Revision 5.0
ST 800 Series HART/DE Option User’s Manual
Page 17
Notes on Conformity
You can select 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 800 Transmitter automatically drops square
root conformity and changes to linear conformity for low differential pressure readings. As shown in
Figure 7, the dropout point is between 0.4% and 0.5% of differential pressure, depending on direction
.
Figure 7 – Differential Pressure Square Root Conformity Curve
