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ST 700 Series HART/DE Option User’s Manual
Revision 4.0
5.3.4.1 Notes on Damping (Digital Noise Reduction)
You can adjust the damping time to reduce output noise. By way of suggestion, set damping to the
smallest value reasonable for your process.
The electrical noise effect on the output signal is partially related to the turndown ratio of the
Transmitter. As the turndown ratio increases, the peak-to-peak noise on the output signal increases.
You can use the following formula to find the turndown ratio using the range information for your
Transmitter.
Turndown Ration =
Upper Range Limit
(Upper Range Value − Lower Range Value)
Note:
ST 700 Transmitters are specified with a minimum of 100:1 turndown capability.
5.3.4.2 Notes on Conformity
You can click the Transmitter output to represent a straight linear or a square root calculation for flow
measurement applications with a differential pressure (DP) unit.
For a differential pressure (DP) Transmitter, measuring the pressure drop across a primary element,
the flow rate is directly proportional to the square root of the pressure drop. The Transmitter output
automatically converts to percent-of-flow when its output conformity is configured as square root.
You can use the following formulas to manually calculate the percent of flow for comparison
purposes:
∆P
Span
× 100 = %P
Where
P
= Differential pressure input in engineering units
Span
= Transmitter measurement span (URV- LRV)
%P
= Pressure in percent of span
Therefore:
√
%P
100
× 100 = % Flow
Also, you can use the following formula to determine the corresponding current output in
Milliamperes DC.
(% Flow X 16) + 4 = mA DC Output
To avoid unstable output at readings near zero, the ST 700 Transmitter automatically drops square
root conformity and changes to linear conformity for low differential pressure readings. As shown in
, the dropout point is between 0.4% and 0.5% of differential pressure, depending on direction
