The basis for the classification of flowmeters can be various criteria: installation methods, measurement principle, measured medium, etc. This article will consider the classification of gas flowmeters by operating principle.
Gas Flow Measurement Device
Diaphragm Flowmeter
These are among the simplest flow measurement devices. Their operating principle is based on the movement of the diaphragms of the measuring chambers as gas flows into them. The inlet and outlet of gas cause the chamber walls to move, which in turn drives the counting mechanism. The number of contractions and expansions of the chambers is proportional to the volume of gas passing through the device.
These devices have a wide range and are relatively inexpensive; however, due to low accuracy, sensitivity to high pressure, and inability to measure large flows, they are practically unsuitable for industrial use.
Rotary Flowmeter
Inside the measuring chamber of a rotary flowmeter are two rotors positioned across the flow and connected by gears so that one edge of each rotor touches the chamber wall, and the opposite edge touches the other rotor. When air flows in, the rotors move under its pressure and begin to roll over each other, trapping specific portions of gas so that each rotation corresponds to a certain volume. The counter mechanically records the number of rotor rotations, which is then converted into volume.
These flowmeters have a wide range, low error, and high stability, but are highly susceptible to contamination, have moving parts, and can only be used for relatively small diameters.
Measurement by Pressure Drop
Most often, this method involves the use of a diaphragm. In this case, a diaphragm is installed in the pipeline to narrow the flow, usually a plate with a hole in the center. The pressure of the gas passing through the diaphragm drops, and the pressure difference before and after the narrowing is proportional to the velocity, and thus to the flow rate of the gas. Using differential pressure sensors, the pressure difference can be measured and converted into flow rate.
Another device using pressure drop to measure flow is the Venturi tube. In this device, the narrowing and widening of the pipeline occur gradually. The tube consists of two truncated cones connected at their narrow ends. The expansion cone is longer than the contraction cone.
A subtype of the Venturi tube is the measuring nozzle, which has a contraction cone but, unlike the Venturi tube, lacks the expanding section. This device is used when flow turbulence is very high.
There are also flowmeters where the flow narrowing is created by a wedge-shaped restrictor. Otherwise, these flowmeters are similar to other devices using the pressure drop measurement principle.
The advantage of these flowmeters is relatively high measurement accuracy and not a significant cost increase with increasing pipeline diameter. The main disadvantage is that the installed diaphragm causes significant pressure losses in the gas flowing through the pipeline. The Venturi tube causes much less loss than the diaphragm but is quite bulky and expensive. Additionally, these flowmeters are poorly suited for low flow rates.
The following graph shows pressure loss values for different types of constricting devices:
Continuation:
Classification of Gas Flowmeters – Part 2
Gas Flow Meter Classification – Part 3
You can select a flow meter suitable for your task in the product catalog or by contacting our technical specialists.
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