Max Machinery 276-5XX SERIES 4-20MA TRANSMITTERS - Manual (Page 12)

Noise: Noise can be defined as any change in either the fluid flow or the 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 system that is very
slow to respond.
All positive displacement Flowmeters add noise to a
Flowmetering system. The noise is typically of two
origins. As the elements of the meter rotate, they
require varying amounts of pressure to move (As
shown in illustration, right).
This induces pressure fluctuations between the pump
(or control valve) and the Flowmeter. 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
Flowmeter 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 Flowmeter and the flow
controlling device (a pump or valve).
The second type of noise that must be considered is
a result of Flowmeter 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 the figure to the
right.
To obtain the smoothest output signal, the Transmitter (276-515 or 276-525) 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.
( 12 ) 276-515-350 © 2006 Max Machinery, Inc.
GENERAL CONSIDERATIONS
—Response Rate, Accuracy & Noise (cont.)
PRESSURE DROP VS TIME
(210 SERIES PISTON METER)
TIME
ROTATIONAL SPEED
(OF 210 CRANKSHAFT)
TIME
Discontinued