Four tips for calibrating a HART pressure transmitter
The fundamental purpose of a HART transmitter is to measure the input process signal and convert it into an accurate output signal and calibration is key.
Pressure transmitters are one of the most common instruments found in a process plant, and the most common type of smart pressure transmitter is a HART transmitter. Whether it is wired or wireless, the fundamental purpose of a HART transmitter is to measure the input process signal and convert it into an accurate output signal. Calibration is key to maintaining this accuracy, but what is the best way to do it? Let’s start with the basics.
Calibration is essential: Even the very best calibrators drift over time, becoming less and less accurate. To ensure that a HART transmitter is working correctly and accurately it needs to be regularly calibrated. To perform a calibration it is necessary to measure (or generate) an accurate input and then accurately measure the output, most often mA output. Then you repeat this process for several points across the whole range – for example at 25% steps. The input and output should be measured with an accurate and traceable reference standard or calibrator. It is also worth noting that wireless HART transmitters only transmit infrequently, so for calibration it is best to connect via the screw terminals rather than reading the wireless signal, otherwise it will be a very time-consuming process.
1. Configuration is not calibration: Changing settings on the HART transmitter using a communicating device that supports the HART protocol is known as configuration. Even though this can be done with a calibrator that supports HART communication, configuration does not ensure transmitter accuracy. It is also worth pointing out that a HART communicator cannot calibrate a HART transmitter.
2. Deal with errors: If you find an error when you calibrate, the HART transmitter needs to be trimmed (or adjusted) to measure accurately. Sometimes people think that if they only use the mA output, they only need to adjust the output/analog section. But because the input section and output section are in series, both should be adjusted if mA output is used. It is possible to just adjust one to compensate for the error in the other, but it is not recommended. It is also important to note that a HART communicator alone cannot trim a HART transmitter – you always need a reference standard (a calibrator) to make the measurements required for the trimming.
3. A calibrator (or mA meter) is needed to measure mA: Another common misconception is that a HART communicator can measure the transmitter’s mA output signal. In reality a communicator typically shows the AO value, which is the digital representation of the nominal mA value. This means it is not a true measured mA current signal, and does not show what the mA transmitter is really outputting. Even if the AO value in the HART communicator shows 4,000 mA, it does not mean the output current is actually 4,000 mA. This means you need an mA meter or a calibrator to measure the true mA output.
4. Watch out for damping: Many HART transmitters support damping, which adds a delay between a change in the transmitter input and when that change affects the input reading and corresponding output value. Damping needs to be set to zero before calibration, then returned to the required value when the process is finished.
To properly calibrate a HART transmitter a calibrator and a communicator are required – or a device which combines the two, such as the field calibrator which contains a fieldbus communicator for HART, as well as for Foundation Fieldbus and Profibus PA instruments. It offers calibration capabilities for pressure, temperature and various electrical signals and can be used with calibration software for a paperless calibration process.