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

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

Pressure Definition• Pressure (P) is the measure of force

(F) exerted over an area (A)

P = F/A AF

F

F

F

F

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

• 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

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

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.

• 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

VnRTP

• 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

The Calibration Process

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

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

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

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

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)

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

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

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

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.

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

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

Equipment Selection

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

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

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

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

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

Pressure Generation

Pressure Generation/ControlThe Ideal Gas Law:

PV = nRT

Volume

Pressure Generation/ControlThe Ideal Gas Law:

PV = nRT

Number of Molecules

DUTGas In Exhaus

t

Pressure Generation/ControlThe Ideal Gas Law:

PV = nRT

Temperature

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

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

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

Examples of Comparators

P5515 Hydraulic Pressure Comparator

P5510 Pneumatic Pressure Comparator

2700G Reference Pressure Gauge

Conclusion

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

and application.

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.

Questions or Comments?Email the moderator, Nicole VanWert:

nvanwert@Transcat.com

Transcat: 800-800-5001www.Transcat.com

For related product information, go to:

www.Transcat.com/Fluke