Max Machinery 284-512 SERIES TRANSMITTERS - Manual (Page 10)

Page 10 284-512-350 © 1990 (Rev 6/97) Max Machinery, Inc.
Response Rate, Accuracy and Noise
There is always a trade off in a metering system between response rate, accuracy and noise.
The three are related such that their product equals a constant. If any one of them is made
smaller, the others can be made larger.
In most metering systems, response rate and accuracy are desirable characteristics. To
maximize one or both of these parameters, noise should be reduced to a minimum. Once
noise has been minimized, there is a trade off between accuracy and response rate.
Response Rate: When discussing response rate there are three facets to consider. They are:
the response of the flow to a change in the system setpoint, the correction of the flow to an
error induced in it, and the response of the flow rate display to a change in flow rates. These
responses are all purposely slowed down by filtering or damping so the system only reacts to
meaningful flow changes and not to such things as pump pulsations or flow meter ripple.
More damping means slower response.
Accuracy: There are three topics to consider when looking at accuracy. The first being the
display; which can typically have anywhere from two digits (1 to 99) to 4-1/2 digits (19,999)
of information. This equals a resolution of 1% to a maximum of 0.005%, respectively. The
display steadiness is also directly related to it’s accuracy. For instance, a display that jitters
from 95 to 105 in a meaningless way is not accurate to one part in 100 (1%) but only to about
10 parts in 100 (10%).
The basic accuracy of the flow meter is a prime consideration. Typically, the accuracy of a
positive displacement meter is not as good for a fraction of its cycle as it is for one or more
complete cycles. If a system is dampened so that the response rate is longer than the period
of one revolution of the meter, the accuracy of the display is increased. The accuracy of the
system can never be better than that of the flow meter.
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 very
slowly responding system.
General Flow Metering Considerations
Discontinued