What is sensor trim in a smart transmitter?

In a conventional analog electronic process transmitter typically have only two calibration adjustments: one for zero and another for span, modern transmitter offers more components in need of adjustment.

Smart transmitter:

The aim of the analog-to-digital converter (ADC) is to transform the electrical output signal of the pressure sensor into a digital number understandable by the microprocessor.

The aim of the digital-to-analog converter (DAC) is likewise to transform the microprocessor’s digital output into a 4 to 20 mA DC current signal representing measured pressure. The ADC calibration technique is called a sensor trim, whereas the DAC calibration process is referred to as an output trim.

Explain the importance of implementing both a sensor trim and an output trim every time a “smart” transmitter is calibrated. In other words, explain why encoding LRV and URV values into the microprocessor (e.g. LRV = 0 PSI; URV = 30 PSI) is not necessary and announce the job done.

In addition, clarify which external calibration equipment has to be connected to the transmitter to complete a sensor trim procedure, and also which external calibration equipment has to be connected to complete an output trim procedure.

Sensor Trim and Output Trim

Simply setting the LRV and URV values does not necessarily calibrate the transmitter to precisely match truth.

Simply setting the LRV and URV values does not necessarily calibrate the transmitter to precisely match truth.

If this idea is difficult to grasp, consider a transmitter whose LRV and URV values are ideally fixed, and whose DAC is correctly tuned, but whose ADC suffers from a zero change. The microprocessor will “say” that the voltage is different from what it really is, resulting in an erroneous (zero-shifted) milliamp signal being emitted.

To conduct a sensor trim, you must attach a known pressure source (a normal) to the input port of the transmitter and compare the specific pressure to the microprocessor recorded pressure value.

Once the voltage is trimmed, you will attach a specific milli-ameter in series with the output current of the transmitter to equate the microprocessor’s expected current signal with the real current.

News Reporter
Instrumentation Engineer