DP Transmitter Elevation and Suppression Corrections for Level Measurement

When installing a DP transmitter to measure liquid level, configuration of the transmitter often must include an adjustment for one of two conditions introduced by the mounting arrangement.

• Elevated Span – The Lower Range Value (LRV) of the transmitter needs to be configured above “0.”
•  Suppressed Span – The LRV of the transmitter needs to be configured below “0.”

A brief discussion of how to make adjustments for elevation and suppression follows. Then two examples of the calculations needed to determine configuration parameters are given. Finally, a brief procedure that does not involve calculations is provided.

The range of the transmitter can be set anywhere, forward or reverse acting, as long as the following criteria are met:

LRL ≤ LRV ≤ URL
LRL ≤ URV ≤ URL
Span = [URV – LRV] ≥ Min Span

Note that the URL (Upper Range Limit), LRL (Lower Range Limit), and Min Span are transmitter dependent.

 ELEVATION CALCULATION EXAMPLE 

The below figure shows a sample transmitter installation.

1. Calculate the differential pressure as follows. 

Pressure_@DP = (H × SpG)_{High Side} – (H × SpG)_{Low Side}

where H = Height

2. Calculate the LRV when the tank is empty. 

LRV = (120 x 1.0)_{High Side} – (0 x 1.0)_{Low Side}

LRV = +120 inH2O

3. Calculate the URV.

URV = LRV + Span
URV = +120 + 100
URV = 220 inH2O

Therefore, transmitter range should be 120 to 220 inH2O

 SUPPRESSION CALCULATION EXAMPLE

The below figure shows a sample transmitter installation.

1. Calculate the differential pressure as follows. 

Pressure_@DP = (H × SpG)_{High Side} – (H × SpG)_{Low Side}

where H = Height

2. Calculate the LRV when the tank is empty. 

LRV = (0 x 1.0)_{High Side} – (100 x 1.0)_{Low Side}

LRV = -100 inH2O

3. Calculate the URV.

URV = LRV + Span
URV = -100 + 100
URV = 0 inH2O

Therefore, transmitter range should be -100 to 0 inH2O

Source : Siemens Pressure Transmitter