The deadweight tester has long been the standard for pressure calibration.  However, advancements in technology have led to the development of digital pressure standards worthy of consideration in lieu of deadweight testers.  Understanding how to contrast the two technologies is key to selecting the appropriate solution.

                                         

          Type T Deadweight Tester                    GaugeCalHP Pressure Comparator & nVision Reference Recorder

 

Accuracy

Deadweight testers are systems that physically generate a known pressure.  They may also be used as gauges to accurately measure system pressure.  These devices do not require a display, as the combination of the masses is used to determining the output pressure.  They operate under the simple formula that pressure is equal to force applied over a known area.  The deadweight tester output is typically very accurate, even at its lower ranges.  Industrial deadweights are available with accuracies to ±0.0015% of reading.

By contrast, digital pressure standards must be combined with a pressure source to generate a known pressure.  Without the capability of producing pressure, the digital standards are technically gauges.  However, in the market, they may be called calibrators to distinguish them from the lower classes of digital indicators.  These digital devices are typically available in accuracies as a function of their full scale, such as ±0.050% of full scale (FS).  However, advancements in technology have led to some instruments specified as a function of the reading, like deadweight testers.  Accuracies are available as low as ±0.025% of reading.

Site Corrections

When comparing accuracy or uncertainty, an important factor to consider is site corrections.  Because deadweight testers are physical standards, they are subject to effects that digital standards are not.  One major effect is gravity.  The force of gravity on the masses of deadweight tester varies based on distance from the Equator and elevation.  For example, a deadweight using the same exact mass will generate a different pressure at Houston, TX than it does at Denver, CO.  The effect is substantial enough that it can alter the output to a value that is outside of the tolerance of the tester.  The user has two options to correct for this.  They can either have the unit calibrated to their local gravity, or to International Mean Gravity (980.665 gals) and then calculate a correction factor for the work site.  Digital standards are not affected by gravity, so such correction is not necessary.

A second site factor to consider is temperature.  While the temperature effect on a deadweight tester is not considerable, the additional error should be calculated and accounted for.  Many digital gauges and calibrators are subject to temperature effects, which may be significant.  The manufacturer’s specifications should offer this information, allowing users to calculate a total error for their local conditions.  Higher quality digital standards include temperature compensation so that there is no effect on the accuracy of the device.

Other Considerations

Digital devices will typically have other functions that are very beneficial on completing certain tasks.  These may include the ability to measure mA in a loop, source and measure the loop, and read temperature.  Firmware functions may include special modes for relieve/safety valve testing, peak measurement recording, scaling, error calculations, or data logging.  In addition to the onboard functions, manufacturers may include software with these devices to allow for automated recording of test results, generation of calibration records, or review and analysis of data.  Deadweight testers do not offer such additional function so additional equipment may be necessary to complete these tasks.

Additionally, digital pressure gauges will typically offer the capability to easily change engineering units (for example, psi, bar, kPa, “H20).  This is particularly useful in workshop or lab settings where various devices using different engineering units may be tested.  Because deadweight testers utilize specific masses to produce an output, those masses are dedicated to a specific engineering units and other mass sets are required to produce useable values of other engineering units.

White paper written by Ametek Sensors, Test & Calibration

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