284-512-350 © 1990 (Rev 6/97) Max Machinery, Inc.
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Response Rate, Accuracy and Noise (cont.)
All positive displacement flow meters add noise to a flow metering system. The noise is
typically of two origins. As the elements of the meter rotate, they require varying amounts of
pressure to move (See Fig. 13). This induces pressure fluctuations between the pump (or
control valve) and the flow meter. If there is any air trapped in the line, the fluid flow will
vary as the air compresses and expands. This will be sensed as a changing flow by the flow
meter and the output will contain unwanted ripple or noise. Plumbing in a flow system
should be sized and laid out to avoid air being trapped between the flow meter and the flow
controlling device (a pump or valve).
The second type of noise that must be considered is a result of flow meter geometry and
design. Because of features such as an oval gear, or a piston/crankshaft configuration, or due
to manufacturing tolerances, the rotation of the metering elements is not completely uniform.
For example, the 210 series meters utilize four pistons connected to a crankshaft. The
varying rotational speed of the crankshaft is shown in Fig. 14. To obtain the smoothest
output signal, some amount of damping will be necessary at the indicator.
The electronic converter of any meter will add its share of noise. For instance, DC
transmitters produce some ripple in their output due to the sinusoidal nature of the induced
voltage in the armature coils.
General Flow Metering Considerations
Discontinued
