires mass or volumetric flow meters?
As an engineer or technician, getting repeatable and accurate measurements from your flow meter is critical. However, this cannot happen if you are using the wrong type of flow meter or incorrectly installing your instrument. Therefore, it is just as important to determine if your application requires a mass flow meter or a volumetric flow meter. Each type has its own set of benefits and can be used across many industries and for various applications.
As part of McMillan’s mission to solve the many challenges encountered in instrumentation and equipment design, this helpful article will serve as a reference to help you distinguish the differences between mass and volumetric flow meters:
Differences in Flow Measurement
For starters, let’s review the differences between mass and volumetric flow rates as it is common for some people to confuse mass flow rates and volumetric flow rates with velocity or air speed.
In mass flow meters, the flow rate is calculated by measuring the amount of mass of a substance passing through a device for a given amount of time.
In volumetric flow meters, the flow rate is calculated by measuring the volume of a substance through a device over a given period.
It is also important to keep in mind that there are various types of substances that can pass through a flow meter such as liquids and gases.
Various Types of Mass Flow and Volumetric Flow Meters
Another key difference when it comes to mass and volumetric flow meters are the various types of instruments available for each flow meter and are selected depending on the desired application.
Some examples of devices that measure mass flow rates include:
- Coriolis mass flow meters
- Thermal mass flow meters
Some examples of devices that measure volumetric flow rates include:
- Turbine flow meters
- Positive displacement flow meters
- Vortex meters
Principles of Operation
Mass and volumetric flow meters also vary in the way they operate and function to measure flow.
In the following comparison, we will primarily focus on the principles of operation for thermal mass flow meters and turbine flow meters for volumetric flow.
Thermal Mass Flow Meters
Thermal mass flow meters operate by utilizing thermal sensing technology. As flow enters the device, a small portion is redirected into a small tube containing two coils – one downstream from the other. The first coil introduces a small amount of heat into the gas stream. As the gas passes through the tube, the smart electronics sense the amount of heat transferred from one coil to the other.
McMillan’s patented system insures that the zero remains stable and the sensor is extremely repeatable.
The output of the thermal mass flow sensor is directly related to the specific heat characteristic of the gas being measured. Therefore, if a unit is calibrated for air, it is a relatively simple calculation to figure the calibration for nitrogen or some other similar gas. This advantage offers flexibility not found on many other types of flow sensors.
Volumetric Flow Meters (Turbine)
On the opposite end, turbine flow meters operate by utilizing the Pelton turbine wheel concept. As flow passes through the device, it is directed onto the very small teeth of the wheel using a precision-machined nozzle. This nozzle is sized per the flow range of the unit.
The rotational speed of the turbine wheel increases proportionally to the volumetric flow rate. The microturbine wheel has alternating white and black sections evenly spaced on one surface of the wheel. As the wheel rotates, an infrared beam is reflected off each white section and is directed to a phototransistor which detects each reflected beam and converts them into pulses. As the wheel spins faster, pulse rate increases. When the wheel stops (under zero flow conditions), no pulses are generated. Consequently, zero drift is not possible and zero adjustments are never required.
Whether searching for a low-flow liquid flow meter for to measure liquid flow rates or a thermal mass flow meter to support your gas flow measurement needs, let this article help you determine the ideal flow meter type you will need to achieve precise results.