The document provides an overview of high-pressure calibration techniques and considerations, defining pressure and its reference modes: gauge, absolute, and differential. It emphasizes the importance of understanding pressure stability, influenced by factors like the number of fluid molecules, volume, and temperature, while also addressing safety, contamination, and equipment selection for high-pressure measurements. Examples of equipment include the Fluke 700HPPK pneumatic pressure pump and the P3100 hydraulic deadweight testers.

1. Tips and Techniques for High-
Pressure Calibration

2. Agenda
• Basic vocabulary and the physics of pressure
• Special considerations when working with high pressure
• Equipment determination for high-pressure calibration

3. Definition of Pressure
• Pressure is a derived measurand. It uses the base
quantities, mass length and time.
P=F/A (Pounds per Square Inch)
• It is a state variable where the quantity values
describe the condition of a fluid much like
temperature.
• Most pressure measuring devices measure over a
range.
• Measurements are made in either some type of gas
or hydraulic fluid.

4. • No Strict Definition, it is a relative term – It depends on one’s
experience, the application and specific governing regulations
• For the scope of this discussion, we will consider pressures of
3000 psi (20 MPa) and above high pressure.
- Limit of many gas cylinders
- Common breakpoint for instrumentation
What is high pressure

5. Reference Mode
• 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.

6. 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

7. 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

8. 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

9. Head Pressure
• The weight of a column of fluid will generate a pressure
P = height X density X acceleration of gravity
• If the reference device and the device under test (DUT) are at different
vertical positions, the pressure at each of them will be different.
• With liquid, the density is constant with pressure, resulting in a constant
offset at all pressures
• With gas,
– the density increases with pressure (gas compresses). The error is a function of
the pressure.
– Gas is less dense than liquid, resulting in a smaller head height error
• Some calibrators will make the correction for you

10. PV=nRT
Where:
• P = Pressure
• V = Volume
• n = Number of fluid molecules (mass)
• T = Temperature
• R = Ideal Gas Constant
Understanding Pressure Stability

11. PV=nRT
Pressure is impacted by 3 things
• Number of fluid molecules
• Volume
• Temperature
Understanding Pressure Stability

12. Number of Molecules
• Keep leaks to a minimum
– Leads to instability
– Could possibly result in measurement
errors
• Method for generating pressure in a
gas system
– Push more molecules in the system to
increase the pressure
– Exhaust molecules out of the system to
decrease pressure

13. Volume Changes
• Inverse relationship – As volume
decreases, pressure increases
• As pressure increases, it pushes on
the inside of tubing and manifold
walls, causing the volume to
increase
– Causes the pressure to decrease
– May look like a leak
• Used to generate hydraulic
pressures using a “screw press” or
“variable volume”

14. Temperature
• In an enclosed system, temperature has the biggest impact on
pressure stability
• As pressure is increased, the temperature of the fluid also
increases.
• When the pressure set point is reached, the temperature
decreases, returning to ambient.
• This causes the pressure to decrease with it.
• Appearance of a leak, but the pressure drop will eventually
stabilize out

15. Considerations with High Pressure
• Safety
• Contamination
• Pressure Stability

16. Safety
• There are inherent danger with high pressure, but the risks can
be mitigated.
– Use Personal Protective Equipment (PPE)
– Ensure all pressure lines and fittings (and other components) are
properly rated for the application and are in proper working order
– Minimalize volumes where possible
– For highest pressures, consider using a liquid media

17. Contamination
• But there are drawbacks to using liquid as the media
• Leads to contamination of the DUT
• Liquid in the DUT can be different than the liquid in the
calibration system, contaminating the calibration system
• If contamination is a concern, a gas system might be preferable

18. Pressure Stability
• Pressure stability can be a bigger issue at higher pressures
• Temperature is more of an influence with liquids than gasses
• More expansion and contraction of pressure lines
• Can be minimized by using the appropriate equipment
– Gas devices will provide a more stable pressure
– Deadweight Testers provide stable pressures (the sinking piston acts
as a natural pressure regulator)

19. Equipment Determination
• Benchtop versus Portable
• Hydraulic versus Pneumatic
– Contamination Prevention
• Ease-of-use
– Physical effort
– Stability of pressure

20. Example – High Pressure Gas
• Fluke 700HPPK Pneumatic Pressure Pump
and Calibration Kit
• Can be combined with a Reference
Pressure Gauge
• Easy-to-use pump generates up to 3000
psi in 30 seconds
• Ideal for operation in the field, no
benchtop required

21. Example – High Pressure Liquid
• P3100 Hydraulic Deadweight Testers
• Dual Piston design covers a wide
pressure range in one instrument
• Floating piston provides a stable,
precise, and accurate pressure

22. Questions or Comments?
Email Nicole VanWert-Quinzi
nvanwert@Transcat.com
Transcat: 800-800-5001
www.Transcat.com
For related product information, go to:
www.Transcat.com/Fluke©2016 Fluke Calibration