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Smart or intelligent transmitters are now the industrial standard for almost all pressure and flow measurement. These transmitters are required to be calibrated regularly to guarantee that they deliver the set measurement objective. What are the recommended calibration practice for these type of transmitters?
Calibration Cycle of a Typical Process Transmitter
Right from when the smart transmitter leaves the factory floor to the plant environment, below is illustrated the typical calibration cycle when applying the transmitters for pressure and flow measurement.
Factory Calibration
Factory calibration is usually done at the factory as part of standard industrial
practice and to conform to relevant standards and codes. A transmitter has an inherent factory Sensor characterization curve similar to the one below:
This characterization curve defines the sensor input and output relationship that can be adjusted any time to ensure the sensor continuous to perform according to its defined specifications.
Installation Zero Trim
After the installation of a transmitter – pressure or flow, a zero trim calibration procedure must be performed. The zero trim procedure eliminates any effect on transmitter performance due to the instrument’s mounting orientation.
To remove zero errors due to process static line pressure, there may be need to carry out the zero trim procedure at line pressure however all necessary safety precautions needed to work on a live line must be imbibed.
In-use or Service Calibration
When a smart transmitter is already in use or installed in a plant there are three common calibration practice recommended:
1. Zero Trim
2. Sensor Trim
3. Analog Output Trim
Perform a Zero Trim After Transmitter Has Been in Use
After the transmitter has been in use for sometime, vibration and other mounting problem may have taken their toll on the transmitter performance. A zero trim procedure should then be performed to correct any zero error observed. The zero trim is a single-point adjustment. It is useful for compensating for mounting position effects. Where a multivariable mass flow transmitter is being used to measure flow, the zero trim should be done on the differential pressure sensor as well as the static pressure sensor.
Sensor Trim Procedure
First verify the calibration of the transmitter sensor at the upper and lower trim points using an external pressure source. In the case of a multivariable transmitter with a temperature input, use a decade box for the temperature sensor verification. Sensor trim should be done on the differential pressure sensor, static pressure sensor as well as the temperature sensor where a multivariable flow transmitter is being used. If the sensors outputs are within acceptable specifications, no sensor trim procedure are required, however if any of the sensors output are outside the acceptable specifications, a sensor trim procedure should be performed on the affected sensor.
Analog Output Trim
This process adjusts the transmitter’s current output at the 4 and 20 mA points. Verify the analog output of the transmitter by doing a loop test. The test requires an accurate current meter and is used to match the transmitter’s analog output to the loop readout meter. If the anlog output of the transmitter corresponds with the correct values (4m A and 20m A), then no output trim procedure is required. If the analog output of the transmitter does not correspond with the correct value, carry out an analog output trim on the transmitter.
