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Procedure to Calculate Accuracy of Pressure Transmitter Discussed

Brian Craig

April 30, 2021

The accuracy of pressure transmitters plays a crucial role in-process monitoring and control across industries. Since pressure is an essential parameter of industrial processes, continuous and accurate monitoring of pressure is of utmost importance. Slightly incorrect measurement in value of pressure may result in pressure shoot-up, pressure drops, and eventually abrupt damage to industrial systems. Needless to say, such incidents pose hazards to operators. In high-pressure hydraulic and pneumatic systems, this can lead to fatal accidents. This is why accuracy of pressure transmitters cannot be jeopardized. Thus, calculation of accuracy of pressure transmitters should be done while selecting and troubleshooting the equipment. This post discusses the process and considerations of calculating accuracy of transmitters.

An Overview of Accuracy of Transmitters

Accuracy of a transmitter is a measure of error that occurs during the measurement of pressure at zero point and measurement of pressure at variable points. If this concept is graphically presented, the measurement at zero point would display a gradually increasing linear graph. However, for real-time pressure measurement at variable points, the graph would represent a slightly curved nature that does not follow linearity. The average deflection between the two graphs is identified as the accuracy of the transmitter.
 
Note: The deflection/error between two curves cannot be taken as a precise value of accuracy. It is affected by various factors which are to be taken into consideration during calculations.

Often, the accuracy of transmitters is misunderstood as non-linearity. However, non-linearity is just one of the parameters that affect accuracy.

Let us discuss all the parameters that contribute to the calculation of accuracy of pressure transmitters.

Parameters of Accuracy to Consider While Calculating Accuracy

Ideally, two types of errors are needed to calculate the accuracy of transmitters. The variable parameters of transmitter accuracy are discussed below.
  • Non-Linearity: Non-linearity is the primary error that contributes to the calculation of accuracy. This error simply indicates the deflection between the zero span graph and actual measurement graph. Non-linearity can be calculated by using two methods as follows.
  • End Point Method/ Terminal Method: In this method, the maximum deflection between graphs at zero point measurement and full-scale measurement is considered non-linearity.
  • Five Point Method/Best Fit Straight Line (BFSL) Method: In this method, the curve of actual values is checked for deflection against zero span linear graph at five specific points. Ideally, from zero value to full scale value, five values are selected such as 0% (zero span), 25%, 50% 75% and 100% (full scale). A best fit straight line is drawn through the five points (the line should pass through maximum points out of 5). The deflection between the zero point graph and BFSL graph is taken as non-linearity.

Out of both the above-mentioned methods, BFSL is considered more accurate for calculating non-linearity.
  • Hysteresis: Hysteresis is the fluctuation in two or more pressure readings at the same point. Hysteresis is also calculated at 5 calibration points of BFSL process. The mean value of hysteresis values is taken into the calculation of accuracy.
Although the further mentioned variable parameters make minimal impact on the accuracy, still, the slightest error may cost a lot when it comes to industrial process control. Therefore, the following points should be considered while calculating the accuracy.
  • Offset Error: Offset error is the difference between the zero point of zero span reading and zero point of actual reading.
  • Span Tolerance: The difference between the zero span and full-scale span is span tolerance.
  • Non-Repeatability: It is the measure of the difference in multiple readings taken at the same pressure. The difference in signal strength and readings is considered in non-repeatability.

The overall calculation of accuracy of pressure transmitter is the addition of all the above-mentioned parameters.

Now that accuracy calculations and parameters are discussed, you can consider those points while selecting transmitters. However, quality affects accuracy and performance therefore you should source transmitters from reliable suppliers like The Transmitter Shop. The company offers transmitters from reliable suppliers such as Rosemount pressure transmitter, Honeywell Pressure Transmitters, etc


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