Flow Measurement of Compressed Air and Gases

Which Flow Measurement Method?

There are a variety of measurement principles that can be used to measure gas flow as follows.

Variable Area meters, also referred to as VA meters or rotameters, have a slim taper conical tube containing a float or piston. As the flow increases, the piston moves towards the larger end of the tube such that the annulus between the float and tube has an increasing area. This increasing area is directly proportional to the flow and is often determined through graduations on the outside of the tube. Simple tube VA meters are very cost effective and accurate enough for indications of relative flow. More sophisticated designs increase the accuracy through gearing and more sensitive meters or by electronic sensing.

Orifice plates and venturi tubes rely on detecting the differential pressure across a restriction in the pipe and this pressure is directly proportional to the media velocity. Given knowledge of the pipe dimensions and media characteristics, a differential pressure instrument can give a reading directly in flow units. The accuracy of these systems are reliant on very fine tolerance machining of the orifice or venturi and the pressure drop incurred through the installation of these devices may not suit all situations. These can be highly accurate and reliable but are expensive and complicated to install and maintain.

Pitot tube systems work on a very similar principle and remove the issue of the pipe restriction as there is only a small diameter tube inserted at right angles to the flow. These can be inserted through ball valves, making removal for service or calibration very simple. This method does not need perfectly dry and pure gases to be accurate and is very tolerant of wet gases and contaminants which makes it ideal for measuring in difficult places such as compressor outlets.

Vortex meters place a bluff restriction in the pipe that causes vortices to form alternately on the 2 rear edges. Sensors are used to detect the formation of these vortices, the frequency of which is directly proportional to the flow rate. These instruments are accurate but again introduce a large restriction into the pipe and are often employed on steam applications.

What about Thermal Mass?

Thermal mass meters and calorimetric flow sensors sense the cooling effect on a heated element which is directly proportional to the mass flow rate rather than the volumetric flow rate. This removes the subsequent calculations needed to determine mass flow rate using other methods of flow measurement and there is no pressure or temperature compensation necessary.

The actual sensing tips are small and thus the restriction placed in the pipe produces very little back pressure, making this method ideal for very low and very high flow rates.

Thermal mass meters are available in a range of accuracies and are ideal for dry, clean gas flow measurement. Some designs have all stainless steel wetted parts making them better suited to contaminated or corrosive gases which would damage the exposed sensing elements of unprotected designs. They are not suitable for wet gases or steam. For those applications a pitot tube design or possibly a vortex meter would be the better choice.

Thermal mass is also an excellent principle for use in flow switches as there are no moving parts in the media and the sensor produces very little back pressure due to its minimal size. These flow switches are very sensitive and some models can be used to determine flow direction as well as flow/no flow.

Compressor Flow and FAD

Flow Meters and Switches