1. Pressure Measurement and Calibration for Process Instrumentation

2. Pressure units Pressure = Force / Area
SI units: Newton/square meter = Pa (Pascal)
kg/cm2 is based upon SI units, but Pa is the officially recognized unit.
British units: pounds/square inch = psi
Many units are based upon the pressure exerted by a column of liquid.
inHg, mmHg, inH2O, etc.
Converting between units is easily done by multiplying by an appropriate factor.

3. Ideal Gas Law
Combining Charles’ and Boyle’s Law results in the Ideal Gas Law
P=kT/V
Chemists often use PV=nRT
k (or nR) is a property of the gas
Example:
Volume smaller and/or
temperature higher will
increase pressure.
이상 기체 상태 방정식은 다음과 같은 식으로 표시된다.
PV = nRT (P : 압력 V : 부피 n : 기체의 몰수 R : 기체상수 T : 온도(K) )
이 식은, 기체의 법칙, 즉 다음과 같은 법칙을 일반화시킨 것이다.
보일의 법칙 : 온도가 일정하면 압력과 부피는 반비례한다.
샤를의 법칙 : 압력이 일정하면 부피는 온도에 비례한다.
보일-샤를의 법칙 : 부피는 압력에 반비례하고 온도에 비례한다.

4. Types of pressure measurement
Gage
Pressure relative to atmospheric pressure
“PSIG” indicates this type of pressure
Differential
Pressure difference
“PSID” indicates this type of pressure
Absolute
Pressure relative to a perfect vacuum
“PSIA” indicates this type of pressure

5. Definition of Pressures
Every pressure is the difference between two pressures
Pressure that is measured relative to vacuum is:
Absolute
Vacuum
Atmospheric Pressure

6. Definition of Pressures
Every pressure is the difference between two pressures
Pressure that is measured relative to atmosphere is:
Positive and Negative Gauge (Relative)
Vacuum
Atmospheric Pressure

7. Definition of Pressures
Every pressure is the difference between two pressures
The difference between two pressures, neither of which is necessarily vacuum or atmosphere is:
Differential
(Relative)
Vacuum
Atmosphere Pressure

8. Pressure measurement The manometer gage
Atmospheric
Pressure
Atmospheric
Pressure
The manometer gage
Under static conditions, both sides will be at equal heights.
Pascal’s Law

9. Gage pressure
Applied
Pressure
Atmospheric
Pressure
Apply pressure to one side of manometer, leave other side vented to atmosphere h h g * r = P
   ρ : 유체밀도      g : 중력가속도      h : 높이의 차

10. Differential pressure
(High)
Pressure
(Low)
Apply two pressures to both sides of the manometer
again... h h g * r =
D P

11. Absolute pressure Pressure relative to a vacuum
Barometers by definition measure absolute pressure
Vacuum
760 mm
Atmospheric
Pressure
Mercury

12. Pressure calibration basics
Pressure is the second most commonly measured PV.
Pressure controls used in virtually all process and manufacturing industries
Petrochemical, food and beverage, pharmaceutical, etc.
Environmental, motor and process pressures
Many different measurement technologies
Strain gauge
Capacitive
Piezoelectric
Deadweight balances

13. Pressure transmitter linear
Converts pressure to linear or square root responding mA or dc voltage
Millions in service, require service and calibration
“Pressure controlled current regulator” + –
Sensor Input
Gage
Absolute
Differential
24 V Loop Supply
Pressure Transmitter
3 to 15 PSI 7 6

14. Pressure transmitter, square root
Converts pressure to square root responding mA or dc voltage
The square of the input % of span produces that percent output
With 25 % of span applied, 50 % output + –
Sensor Input
Gage
Absolute
Differential
24 V Loop Supply
Pressure Transmitter
0-100 inH20 7 6

15. Analog pressure transmitters
Fixed ranged
Some turn-down and re-ranging via jumper settings
Adjustment manual, zero (4 mA) and span (20 mA)
Some include linearity and dampening adjustment
Signal conditioner technology, little or no digitization
Analog Transmitter
Zero
Span
Analog Electronics
Input Sensor
4-20 mA Output

16. Fluke 740B Series Documenting Process Calibrators
Pressure transmitter calibration using a portable calibrator: connection 49

17. Configure the calibrator
Fluke 740B Series Documenting Process Calibrators
Configure the calibrator
Set the correct source and measure functions.
The measure portion of the screen should be in mA.
The source portion of the screen should be in pressure.
Define the span parameters, acceptable tolerances and test strategy (example: 0 – 50 – 100 %) for the test. 49

18. Documenting the as found test
For each test step:
Capture the applied pressure.
Measure the actual mA value.
Calculate and document the error % of span for the measured mA.
Record the tag, serial number and operator for the as found result.

19. Adjusting to within tolerance, as left test
If the as found test passes within limits:
No adjustment required.
Return transmitter to service.
If the as found test fails:
Adjust the zero and span.
Run the as left test, confirm, document adjustments.

20. I to P Transmitters
I to P converts a 4-20 mA signal to a 3-15 PSI signal
Often used with control valves
Used as a bridge between 4-20 mA loop and 3-15 PSI pneumatic technology
Typically operate from a 20 PSI or greater pressure supply
“A current controlled pressure regulator”
Pressure Output
4-20 mA Current input
Supply
Pressure
~20 PSI

21. I to P calibration using a portable calibrator: connection

22. Configure the calibrator for the test
Select measure pressure and source mA.
Define the test parameters.
Document the results.

23. Adjust and as left test
Apply the 0 % pressure value and adjust the zero.
Apply the 100 % pressure value and adjust the span.
Verify 50 % linearity.
Run the as left test.
Document the results.

24. Smart transmitter calibration
Smart pressure transmitters have no physical zero and span adjustment.
Smart Transmitter
PV URL
PV LRL
Sensor Trim
Pressure Zero
Loop Test
Output Trim
Input
Sensor
Sensor
Input
Range Computation
Instrument Output
4-20 mA
Output PV
PVAO

25. Smart pressure transmitter
Three stages
Sensor block
Converts pressure signal to digital (A/D)
Range computation
Holds the programmed settings for zero (LRV) and span (URV)
Outputs PVAO digital mA output signal
Output section
Receives the PVAO command and output the mA signal (D/A)
Input and output section are individually calibrated to provide overall performance.
Sensor
Input
Range
Computation
Instrument
Output
4-20 mA PV
PVAO
Sensor Trim
PV URV
PV LRV
Loop Test
Output Trim
A HART instrument is more complex, having three distinct stages.

26. Example smart calibration
Almost identical to analog transmitter connections
HART cable connection required for digital adjustment.
Rosemount 275 or 375 can be used in place of Fluke 744 for digital adjustment (requires an analog calibrator).

27. Configure the calibrator
Acquire the test parameters (HART query).
Configure the input/output of the calibrator.
Define the test parameters.
Configure the measure and source function of the calibrator.
Define the performance test.
Acquire HART configuration.

28. Calibration, as found test
Document the pre-adjustment transmitter condition.

29. Calibration, digital adjustment
Perform HART trim on the transmitter.

30. As left test Verify post adjustment condition of transmitter.
If still out of tolerance (OOT)
Repeat adjustment.
Re-run as left until it passes within tolerance.
If it cannot be satisfactorily adjusted
Was the test tolerance too tight?
Were there leaks in the fittings?
Repair or replacement if necessary

31. Summary Pressure is sensed via a number of different methods.
Pressure indicators vary in functionality.
Many configured as switches
Pressure transmitters vary in range and type.
Analog and digital
Gage, differential, absolute
Analog transmitters are adjusted with a manual tool.
Smart transmitters need a digital adjustment tool.