Choosing a Trim Procedure Sensor Trim

Reference Manual

00809-0100-4027, Rev AA December 2005

Rosemount 4500

Choosing a Trim Procedure

Sensor Trim

To decide which trim procedure to use, you must first determine whether the analog-to-digital section or the digital-to-analog section of the transmitter electronics need calibration. Refer to Figure 4-1and perform the following procedure:

1.Connect a pressure source, a HART Communicator or AMS, and a digital readout device to the transmitter.

2.Establish communication between the transmitter and the HART Communicator.

3.Apply pressure equal to the upper range point pressure.

4.Compare the applied pressure to the Process Variable (PV) line on the HART Communicator On-line Menu or the Primary Variables screen in AMS.

a.If the PV reading does not match the applied pressure (with high-accuracy test equipment), perform a sensor trim.

5.Compare the Analog Output (AO) line, on the HART Communicator or AMS, to the digital readout device.

a.If the AO reading does not match the digital readout device (with high-accuracy test equipment), perform an output trim.

Trim the sensor using either full or zero trim functions. Trim functions vary in complexity and are application-dependent. Both trim functions alter the transmitter’s interpretation of the input signal.

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. Since this correction maintains the slope of the characterization curve, it should not be used in place of a full 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.

NOTE

Do not perform a zero trim on 4500 Absolute pressure transmitters. Zero trim is zero based, and absolute pressure transmitters reference absolute zero. To correct mounting position effects on a 4500 Absolute Pressure Transmitter, perform a low trim within the full sensor trim function. The low trim function provides a “zero” correction similar to the zero trim function, but it does not require zero-based input.

Full trim is a two-point sensor calibration where two end-point pressures are applied, and all output is linearized between them. Always adjust the low trim value first to establish the correct offset. Adjustment of the high trim value provides a slope correction to the characterization curve based on the low trim value. The factory-established characterization curve is not changed by this procedure. The trim values allow you to optimize performance over your specified measuring range at the calibration temperature.

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