Pressure gauges, sensors, transmitters and transducers, like all measuring tools, have a tendency to wear over time and become less accurate. Because accurate readings are a critical component of many industrial processes, a number of technologies have been developed to calibrate pressure devices.
Calibration is a comparison between two devices. The first device is the unit to be calibrated, often called the unit under test. The second device is the standard, which has a known accuracy. Using the standard as a guide, the unit under test is adjusted until both units display the same results while under the same pressure.
Not all standards are created equally. While all standards have a known accuracy, there are some—known as primary standards— that are sufficiently accurate such that they are not calibrated by or subordinate to other standards. Primary standards achieve their high accuracy by relying upon measurement technologies that do not drift such as length, mass or time. These fixed values remove uncertainty, making primary standards the most accurate calibration tools. Due to the high accuracy of primary standards, they are also used to calibrate other calibration equipment, known as working standards, that—while remaining highly accurate— use measurement technology with higher levels uncertainty and which are more prone to drift.
Traceability
To improve the quality of a calibration to levels acceptable to outside organizations, it is generally desirable for the calibration and subsequent measurements to be traceable to internationally recognized standards. Establishing traceability is accomplished by a formal comparison to a standard which is directly or indirectly related to national standards (such as NIST in the USA), international standards, or certified reference materials.
Types of Pressure Calibrators
Deadweight Testers
Deadweight testers use known, traceable weights and a precision machined piston and cylinder assembly to check the accuracy of pressure gauges, sensors, transmitters and transducers. The unit under test is hooked up and the weights, which correspond to specific pressures, are added to the top of the piston assembly while a pump generates pressure from the bottom ultimately creating a balance between the force made by the weights and the force made by the pressure. Once balance has been achieved the exact pressure is determined by the amount of weight used. That pressure is compared to the pressure shown on the unit under test and corrections can be made.
No other type of pressure calibration equipment can match the stability, repeatability and accuracy of deadweight testers. As they rely on weight to calculate pressure, they are considered to be primary standards and are accepted world-wide as the most accurate measurement of pressure.
Calibration Pumps and Pressure Sources
Calibration pumps and other pressure sources provide a means for side-by-side comparison of readings from both the unit under test and a traceable reference gauge working as the standard. Compatible with multifunction calibrators or used as standalone devices, calibration pumps include mounts for both gauges. As pressure is generated, the unit under test can be directly compared to the reference gauge and adjustments made as needed.
Rugged and portable, pressure pumps serve as an excellent means to spot check the accuracy of pressure gauges, sensors, transmitters and transducers in the field for applications in which ultimate accuracy is not required.
Multifunction Calibrators
Multifunction calibrators are the do-everything calibration instrument. Capable of accepting input from a wide range of sensors, many are also able to calibrate pressure equipment. Generally speaking, pressure calibration using a multifunction calibrator is similar to calibrating pressure with just a hand pump and reference gauge. The multifunction calibrator serves as the display and accompanying electronics while internal or external generation devices create pressure while pressure modules serve as the standard.
Despite the similarities, a multifunction calibrator is more sophisticated than a hand pump and offers features that make calibration more accurate. For example, multifunction calibrators often include internal sensors to compensate for ambient temperature and pressure thus mitigating two of the most common reasons for uncertainty in calibration.
Pressure Calibration Kits / Systems
Pressure calibration kits / systems are generally based around multifunction calibrators or calibration pumps and include all the items needed to perform pressure calibrations. Generally, when purchasing multifunction calibrators or calibration pumps that are not part of a system, they often do not include the fittings, hoses, reference gauges, etc. that are required. Kits and systems take all the guesswork out of matching components. All the items will be fully compatible and ready to perform calibrations.
Questions to Consider When Buying a Pressure Calibrator:
- What devices are to be calibrated?
- What level accuracy is required?
- What pressure range is required?
- Where will calibrations be performed? In a lab? The field?
- Do calibrations need to be traceable to NIST or another organization?
- What fitting, hoses, adaptors and other accessories are needed?
If you have any questions regarding pressure calibrators please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
