Pressure Measurement in Q&A-(Part 3)

Reviewing questions in part (1) which is covering famous techniques for pressure measurement, and part (2) which is covering the most important accessories used with pressure instruments. Then and we could continue with this last part ..

The following questions will cover the discrete pressure instruments ..

Q.15 Quick (True/False) questions:

1. In industrial engineering .. Discrete refers to true or false conditions. (True/False)
2. "Normal" status for Pressure Switch is the status of its electrical contacts during "Physical Stimulation" condition. (True/False)
3. Pressure switches often use  "diaphragms" or "bellows" as a pressure-sensing element, the motion of which actuates one or more switch contacts. (True/False)

Answers:

1. True
2. False

"Normal" status for Pressure Switch is the status of its electrical contact during "No Physical Stimulation" condition .. Or simply the switch is "in rest"

We have here the pressure switch symbols with description for Normal Open (NO) and Normal Close (NC) cases:

3. True

Another design also using "Bourdon tube" as the pressure-sensing element. It's brand famous name is "Mercoid" as it uses the mercury drop to be tilted with bourdon tube to connect/disconnect the electric contacts - as per below:

 Q.16 What is pressure switch deadband?

 It is a pressure gap (also called differential pressure) between the "set pressure" and the "pressure" value required to re-set the switch to its normal state after it has tripped.

 For example, a high-pressure switch with a trip point of 67 PSI (state changes at 67 PSI, increasing) that re-sets back to its normal state at a pressure of 63 PSI decreasing - Its “deadband” or "pressure gap" or "differential pressure" is 4 PSI (67 PSI − 63 PSI = 4 PSI).

The below graph indicates it all:

Not to confuse between "differential pressure" adjustment for a pressure switch and "differential pressure switch" .. The second is a separate/different device used to switch on/off at a specific differential value between 2 pressure inputs (High and Low) - as illustrated below:

Now switching to the questions that cover "calibration" for pressure instruments

First, we need to get familiarized with some terminology .. So let's have the below question:

Q17. Consider the following case:

Operation team in an industrial facility requested a PT to be installed on a process vessel with a measurement range of 50 PSI to 90 PSI.

At the warehouse, there was a PT with a range of zero to 1000 PSI, and a turndown ratio of 20:1.

Answer the following:

a. PT is to be calibrated for URV of 1000 PSI. (True/False)

b. PT is to be calibrated for URV of 90 PSI. (True/False)

c. PT is to be calibrated for LRV of 50 PSI. (True/False)

d. PT is to be calibrated for LRV of 0 PSI. (True/False)

e. PT zero is 0 PSI. (True/False)

f. PT span will be 40 PSI. (True/False)

g. PT found in the warehouse is valid to be installed on the process vessel. (True/False)

 Answer:

b, c and f are correct, while a, d, e and g are incorrect.

PT has a manufactured range of 0 - 1000 PSI and will be calibrated from 50 - 90 PSI, so its LRV (Lower Range Value) will be 50 PSI and URV (Upper Range Value) will be 90 PSI.(b, c correct and a, d is incorrect)

LRV = Zero of the PT (So e is incorrect)

Span is the calibrated range width which equal (URV - LRV) = 90 - 50 = 40 PSI. (f is correct)

Turndown Ratio is (Maximum Span):(Minimum Span) = 20:1

Maximum Span for this PT is 1000 (its full fabricated range) .. So getting its Min. Span by dividing 1000 by 20 = 50 PSI.

So the Min. span for this PT is 50 PSI, and this way it will not be valid for our request which is (40 PSI) .. and g is not correct.

Q18. What is the difference between "instrument calibration" and "instrument ranging"?

"Calibrate an instrument" means to check and adjust (if necessary) its response .. So the output accurately corresponds to its input throughout a specified range.

Instrument is to be exposed to input stimulus of a precisely known quantity.

For a pressure instrument (gauge, transmitter or switch), it would be subjected to known fluid pressures and comparing the instrument response against those known pressure quantities.

"Range an instrument" means to set the lower and upper range values so it responds sensitively to changes in input.

For example, a pressure transmitter set to a range of 0 to 200 PSI (0 PSI = 4 mA output ; 200 PSI = 20 mA output) could be re-ranged to respond on a scale of 0 to 150 PSI (0 PSI = 4 mA ; 150 PSI = 20 mA).

 In classic analogue instruments, re-ranging could (usually) only be accomplished by re-calibration.

In digital instruments, calibration and ranging are typically separate adjustments (i.e. it is possible to re-range a digital transmitter without having to perform a complete recalibration).

Here is a good video including re-ranging of smart PT and then testing it with a pressure pump

Q19. What is the "Digital Trimming" of a Pressure Transmitter?

 Suppose we have a smart PT ranged for 0 to 100 PSI. This PT sensor is fatigued from years of use .. such that an actual applied pressure of 100 PSI generates a signal that the analogue-to-digital converter interprets as only 96 PSI - As per the following figure:

"Digital trim" is a mean to calibrate both ADC and DAC circuits, to ensure the microprocessor “sees” the correct representation of the applied stimulus and to ensure the microprocessor’s output signal gets accurately converted into a DC current, respectively.

 The following video sum it all up

Q20. What is the "Damping Adjustment" for Pressure Transmitter?

 "Damping" feature of PT is simply enabling a "low-pass filter" of the pressure transmitter in order to reduce the process noise which is reported by this transmitter .. This is achieved through the internal configuring of the PT itself.

During PT calibration Damping should be set to minimum.

Also, excessive damping could cause the PT to "Lie/Trick" the control system .. So "Damping" feature should be used carefully.

 Q21. What are the famous calibrators for pressure instruments?

1. Deadweight Tester

It is a simple tester with no electronic stuff and usually used for gauges testing in the workshop.

Here is a quick video about using it

2. Hand Pump with built-in electronic "test gauge" which is almost used for field/workshop calibration.

Coming to the end of this series and hope it is useful ..