SHANGHAI SANZHOU AUTOMATION DASH CO.,LTD

In general, the pressure transmitter manufacturer is stating their accuracy as a total of their reference accuracy, ambient temperature effect on accuracy, voltage effect, etc. We must carefully calculate their ‘true’ or total accuracy based on information given in their catalogue rather than just use reference accuracy as their accuracy. The accuracy words itself is also ambiguously used by the manufacturer to present their equipment inaccuracy or uncertainty. Thus from now on we will call the accuracy as uncertainty.

How to calculate pressure transmitter uncertainty?

Basically we can express the total uncertainty of the equipment from its individual component uncertainty as below:

Ux = ((c1*Ux1)^2 + (c2*Ux2)^2 + … + (cn*Uxn)^2)^0.5

Where cn is a sensitivity coefficient and Uxn is a standard uncertainty of its individual component and Ux is a total standard uncertainty.

Let’s take an example of how to calculate the pressure transmitter uncertainty (inaccuracy) as below.

Pressure Transmitter Accuracy Requirement = 0.5% of Span

Ambient Temperature = 30 C

Process Operating Pressure = 200 psig

Selected Pressure Transmitter Range = 600 psig

Calibrated Range or Span (that will represent 4 – 20 mA output) = 0 to 300 psig.

The uncertainty statement from vendor catalogue:

Reference accuracy (including hysteresis, repeatability, linearity etc) = +/- 0.06% of Span (99% confidence level or 3 times standard uncertainty)

Ambient Temperature Effect per 30 C changes from calibration temperature which is 20 C = +/- 0.03% of Max Range + 0.15% of Span (99% confidence level or 3 times standard uncertainty)

Vibration Effect = +/- 0.05% of Max Range (99% confidence level or 3 times standard uncertainty)

Power Supply Effect = +/- 0.01% of Span per Volt (99% confidence level or 3 times standard uncertainty)

Then we can get the pressure transmitter uncertainty budget as below: