Deciding what instrument to use for calibrating pressure measuring devices depends on the accuracy of the DUT. For devices that ascribe to the highest accuracy achievable, the reference standard used to calibrate it should also have the highest achievable accuracy.
Accuracy of DUTs can range widely but for devices with accuracy greater than 1-5% it may not even be necessary to calibrate. It is completely up to the application and the discretion of the user. Calibration may not be deemed necessary for devices used only as a visual "ballpark" indication and are not critical to any safety or process concern. These devices may be used as a visual estimate of the process pressures or limits being monitored. To calibrate or not is a decision left to the owner of the device.
More critical pressure measuring instruments may require periodic calibration because the application may require more precision in the process pressure being monitored or a tighter tolerance in a variable or a limit. In general, these process instruments might have an accuracy of 0.1 to 1.0% of full scale.
The Calibrator
Common sense says the device being used to calibrate another device should be more accurate than the device being calibrated. A long-standing rule of thumb in the calibration industry prescribes a 4 to 1 test uncertainty ratio (TUR) between the DUT accuracy and the reference standard accuracy. So, for instance, a 100 psi pressure transducer with an accuracy of 0.04% full scale (FS) would have to employ a reference standard with an accuracy of 0.01% FS for that range.
Knowing these basics will help determine the equipment that can deliver the accuracy necessary to achieve your calibration goals. There are several levels of calibration that may be encountered in a typical manufacturing or process facility, described below as laboratory, test bench, and field. In general, individual facility quality standards may define these differently.
Laboratory
Laboratory primary standard devices have the highest level of accuracy and will be the devices used to calibrate all other devices in your system. They could be deadweight testers, high accuracy piston gauges, or pressure controllers/calibrators. The accuracy of these devices typically range from about 0.001% (10 ppm) of reading to 0.01% of full scale and should be traceable to the SI units. Their required accuracy will be determined by what they are required to calibrate to maintain a 4:1 TUR. Adherence to the 4:1 rule can be relaxed but it must be reported on the calibration certificate. These laboratory devices are typically used in a controlled environment subject to the requirements of ISO 17025, which is the guideline for general requirements for the competence of testing and calibration laboratories. Laboratory test standards are typically the most expensive devices but are capable of calibration a large range of lower accuracy devices.
Test Bench
Test bench devices are used outside of the laboratory or in an instrument shop, and are typically used as a check or to calibrate pressure instruments taken from the field. They possess sufficient accuracy to calibrate lower accuracy field devices. These can be desktop units or panel mount instruments like controllers, indicators or even pressure transducers. These instruments are sometimes combined into a system that includes a vacuum and pressure source, an electrical measurement device and even a computer for indication and recording. The pressure transducers used in these instruments are periodically calibrated in the laboratory to certify their level of accuracy. To maintain an acceptable TUR with devices from the field, multiple ranges may be necessary or devices with multiple and interchangeable transducer ranges internally. The accuracy of these devices are typically from 0.01% FS to 0.05% FS and are lower cost than the higher accuracy instruments used in the laboratory.
Field
Field instruments are designed for portable use and typically have limited internal pressure generation and the capability to attach external higher pressure or vacuum sources. They may have multi-function capability for measuring pressure and electrical signals, data logging, built-in calibration procedures and programs to facilitate field calibration, plus certifications for use in hazardous areas. These multi-function instruments are designed to be self-contained to perform calibrations on site with minimal need for extraneous equipment. They typically have accuracy from 0.025% FS to 0.05% FS. Given the multi-function utility, these instruments are priced comparable to the instruments used on the bench and can also be utilized in a bench setting.
In general, what is used to calibrate your pressure instruments in your facility will be determined by your established quality and standard operating procedures. Starting from scratch will require an analysis of the cost given the range and accuracy of the pressure instruments that need to be calibrated.
