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.
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.
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