Accuracy of Pressure

General George S. Patton once said that “Pressure makes diamonds.” From a forklift’s hydraulic pressure to the differential pressure of a diving submarine, in many industries pressure measurement is essential to operations and being able to do it accurately is paramount.

The accuracy of pressure classification was not really appreciated until the moment it could be rendered into quantifiable values. The complete pressure measurement system is constituted by the primary element that will be directly or indirectly contact with the process where the pressure changes occur and the second element (the pressure transmitter) that will translate this change into quantifiable values to be used on an indication, monitoring, and control.

Static performance or accuracy depends on how well the transmitter is calibrated and how long it can maintain its calibration. Many times accuracy is mistaken for precision, where accuracy is associated with the proximity of the value and precision is related to the dispersion of the resulting values of a series of measurements. The calibration of a pressure transmitter involves zero and span adjustment. Exactness normally includes non-linearity, hysteresis and repeatability effects.

Normally the accuracy is indicated in % of the calibrated span. Usually, the relation between a pressure transmitter input and output is predominantly linear (Y = ax + b), where a is known as gain and b is zero or offset, as explained in figure 1.

URL (Upper Range Limit): is the highest pressure at which the transmitter was set to measure, respected the sensor upper range limit.

LRL (Lower Range Limit): is the lowest pressure at which the transmitter was set to measure, respected the sensor lower range limit.

Span (Range Calibrado): the working range where the calibration is done is known as span, for example, from 500 to 3000 mmH2O, where the span is 3000-500 = 2500 mmH2O. The Span is equal to URL – LRL.

Zero Suppression (is the quantity with which the lower value surpasses the pressure zero value): the suppression occurs when the transmitter indicates a level above the real. On level measurements, where the transmitter is not installed at the same level than its high socket and there is the need to compensate the liquid column at the transmitter socket. This type of installation is required where the transmitter is at a lower level, which, in practice is the chosen way to facilitate access, visualization and maintenance. In this case, a liquid column is formed with the same height as the liquid inside the impulse socket and the transmitter will indicate a level above the real. This must be taken into consideration and is called Zero Suppression.