In pressure transmitter calibration, sensor trim can be performed using either sensor or zero trim functions. Both trim functions alter the transmitter’s interpretation of the input signal. Also analog output trim is required to calibrate the output section of the transmitter.
Zero Trim
Zero trim is a single-point adjustment. It is useful for compensating for mounting position effects and is most effective when performed with the transmitter installed in its final mounting position. Zero trim should not be used in place of a sensor trim over the full sensor range. When performing a zero trim,
ensure that the equalizing valve is open and all wet legs are filled to the correct levels.
ensure that the equalizing valve is open and all wet legs are filled to the correct levels.
Sensor Trim
Sensor trim is a two-point sensor calibration where two end-point pressures are applied, and all output is linearized between them. It allows the user to select low trim and high trim end point values to provide a digital calibration over the required measurement range. The low trim value is always adjusted first as it determines the offset or zero adjustment to the characterization curve by correcting all points along the curve equally.
For example, suppose a negative 150inH2O has been applied to a pressure transmitter with a highly accurate pressure source, and the process variable reading shows negative 152inH2O. If corrections are made using the low trim procedure, the characterization curve can be adjusted so that the digital process variable will match the input. The effect of this trim is that the entire characterization curve shifts the same amount at every point.
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| Low sensor trim in pressure transmitter calibration |
The high trim value determines the span or slope adjustment to the characterization curve based on the low trim value. These values should never be changed without a highly accurate pressure source. The graph below shows a high value trim correction based on the low trim adjustment made in the example above:
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| High sensor trim in pressure transmitter calibration |
As seen above, the high trim value has been set at 150in H2O. This has the effect of altering the slope of the characterization curve from the low point to the high point.
The trim values (low and high) allow the transmitter performance to be optimized over a specified measuring range at the calibration temperature.
Analog Output Trim
This process adjusts the transmitter’s current output at the 4 and 20 mA points to match the plant standards. The analog output trim procedure requires an accurate current meter and is used to match the transmitter’s analog output to the loop readout meter. It is essentially a calibration of the output electronics of the smart pressure transmitter.
