In the Field or in the Shop: Overcoming the Challenges in Pressure Calibration

Presenter: Tim Francis, Fluke Calibration

Principles and practical tips about electrical, flow, pressure,

RF and temperature calibration

Agenda

• The Vocabulary – Definition of some basic terms– Reference Mode – Gauge versus absolute versus

differential– Pressure Ranges – How high is high and how low is low?

• Challenges and how to overcome them– Safety Considerations– Environmental Impact– Contamination– The right process and equipment for the job

• Question and Answer©2016 Fluke Corporation.

Agenda

• The Vocabulary – Definition of some basic terms– Reference Mode – Gauge versus absolute versus

differential– Pressure Ranges – How high is high and how low is low?

• Challenges and how to overcome them– Safety Considerations– Environmental Impact– Contamination– The right process and equipment for the job

• Question and Answer©2016 Fluke Corporation.

Reference Modes• Pressure values are sometimes followed by the words

“gauge” or “absolute” (or possibly “psig” or “psia”)• What do these words mean and how do they affect the

calibration process?• Measurements are relative

– You don’t say, “I live 5 miles.” You say, “I live 5 miles from here” or “I live “5 miles from my work.” Everything has a starting point.

– Pressure is no different. Saying “The pressure is 5 psi” doesn’t say the whole story. More proper to say “The pressure is 5 psi above atmosphere.”

– Gauge, absolute, and differential are simply shorthand for this.

©2016 Fluke Corporation.

Gauge Mode• Referenced to atmospheric

pressure– 0 means that your test pressure

is the same as atmosphere– Most common reference mode– Can be both positive and

negative (partial vacuum)– Usually the easiest to measure

• Examples– Tire Pressure– Many Process Measurements

Gauge (+ or -)

Barometric

Zero pressure, no molecules©2016 Fluke Corporation.

Absolute Mode• Referenced to a perfect vacuum

– 0 means that it is a perfect vacuum (absolutely no molecules)

– Can only be positive numbers – You can’t have negative molecules in a vessel

– More difficult measurement – How do you re-zero the reference?

• Examples– Barometric pressure measurement– Airplane altitude measurement– Downhole tools

Gauge (+ or -)

Absolute

Barometric

Zero pressure, no molecules©2016 Fluke Corporation.

Differential Mode• Referenced to another pressure (line

pressure)– 0 means that there is no difference in the

two pressures– Gauge mode can be thought of as a

special case of differential mode.– Can be a difficult measurement to make.

Few calibrators support high line pressure.

– Can be both positive or negative (above or below line pressure)

• Examples– Flow measurements in a pipeline– Draft Range measurements – difference in

pressure between two rooms

Gauge (+ or -)

Absolute

Differential

Barometric

Zero pressure, no molecules©2016 Fluke Corporation.

How high is high and low is low?

• Pressure measurement covers a wide range, from vacuum to > 60,000 psi– Vacuum – Less than atmosphere– Draft Range – low differential/gauge pressures (< 2 psi)– Low Pressure – Normally pneumatic pressures (<= 300 psi)– Medium Pressure – Could be pneumatic or hydraulic (<= 2000

psi)– High Pressure – Almost always hydraulic (2000 psi to 20,000

psi)– Very High Pressure - > 20,000 psi

• General statements – no official rules©2016 Fluke Corporation.

Challenges created by Reference Mode

• I only have a gauge standard. Can I use it to calibrate an absolute device?

• The spec sheet says that the range is -15 to 30 psi, but I can’t get down to -15. Why not?

• Is it okay to calibrate a differential device as though it was a gauge mode device?

• What is sealed gauge and when can I use it?

©2016 Fluke Corporation.

Using a gauge standard to measure absolute mode

• There will be an offset equal to barometric pressure (~14.7 psi)

• The offset will change with barometric pressure – if there is a storm that causes barometric pressure to drop 0.4 psi, then your error will change by 0.4 psi.

• If working with high pressures, you may be able to estimate barometric pressure and add it to your standard reading.

• If working with lower pressures, you will need to measure barometric pressure and continuously adjust your standard reading to compensate.

©2016 Fluke Corporation.

-15 psi isn’t reachable

• At barometric pressure, 0 psi gauge = ~14.7 psi absolute.• -15 psi gauge = -0.3 psi absolute – NOT POSSIBLE• The lowest you can possibly go is -1 x barometric

pressure– Denver, Colorado = ~12.1 psi absolute– Honolulu, Hawaii (at the beach) = ~14.7 psi absolute– Mauna Kea, Hawaii = ~8.7 psi absolute

• Even at sea level, it may not be possible to get all the way to -14.7 psi – you have to get all the molecules out.

©2016 Fluke Corporation.

Calibrating High Line Differential

• Example: Pressure transmitter is used on a pipeline that has a pressure of ~1500 psi absolute. It’s measuring the drop in pressure across an orifice, ~30 psi.

• To calibrate it like it is used, we need to apply 1500 psi to one side of the transmitter and 1530 psi to the other side.

• Difficult to do, so we normally leave one side open to atmosphere and apply 30 psi to the other side.

• Is this acceptable? Depends on who you ask.©2016 Fluke Corporation.

Sealed Gauge• Special case that is a hybrid of gauge mode and absolute

mode• If the test port is open to atmosphere, it reads 0 (like gauge

mode)• If test port is open to atmosphere and barometric pressure

changes, then the measurement will change with it (like absolute mode)

• Similar to using an absolute mode device and subtracting a default barometric pressure value from it.

• Useful for high pressure applications where variations in barometric pressure

©2016 Fluke Corporation.

Agenda

• The Vocabulary – Definition of some basic terms– Reference Mode – Gauge versus absolute versus

differential– Pressure Ranges – How high is high and how low is low?

• Challenges and how to overcome them– Safety Considerations– Environmental Impact– Contamination– The right process and equipment for the job

• Question and Answer©2016 Fluke Corporation.

Safety Considerations

• A pressurized vessel has some inherent safety risks• Make sure all equipment is rated to the proper

pressures for the work being performed.– Some shops only stock pressure lines and fittings that are

rated to the highest pressure they work to. This works if the highest pressure is relatively low. Doesn’t work if the highest pressure is 10,000 psi or higher.

• Use the proper Personal Protection Equipment (PPE)• When working in the field, be cognizant of other

dangers around you.©2016 Fluke Corporation.

Impacts from the Environment• A pressure calibration can be impacted by the environment around it.• Temperature – Almost everything that measures pressure is also

impacted by temperature. Pressure Calibrator manufacturers will characterize and compensate, but they still perform best over a given temperature range.

• Gravity – Acceleration of gravity varies by location. A traditional deadweight tester is impacted by this.

• Head height – A large difference in elevation between the reference and the device under test will create an error– 3 feet in gas = ~0.01% reading– 3 feet in oil = ~ 10 psi

• Large drafts, temperature swings, or changes in barometric pressure can make it difficult to get pressure to stabilize

©2016 Fluke Corporation.

Contamination

• Dirt, debris, and liquids from the Device Under Test can cause problems:– Damage your calibration equipment– Cross contaminate other Devices Under

Test• How do we combat it?

– Clean the devices before we connect to them

– Make use of filters, traps, or separators

In the field or in the lab?

• Need to consider a few questions– Can the work be done safely in the field?– Does the environment allow for the calibration to be

done properly?• Is the temperature stable and within the allowed for

operating range of the standard?• If you are using a deadweight tester, do you know the local

gravity sufficiently?• Can you easily connect to the device? Or is it better to

calibrate a similar device in the shop and rotate it into the field?

©2016 Fluke Corporation.

The Right Equipment

• Portable versus Bench– Portable type instruments, like the Fluke

718 can be used on the bench, but…– They are designed for the field, so they

aren’t always ergonomic for the bench.©2016 Fluke Corporation.

• Make sure you have devices that cover the proper range and mode– Add pressure modules for absolute mode– Not always appropriate to calibrate low

pressure with a higher range instrument– Example: Fluke 3130 with 750P Pressure

Modules

Conclusion

Pressure calibrations can be challenging, but these challenges can be overcome with the right processes and the right equipment.

©2016 Fluke Corporation.

Special limited-time offer!

• Gift with purchase – up to $1200 value• +Go to www.transcat.com/deals for more

details

Questions or Comments?Email the moderator, Nicole VanWert:

nvanwert@Transcat.com

Transcat: 800-828-1470www.Transcat.com

For related product information, go to:

www.Transcat.com/Fluke