Types of Flow Transducers
There are many hundreds of types of [ithoughts_tooltip_glossary-glossary]flow[/ithoughts_tooltip_glossary-glossary] meters depending on the make and application. They may be classified roughly as follows.
Positive Displacement Type
Differential Pressure Type
Positive Displacement Type
These types have a mechanical element that makes the shaft of the meter rotate once for an exact known quantity of fluid. The quantity of fluid hence depends on the number of revolutions of the meter shaft and the flow rate depends upon the speed of rotation. Both the revolutions and speed may be measured with mechanical or electronic devices. Some of the most common listed below.
The MESHING ROTOR type consists of two rotors with lobes. When fluid is forced in, the rotors turn and operate the indicating system.
Measurement is based on a vane wheel rotating at a speed proportional to the flow velocity of the fluid which surrounds it. The rotational speed is virtually independent of density, pressure and temperature of the medium. Vane wheel anemometers can be used in gases and liquids.
The flow of the fluid is inferred from some effect produced by the flow. Usually this is a rotor which is made to spin and the speed of the rotor is sensed mechanically or electronically. Below are some of its type
Turbine rotor type
The turbine type shown has an axial rotor which is made to spin by the fluid and the speed represents the flow rate. This may be sensed electrically by coupling the shaft to a small electric tachometer. Often this consists of a magnetic slug on the rotor which generates a pulse of electricity each time it passes the sensor.
Rotating vane type
The jet of fluid spins around the rotating vane and the speed of the rotor is measured mechanically or electronically.
Variable Area Type
There are two main types of this meter
Float type (Rotameter)
The float is inside a tapered tube. The fluid flows through the annular gap around the edge of the float. The restriction causes a pressure drop over the float and the pressure forces the float upwards. Because the tube is tapered, the restriction is decreased as the float moves up. Eventually a level is reached where the restriction is just right to produce a pressure force that counteracts the weight of the float. The level of the float indicates the flow rate. If the flow changes the float moves up or down to find a new balance position.
When dangerous fluids are used, protection is needed against the tube fracturing. The tube may be made of a non-magnetic metal. The float has a magnet on it. As it moves up and down, the magnet moves a follower and pointer on the outside. The position of the float may be measured electrically by building a movement transducer into the float.
Tapered plug type
In this meter, a tapered plug is aligned inside a hole or orifice. A spring holds it in place. The flow is restricted as it passes through the gap and a force is produced which moves the plug. Because it is tapered the restriction changes and the plug takes up a position where the pressure force just balances the spring force. The movement of the plug is transmitted with a magnet to an indicator on the outside.
Differential Pressure Flow Meters
These are a range of meters that convert flow rate into a differential pressure. The important types conform to BS 1042 and are
The working principle for all these is that something makes the velocity of the fluid change and this produces a change in the pressure so that a difference Dp = p2 – p1 is created. It can be shown for all these meters that the volume flow rate Q is related to Δp by the following formula.
Q = K(Δp)0.5
K is the meter constant. A full explanation of these meters is covered in the tutorials on fluid mechanics. The picture shows an industrial d.p.meter. Extra instrumentation heads can be fitted to produce an electrical output (4 – 20 mA) or a pneumatic output (0.2 – 1 bar).