U-tube heat exchangerWhat is a flow meter?
Flowmeter StylesMass Flow
Differential Pressure Meters
Positive Displacement


Gas Meter Types


In this post, we consider gas meter types often used for flow measurement. I have categorized them as mass flow meters, the velocity flow meter, differential pressure and PD meters.


What is a flow meter?


A flow meter is a precision instrument that measures the rate of gas flow or (liquid flow) in a pipe. There are three main meter styles for flow measurement:


– Positive displacement meters collect a fixed volume of fluid, then release and refill the fluid, then tally how many times the volume is filled to determine flow.
– Flow may be established by measuring the rate of fluid over an area that is known.
– Other methods depend on forces produced by the flowing stream as it overpowers a known constriction, and indirectly calculates flow.


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Flowmeter Styles


Mass Flow


Mass flow meters determine mass flow passing through the meter. Two types deserve mention here:


Coriolis flowmeters provide a direct mass flow measurement based upon the deflection force of the fluid moving through a vibrating tube. These meters are highly accurate with high turndown capabilities and are independent of fluid properties. They are also extremely expensive to purchase and install, and are not suitable for larger pipe sizes.


Thermal mass flow meters measure the mass flow based on heat transfer from a heated element. The measurement is in mass flow, and additional pressure and temperature correction is not required. They also provide excellent accuracy and repeatability and are easy to install.




In a velocity meter type, the rate of the medium passing through the meter determines the measurement.


Turbine flow meters measure volumetric flow based on fluid flowing passed a free-spinning rotor; each revolution corresponding to a specific volume of fluid. The meters have high turndown and accuracy. Unfortunately, because of the meter’s moving parts its use is limited to clean dry gases only, and pressure and temperature compensation are required.


Ultrasonic flow meters measure the difference in transit time of pulses that travel from a downstream transducer to the upstream transducer, compared to the time from the upstream transducer back to the downstream transducer. This style of meter is extremely accurate but extremely expensive, and pressure and temperature measurement are required.


Vortex flow meter has a bluff object or shedder bar that is placed in the flow path, and as gas flows around the shedder bar, vortices are cyclically generated from opposite sides of the bar. The frequency of vortex generation is a function of the gas velocity. The frequency of vortex shedding is independent of fluid composition. The meter, however, requires temperatue and pressure compensation and needs a minimum flow rate to produce vortices.


Differential Pressure Meters


Differential pressure flow meters calculate flow by measuring pressure drop over an obstruction inserted in the flow path. Common types of flow elements are orifice plates, flow nozzles, venturi tubes and averaging pitot tube.


The orifice plate is a differential pressure meter frequently used for natural Barauni gas measurement. It measures volumetric flow, not mass flow. Limitations of this meter include poor low flow sensitivity, limited turndown, and it also creates a pressure drop, which impacts operating costs. Additionally, since it’s a volumetric flow meter it requires correction of temperature and pressure to achieve mass flow.


An averaging pitot tube is a differential pressure flow measurement device commonly used for combustion air measurement. The device has limitations with gas flow; especially low flow sensitivity and turndown. The measurement is contingent upon achieving velocity pressure, and it is possible to have inadequate speed to achieve an acceptable signal at low flow rates.


Positive Displacement


Positive displacement meters require fluid to mechanically displace components, and measure volumetric flow at the operating temperature and pressure. While they have sufficient accuracy, pressure and temperature compensation are required to achieve mass flow, and since they have moving parts gas cleanliness needs to be considered. A PD meter may be called a PD flow meter or a volumetric flow meter. An example of a PD meter is the diaphragm meter.


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