Fluke and Transcat Present How to Calibrate a Pressure Gauge using a Pressure Comparator or Pressure Calibrator Webinar

1. How to calibrate a pressure gauge using a
Pressure Comparator or Pressure Calibrator
Presenter: Tim Francis, Fluke Calibration
Principles and practical tips about electrical, flow, pressure,
RF and temperature calibration

2. Agenda
• The Basics – The physics behind Pressure Measurement
– Pressure defined
– Laws of Physics
• The calibration process
– Steps to Include
– Technique
• Equipment Selection
– Calibration media…gas, oil, or water?
– Specifications…is this accurate enough
– Comparators versus deadweight testers
– Example Equipment

3. Pressure Definition
• Pressure (P) is the measure of force
(F) exerted over an area (A)
P = F/A
A
F
F
F
F
F

4. Hydrostatic Pressure
• When calibration Pressure Gauges, we are
specifically interested in Hydrostatic Pressure
– Hydro – The media is a fluid (gas or liquid)
– Static – The pressure is stable and not changing

5. • Calculating head pressure
– Absolute pressure head calculation.
– Where
• ρ fluid = density of the media
• gl = local gravity
• h = difference in height
Some Physical Principals
hgP lfluid  
TEST GAUGE
Reference

6. Does Head Pressure matter?
• When using liquid, 1 inch = approximately 0.3
psi
– If calibrating a 100 psi, 0.25% gauge, then 1 inch is
greater than the specification, so yes.
– If calibrating a 10,000 psi, 0.25% gauge, then 1
inch is one one-hundredth of the specification, so
no.
• When using gas, 1 inch = 0.0003% reading.
Small height differences normally don’t affect
the measurement

7. Some Physical Principals
• Pascal’s Principal
– In a fluid at rest in a closed container a pressure
change in one part is transmitted without loss to
every portion of the fluid and to the walls of the
container.
– This is what pressure metrologists depend on when
comparing instruments such as a reference and
UUT.
– The time it takes for Pascal’s principal to take affect
can be compromised with improper tubing size.

8. • Ideal gas law – R is the universal gas constant
– Pressure is absolute.
• Re-arranged for pressure – use as an
evaluation tool.
Some Physical Principals
PV nRT
V
nRT
P 

9. • As pressure increases, temperature also
increases
• When the pressure change stops, the
temperature slowly returns to ambient
• As the temperature decreases, the pressure
also decreases
• You chase a pressure leak that doesn’t exist.
Adiabatic Effects

10. The Calibration Process

11. What actions need to be considered when performing a
pressure calibration?
• Pre-test actions
– Clean the DUT
– Leak Test
– Exercise the DUT
• During the test
– Dwell Time
– Cardinal Point versus Nominal Point
– Dithering
• Post-test actions
– Clean the DUT
Calibration Process Considerations

12. Is it important to clean your test instruments?
Why? What are the consequences?
• Contaminate the reference
• Cause errors (fluid head and restrictions)
• Contaminate future test instruments.
But why is that bad?
• Contaminate the pressure medium
DUT Cleanliness

13. What are some ways to clean a test instrument?
• Disassemble the test...
Not always practical or possible. Could damage
test.
• Purge through the test...
Seldom possible. Difficult/risky to modify the
test.
• Fill and drain using a solvent...
Most logical choice but many different
techniques.
DUT Cleanliness

14. Leaks
• Leaks can potentially result in measurement
errors
• Can slow down the process
• To check: Go to the Device Under Test’s full scale
pressure. Wait for a period of time (adiabatic
effects), then measure the pressure drop over a
given period of time (~ 1 min)
• Acceptable leak rate will depend upon volume of
the system and accuracy requirements. No set
rule.
• 100% leak free systems are like unicorns

15. Exercising the DUT/Reference
• Some pressure measurement devices are impacted by
recent usage – they have a memory
• Always want your calibration process to mimic the actual
usage of the device
• Applying pressure causes the elastic element of the gauge
to behave like in normal usage
• Cycle full scale pressure (commonly 3 times)

16. What actions need to be considered when performing a
pressure calibration?
• Pre-test actions
– Clean the DUT
– Leak Test
– Exercise the DUT
• During the test
– Dwell Time
– Cardinal Point versus Nominal Point
– Dithering
• Post-test actions
– Clean the DUT
Calibration Process Considerations

17. Dwell Time
• Change the pressure and wait…but for how long?
• Some devices respond faster than other devices
• Equipment may impact dwell time – restrictive tubing
may increase dwell time
• Wait long enough to insure that the pressure has
stabilized (adiabatic effects) and both the DUT and the
reference are properly reading the pressure
• Usually at least 30 seconds, may be longer

18. How stable is stable?
• Pressure is impacted by changes in volume,
temperature, and leaks. It’s difficult to get
perfectly stable.
• Must be sufficiently stable that the operator
can determine the pressure reading
• Resolution – Extra digits on the reference can
kill you

19. Cardinal Point Calibration
• Technique used to insure best resolution and take
out some of the operator influence.
• Consider case where you are calibrating a
traditional analog dial gauge using a digital
reference gauge as the reference standard.
• If the dial gauge pointer is between two lines, it
can be difficult to interpolate the pressure value
• Adjust the pressure until the analog dial gauge is
on the cardinal point and then read the output
off of the digital gauge.

20. Dithering
• Dithering – Tapping on a mechanical gauge
• Goal – Release any friction that could be holding
mechanical components in place and keeping it from
reading properly
• When should you do it? When it’s reasonable to assume
that the gauge is dithered during its normal operation
– If gauge is mounted behind thick glass behind a block wall, then
it isn’t dithered during usage, so it shouldn’t be dithered during
calibration

21. What actions need to be considered when performing a
pressure calibration?
• Pre-test actions
– Clean the DUT
– Leak Test
– Exercise the DUT
• During the test
– Dwell Time
– Cardinal Point versus Nominal Point
– Dithering
• Post-test actions
– Clean the DUT
Calibration Process Considerations

22. Equipment Selection

23. Choosing a Media
• Gas Media (Pneumatic)
– Normally used for lower pressures
• Cutoff may be as low as 300 psi
• Often used up to 1,000 – 3,000 psi
• May be used for higher pressures (up to 15,000 psi)
– Extra safety precautions required at higher pressures
– Generate pressure by increasing the number of molecules
in the system
• Compress ambient air
• Supply from a bottle
– “Clean” media, used for high pressures when
contamination with oil or water is not allowable

24. Choosing a media
• Oil Media (hydraulic)
– Normally used for higher pressures
• Head height error becomes an issue with low pressures
• Easier to generate high pressures
– Helps lubricate and extend the life of equipment
– Easy to generate higher pressures – just change
the volume in the system
– Purging the gas from the system
• Purging/Priming Pump
• Vacuum fill

25. Why use water?
• Pressure is too high for gas
• Oil contamination of the device under test is
not allowable
• Oil is messy, so water is often preferred, but
has its limitations
– Not a good lubricant
– Not all water is created equal – tap water is
normally not a good idea

26. Is this accurate enough
• Concept of Calibration – compare the output of
the test device against a known reference
standard
• Reference standard must be sufficiently accurate
– Sufficiently accurate can be hard to define
– Normally presented as a test ratio comparing the
uncertainty of the reference standard against the
uncertainty of the DUT
– Acceptable ratio is dependent upon application and
industry. Rule of thumb is 4:1, but that’s often just a
guideline

27. Apples to Apples
• It’s imperative that when comparing the uncertainty of
the reference standard and the DUT that you compare
“apples to apples”
• If the DUT is 0.1% FS and the standard is 0.02% FS:
– Nominally, that is a 5:1 ratio, but what if…
– The reference has a full scale of 1000 psi and the DUT has
a ratio of 100 psi
• Standard = +/- 0.2 psi
• DUT = +/- 0.1 psi
• Ratio is 0.5:1. not 5:1
– Always best to compare in actual pressure units, psi:psi

28. Pressure Generation

29. Pressure Generation/Control
The Ideal Gas Law:
PV = nRT
Volume

30. Pressure Generation/Control
The Ideal Gas Law:
PV = nRT
Number of Molecules
DUT
Gas In Exhaust

31. Pressure Generation/Control
The Ideal Gas Law:
PV = nRT
Temperature

32. Calibration System Components
• Ability to generate the pressure
– Pneumatic Systems
• Supply Bottle or pump to compress ambient air
• Ability to meter the pressure or fine adjust (variable volume)
• Ability to vent the pressure
– Hydraulic Systems
• Reservoir for liquid
• Method for purging gas and priming the system
• Screw Press or similar pump to generate pressure
• Method to fine tune the pressure
• Ability to measure the pressure (Reference)
– Digital Reference Gauge
– Floating Piston

33. Deadweight Tester versus Comparator
• Deadweight Tester
– Pressure measured by a floating piston with masses used
to apply a force
– Generates a stable pressure – sinking of piston offsets
adiabatic effects
– Can be very stable over time
– More accurate – inherently % reading, so can be used on a
wide range of pressures
– Impacted by many influences, including gravity
– Requires using heavy weights
– Difficult to do cardinal point technique

34. Deadweight Tester versus Comparator
• Pressure Comparator
– Uses a digital reference gauge or similar to measure
pressure
– Easy-to-use – Just read the display, no corrections to
apply
– Sufficient accuracy for most applications
– Not impacted by gravity
– No weights to carry around
– Pressure not as stable (adiabatic effects)
– Requires more routine calibration

35. Examples of Comparators
P5515 Hydraulic
Pressure Comparator
P5510 Pneumatic
Pressure Comparator
2700G Reference
Pressure Gauge

36. Conclusion
• Dial Gauges can be properly and efficiently
calibrated by:
– Following common techniques
– Using the right equipment for the pressure range
and application.

37. Special limited-time offer!
• Now is the perfect time to buy a Fluke Pressure Calibrator to calibrate your
sensors and transmitters—because you can receive one or even two FREE
2700G Series Reference Pressure Gauges to expand your pressure
calibration range, expand capacity, act as a check standard or provide
backup when equipment is in the field or out for calibration.

38. Questions or Comments?
Email the moderator, Nicole VanWert:
nvanwert@Transcat.com
Transcat: 800-800-5001
www.Transcat.com
For related product information, go to:
www.Transcat.com/Fluke