Pressure Measurement & Control


Understanding accuracy specifications

May 2019 Pressure Measurement & Control

Pressure measurement device accuracies are commonly specified as percentage of full scale, or percentage of reading, and the difference is significant. If an accuracy statement simply names a percentage (0,1%), it is normally specifying a percentage of full scale.

Accuracy (or really inaccuracy) of pressure instruments is therefore specified in one of two ways:

• Accuracy as a percentage of full scale(% of FS).

• Accuracy as a percentage of reading (% of RD).

If an instrument has an accuracy specified as % of FS, the error will have a fixed value, no matter where the applied pressure is in the measuring sensor’s range. Take, for example, an instrument calibrated for a pressure range of 0-100 kPa, with a stated accuracy of 0,1% of FS. At an applied pressure of 100 kPa (full scale of the instrument) the error will be at most 0,1% of full scale, or ±0,1 kPa.

As the pressure moves down, the error will remain 0,1% of FS. Therefore, at half scale (50 kPa), that error becomes proportionally larger (0,2%). The further down from full scale, the further the accuracy decreases. At 10 kPa the error is 1% and at 1 kPa the error is 10%, which still relates to a ±0,1 kPa inaccuracy.

If an instrument has accuracy specified as 0,1% of RD, the error will always be the same percentage of the actual applied pressure. Using the same example as above of 0-100 kPa, but this time, with a stated accuracy of 0,1% RD, at 10 kPa the error is ±0.01 kPa i.e. still 0,1% accuracy, and 10 times more accurate than an instrument specified as % of FS.

Table 1.
Table 1.

In summary, as per Table 1, 0,1% of FS accuracy is ±0,1 kPa across the scale. With a turndown of 10:1, the % error becomes 10 times as large at the lower end.

In comparison, for 0,1% of RD, the accuracy remains 0,1%, no matter the turndown ratio. However, it is important to note that this cannot continue all the way to zero, as there is a physical limit to how well the sensor can perform.

The latest Schneider Electric IGP50S gauge pressure transmitter range – also available as absolute or differential variants – with FoxCal technology (11 pre-calibrated ranges imbedded in the sensor), are leaders with their stated accuracy of 0,025% of RD and very high stability of <±0,015% of URL (upper range limit) per year.

The 400:1 turn-down ratio, coupled with the built-in calibration curves, allows for a single sensor to cover most ranges of pressure measurement with a single transmitter, resulting in drastically reduced inventory, engineering and configuration time. The robust design includes a standard five-year warranty and SIL2 safety certification.

For more information contact Johan van Jaarsveldt, EOH, +27 87 803 9783, johan.vanjaarsveldt@eoh-pas.co.za, www.eoh-pas.co.za



Credit(s)



Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

Intrinsically safe electronic pressure gauges
January 2020, Instrotech , Pressure Measurement & Control
Instrotech now offers Keller’s range of intrinsically safe electronic pressure gauges for use in areas subject to gas explosion risks. The type approvals are compliant with the ATEX Explosion Protection ...

Read more...
Pressure sensors with touchpads
December 2019, Turck Banner , Pressure Measurement & Control
Turck has launched its new PS+ series pressure sensors, a modular range that enables access to reliable measuring instruments with intuitive operation. The sensors are easily commissioned and overhead ...

Read more...
Pressure measurement converter for harsh conditions
December 2019, Instrotech , Pressure Measurement & Control
Kobold’s PNK pressure measurement converter is used for the remote electrical transmission of pressure signals. For use in heavy machinery, engines, shipbuilding and automotive engineering, this robustly ...

Read more...
Why does water hammer affect pressure gauges and transmitters?
October 2019, SA Gauge , Pressure Measurement & Control
Water hammer is a high pressure surge or wave created by the kinetic energy of moving fluid when it is rapidly forced to stop or change direction, altering the liquid’s velocity. Rapid stopping or starting ...

Read more...
Tough OEM pressure switch from WIKA
October 2019, WIKA Instruments , Pressure Measurement & Control
The PSD-4-ECO pressure switch from WIKA is designed for harsh ambient conditions, and is optimised for OEM integration. With a diameter of 29 mm, the model is very slim and through the arrangement of ...

Read more...
Hygienic pressure transmitter
September 2019, ASSTech Process Electronics + Instrumentation , Pressure Measurement & Control
The pressure transmitter Jumo Taros S46 H – available locally from ASSTech – is designed according to the hygienic requirements for precision measurement and longevity. The high-level protection type ...

Read more...
Electronic pressure gauges for Ex areas
September 2019, Instrotech , Pressure Measurement & Control
Instrotech now offers Keller’s range of intrinsically safe electronic pressure gauges for use in areas subject to gas explosion risks. The type approvals are compliant with the ATEX Explosion Protection ...

Read more...
PS17 differential pressure ­transmitter
August 2019, Vepac Electronics , Pressure Measurement & Control
Differential pressure transmitters are used for regulating HVAC systems and for monitoring filters. The devices can also monitor under and overpressures in cleanrooms or mini-environments. halstrup-walcher, ...

Read more...
Foxboro buoyancy level transmitter
August 2019, I-PAS (Industrial Process Automation Solutions) , Level Measurement & Control
Principle of operation When an element a (displacer rod) is immersed into a liquid, according to Archimedes’ Principle, the displacer will ‘lose’ some of its weight proportional to the level of the liquid ...

Read more...
The dream team for heat and condensate
August 2019, VEGA Controls SA , Pressure Measurement & Control
An innovative duo of ceramic and metallic measuring cells from VEGA optimises pressure measurement in a distillation process at BASF.

Read more...