Pressure calibrations entail more than simply recording the pressure number and comparing it to a recognized standard. Because pressure measurements are affected by several factors such as temperature, local gravity, pressure medium characteristics, and local adjustments, it is critical to account for these external factors and corrections when performing high precision pressure calibrations.
There is a wide range of calibration software on the market that automatically adds these corrections to the recorded pressure values from the reference and DUT. However, understanding the sources of deviation and errors in the calibration setup can be aided by being aware of these adjustments.
Site specific adjustments
Specific modifications are made to the recorded value to account for characteristics that impact the pressure output in respect to the location of the site and the ambient environment. These modifications may include sea level adjustments, local gravity adjustments, and temperature variations in the surroundings.
Typically, sea level adjustments are done to account for variations in height and barometric pressure. To compensate for gravitational forces on masses, local gravity corrections are utilized. Temperature compensation is used to capture the correct reference device output as the ambient temperature changes.
- Sea Level
This adjustment is critical for absolute ranges, especially barometric pressure ranges. Regardless of height, this adjustment gives a common barometric reference. Because all of the barometers are calibrated to sea level, this makes it easy for meteorologists to track weather fronts.
As the altitude of an absolute sensor increases, it approaches absolute zero. However, this might cause issues with a barometric range sensor because the reading will no longer be 14.5 psi when vented to atmosphere. The local barometric pressure may instead be 12.0 psi. This is not the case when a sea level adjustment is applied. The present barometric pressure in Denver, Colorado, for example, will be closer to 14.5 psi than 12.0 psi. This is due to the sea level adjustment done to the barometer sensor.
- Local Gravity
A correction for local gravity is the final site-specific adjustment and perhaps the biggest contribution to inaccuracies, particularly in piston-gauge systems. Gravity is the acceleration that allows mass sets to exert force on the piston area, resulting in pressure. The gravity of the Earth varies over its whole surface, with the lowest gravity acceleration being around 9.7639 m/s2 and the maximum gravity acceleration being roughly 9.8337 m/s2.
The local gravity may be used during the pressure calculation for a piston gauge, and a gravity adjustment may not be required. Many industrial deadweight tests, on the other hand, are calibrated to standard gravity (9.80665 m/s2) and must be rectified.
- Temperature
Temperature fluctuations are another source of mistake in pressure calibrations. While pressure transducers, such as the CPT9000 Premium Pressure Transducer, are temperature adjusted during production, not all transducers are. Manufacturers of such transducers establish the connection of the pressure output’s accuracy to temperature rise or drop through applicable temperature standards.
Temperature adjustment is especially necessary for reference standards requiring temperature monitoring, such as piston gauges. Piston-cylinder systems, regardless of composition (steel, tungsten carbide, etc.), must be temperature corrected during operation since all materials expand or contract in response to temperature changes based on their thermal expansion coefficient.
As the temperature of the piston cylinder rises, the piston-cylinder system expands, increasing the area and decreasing the pressure generated. As the temperature drops, the piston-cylinder system contracts, causing the area to shrink. As a result, the pressure created rises. This modification will be delivered immediately to the piston’s region.
Media related adjustments
Media-related changes may be irrelevant to regular users of pressure controllers or gauges. However, media-related changes are required for primary standards since they affect the desired target specification and associated uncertainty.
- Air Buoyancy
Air buoyancy is one of the most critical modifications that must be made to piston-cylinder systems.
The pressure generated by the air surrounding us acts as a column of air. It also exerts an upward pull on things at the same time. If this modification is not made, the displayed value may be incorrect. Any mass, including the piston, will require what is known as an air buoyancy adjustment.
This adjustment is only required for gauge calibrations in which the reference is exposed to ambient air (atmospheric reference). It is insignificant for absolute since the ambient air is effectively eliminated by using vacuum as a reference.
- Surface Tension
When using oil-lubricated piston-cylinder systems, the surface tension of the fluid must be overcome in order to “free” the piston. Essentially, depending on the diameter of the piston, this results in an extra “phantom” mass load. As the piston’s diameter rises, so does the effect.
This adjustment is more critical at lower pressures and becomes less relevant as pressure increases.
Device specific adjustments
Device-specific modifications, such as head height and distortion correction (piston-cylinders only), are required for precision devices where slight changes in measurement might contribute to inaccuracies that effect the calibration’s total uncertainty.
- Distortion
A distortion adjustment is a comparable correction that must be performed to piston-cylinder systems. As the pressure on the piston-cylinder system grows, the piston area increases, forcing it to create less pressure.
As the pressure grows, the piston area expands, resulting in less pressure than intended. The distortion coefficient is normally supplied by the manufacturer, although it may also be measured experimentally.
- Head Height
If the reference standard is a pressure controller, the only correction that may be required is a head height correction. This adjustment cancels out the discrepancies in height / placement of the sensing elements in the reference relative to the DUT. If the DUT is below the reference level, the value will be positive; if the DUT is above the reference level, the value will be negative. A head height correction must be determined regardless of the pressure medium and dependent on the precision and resolution of the DUT.
Mensor pressure controllers, such as the CPC6050, Modular Pressure Controller, allow the operator to enter a head height and the instrument will compute the corrective head height.
Conclusion
Being aware of and compensating for the elements that influence the output of your pressure calibration may make a substantial difference in the total uncertainty of a calibration. While some of these adjustments, such as local gravity and sea level, will remain constant between calibrations, others will change dramatically depending on the reference device, pressure range, and even the equipment under test.
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