Re-calibrate the Foxboro DP Pneumatic Transmitter

Suppose this Foxboro model 13 pneumatic based DP transmitter has a calibrated range of 0 to 125 inches water column and an instrument air supply pressure of 20 PSI:

Foxboro Transmitter

Identify which way the range wheel would have to be moved in order to re-calibrate the transmitter to a new range of 0 to 180 inches water column (from 0 to 125 ”W.C.), explaining your reasoning.

Identify which way the zero screw would have to be turned in order to re-calibrate the transmitter to a new range of 15 to 140 inches water column (from 0 to 125 ”W.C.), explaining your reasoning.

Answer:

Move the range wheel up in order to re-calibrate from 0-125 ”W.C. to 0-180 ”W.C.

Turn the zero screw for less tension against the range bar (less force pulling the bottom of the range bar to the right) in order to re-calibrate from 0-125 ”W.C. to 15-140 ”W.C.

More Questions:

1. Explain why it is important we know whether this mechanism is motion- or force-balance to be able to correctly determine the effects these changes will have on calibration.

2. What would happen if the capsule were to develop a leak?

3. What would happen if the flexure connecting the tops of the force and range bars were to break in half, leaving those two bars disconnected from each other?

4. What would happen if the air supply pressure were to increase from 20 PSI to 22 PSI?

5. What would happen if the air supply pressure were to decrease from 20 PSI to 18 PSI?

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Credits: Tony R. Kuphaldt