Deadweight Testers and Digital Pressure Calibrators
Primary vs. Secondary Standards
Deadweight testers are primary standards. This is because they are based solely upon physical parameters and the pressure measurement is not translated into an electronic or analog signal. Since they are a physical standard, they can be made to cover wide pressure ranges using different masses and effective area components. Conversely, these units are bulkier and much heavier than most digital standards. They are often more difficult to set up and require more training to become an efficient user than would be the case for a digital pressure calibration system. However, because of the stable, regulated output, technicians can become proficient in the use of these testers and can complete a calibration in a very reasonable time.
Digital pressure gauges are secondary standards, because the pressure is translated into an electronic signal using a transducer. They may also be considered as transfer standards, as they are used on site and then checked against a primary standard on a regular basis. Unlike deadweight testers, digital standards have limited ranges due to the sensors used in their construction. Multiple units may be necessary to cover large pressure ranges. Even if multiple units are needed, the overall size and weight of the digital system will typically be less than that of a deadweight tester. Because they do not generate a pressure, some consideration needs to be given to the portability of the pressure source.
If the pressure source is a handpump or jack pump, care must be taken to ensure a stable pressure is applied to the instrument under test and to the reference standard. Additionally, training is required such that the technician understands system indications such as a temperature change, adiabatic effects, and entrained air are identifiable and understood.
Cost of Ownership
One final consideration when evaluating the deadweight testers versus digital gauge is the overall cost of ownership and long term monetary benefit. Deadweight testers typically cost more to purchase than a digital pressure system. In addition to the initial purchase cost, the calibration cost for a deadweight tester is typically more than a digital standard. However, in general, deadweight testers will last longer than digital devices, and their higher accuracy may result in smaller errors throughout a system.
Conclusion
When considering a change from one technology to the other, all factors should be considered, including a proper support and training program for the technicians and understanding the complete accuracy/uncertainty specifications for the units. A digital to deadweight change would increase stability and accuracy, and cover a wider pressure range. However, corrections for gravity and temperature must be applied. A deadweight to digital change would increase portability, reduce purchasing and operating costs, add the ability to read directly in multiple pressure units, remove requirement to adjust for gravity, may be fully temperature compensated, and will include useful functions and features. However, they are not as accurate, may not last as long, and are not primary standards.
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