Max Machinery 272-5X8 BIDIRECTIONAL TRANSMITTER (210_240 SERIES FLOW METERS) - Manual (Page 14)

Page 14 272-500-350 © 1996 Max Machinery, Inc.
GENERAL CONSIDERATIONS
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Noise can be defined as any change in either the fluid flow or the electrical system that is not a meaningful
change in the flowlow or th electrical system that is not a meaningful change in the flow rate. For instance, the
ripple induced in the flow by a gear or piston pump is noise. The system will typically have to be dampened so
that its’ response time is longer than the tooth to tooth
period of the pump. Piston pumps with fewer than three
pistons create a particularly large amount of bothersome
ripple and result in a very slowly reponding system.
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, the transmitter (272-5X5 or 272-5X8) for these meters can be adjusted
to minimize this characteristic. Additionally, some amount of damping is usually 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.
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