Pressure calibrations are not just about recording the pressure value and comparing it against a known standard. As pressure measurements are dependent on many factors like temperature, local gravity, pressure media properties and local adjustments, it’s imperative to correct for these external factors and corrections when performing high accuracy pressure calibrations.
There is a variety of calibration software on the market that applies these corrections to the recorded pressure value from the reference and DUT automatically. However, being aware of these corrections can help understand sources of deviation and inaccuracies in the calibration setup.
Site specific adjustments
Site specific adjustments are applied to the recorded value to compensate for parameters that affect the pressure output in relation to the site’s location and ambient environment. These corrections can include sea level adjustment, local gravity adjustment, and environmental temperature changes.
Sea level adjustments are typically made to compensate for the variability in altitude and barometric pressure. Local gravity corrections are used to compensate for gravitational forces on masses. Temperature compensation is used to capture the proper reference device output with a changing environmental temperature.
This correction is important for absolute ranges, particularly barometric pressure ranges. This correction provides a common barometric reference regardless of elevation. This makes it easier for meteorologists to monitor weather fronts as all of the barometers are referenced to sea level.
For an absolute sensor, as the sensor increases its altitude, it will approach absolute zero. However, this can be problematic for a barometric range sensor as the reading will no longer be ~14.5 psi when vented to atmosphere. Instead, the local barometric pressure may read ~12.0 psi. However, when a sea level correction is made, this is not the case. For example, the current barometric pressure in Denver, Colorado, will actually be closer to ~14.5 psi and not ~12.0 psi. This is because the barometric sensor has a sea level correction applied to it.
Another source of error in pressure calibrations are changes in temperature. While pressure transducers, like the CPT9000 Premium Pressure Transducer, are compensated over a temperature range during manufacturing, not all transducers are temperature compensated. For such transducers, manufacturers specify the relationship of the pressure output’s accuracy to increase or decrease in temperature through relevant temperature specifications.
The temperature correction becomes particularly important for reference standards such as piston gauges, where the temperature must be monitored. Piston-cylinder systems, regardless of composition (steel, tungsten carbide, etc.), must be compensated for temperature during use as all materials either expand or contract depending on changes in temperature per their thermal expansion coefficient.
As the temperature of the piston cylinder increases, the piston-cylinder system expands, causing the area to increase, which causes the pressure generated to decrease. Conversely, as the temperature decreases, the piston-cylinder system contracts, causing the area to decrease. This causes the pressure generated to increase. This correction will be applied directly to the area of the piston.
The final site specific correction and arguably the largest contributor to errors, especially in piston-gauge systems, is a correction for local gravity. Gravity is the acceleration that enables the mass sets to exert a force on the piston area, leading to pressure. Earth’s gravity varies across its entire surface, with the lowest acceleration due to gravity being approximately 9.7639 m/s2 and the highest acceleration due to gravity being approximately 9.8337 m/s2.
During the pressure calculation for a piston gauge, the local gravity may be used and a gravity correction may not need to be applied. However, many industrial deadweight testers are calibrated to standard gravity (9.80665 m/s2) and must be corrected.
Media related adjustments
For everyday users of pressure controllers or gauges, media related adjustments may not be relevant. However, for primary standards, media related adjustments are a necessity as they impact the desired target specification and associated uncertainty.
A surface tension correction must be made with oil-lubricated, piston-cylinder systems as the surface tension of the fluid must be overcome to “free” the piston. Essentially, this causes an additional “phantom” mass load, depending on the diameter of the piston. The effect grows larger as the diameter of the piston increases.
This correction is more important at lower pressures as it becomes less with increasing pressure.
One of the most important corrections that must be made to piston-cylinder systems is air buoyancy.
The air surrounding us generates pressure acting as a column of air. At the same time, it also exerts an upward force on objects. If this correction is not applied, it can cause an error in the indicated value. Any mass, including the piston, will need to have what is referred to as an air buoyancy correction.
This correction is only necessary with gauge calibrations where the reference is exposed to ambient air (atmospheric reference). It is negligible for absolute as the ambient air is essentially removed with the use of vacuum as reference.
Device specific adjustments
Device specific adjustments like head height and distortion correction (piston–cylinders only) are necessary for precision devices where small differences in measurement could lead to errors that impact the overall uncertainty of the calibration.
If the reference standard is a pressure controller, the only correction that may need to be applied is what is referred to as a head height correction. This correction negates the differences in height / location of the sensing elements in the reference with respect to the DUT. Typically, if the DUT is below the reference level, the value will be positive, and vice versa if the DUT is above the reference level. Regardless of the pressure medium, depending on the accuracy and resolution of the DUT, a head height correction must be calculated.
Mensor pressure controllers like the CPC6050, Modular Pressure Controller allow the user to input a head height and the instrument will calculate the head height correction.
A similar correction that must be made to piston-cylinder systems is referred to as a distortion correction. As the pressure increases on the piston-cylinder system, it will cause the piston area to increase, causing it to effectively generate less pressure.
With increasing pressure, the piston area increases, generating less pressure than expected. The distortion coefficient is typically provided by the manufacturer, but it can also be experimentally determined.
Being mindful of the factors that impact the output of your pressure calibration and correcting for them could make a significant difference in the overall uncertainty of a calibration. While some of these corrections are stationary for all your calibrations like local gravity and sea level, some corrections will vary greatly between calibrations depending on the reference device, pressure range and even the device under test.