Calibrating a smart pressure transmitter involves comparing the device under test to a calibration standard. Typically, the calibration standard will be a process calibrator that can measure both the pressure input and the mA output. To make adjustments if the calibration is out of tolerance, you will also need a HART Communicator. Some process calibrators have this capability, but you will never get the complete functionality of a HART 475. And of course, you will need to generate pressure to simulate pressure throughout the instrument’s calibrated range. A hand pump is the most convenient tool for achieving this in the field.
HART Smart Pressure Transmitter Calibration Procedure for in the field.
- Isolate and vent the pressure transmitter via the block and vent manifold.
- Connect the Process Calibrator’s mA meter in series with the loop (this method ensures accuracy compared to using the test diode terminal).
- Connect the hand pump to the Process Calibrator’s pressure module and the pressure transmitter manifold’s vent port. Be sure to select a pressure module that closely matched the calibrated range of the instrument.
- Configure the process meter to read the measured pressure value and the mA output.
- Test the pressure transmitter’s calibration over five points upscale and downscale. 0% >25% >50%>75%>100%>75%>50%>25%>0% . Verify that the mA output is within the specified error tolerance (e.g +/- 1% or +/- 0.16mA).
- Record the calibration results ‘As Found’.
- If the transmitter’s calibration is within tolerance, there is no need to perform adjustments.
- If the transmitter’s calibration is outside of tolerance, first check the loop for any connection issues that may be affecting the loop current. Then, determine whether a digital-to-analog or sensor trim is required.
- Connect the HART communicator in parallel.
- If the pressure displayed doesn’t match the pressure applied, perform a sensor trim.
- If the mA output displayed doesn’t match the process meter’s reading, perform a digital-to-analog trim.
- After making the adjustments, check the pressure transmitter’s calibration again as per step 5 to confirm that it is within tolerance and record the calibration results ‘As Left’.
- Disconnect the calibration equipment, re-instate the loop, de-isolate, and return to service.
Calibrating a pressure transmitter on a bench.
In some scenarios, a bench calibration is required. For instance, if a new pressure transmitter is being commissioned. Or, if the pressure transmitter has a very low-pressure range that requires a controlled environment to achieve the desired accuracy. The procedure for calibrating a pressure transmitter on a bench is similar to calibration in the field. The main difference is that loop power will need to be supplied to the transmitter. This can be achieved using a workshop power supply. However, most process calibrators these days have loop power and measure functionality. A HART communicator is also required for setting the parameters up in the transmitter, such as LRV, URV, damping, tag number, etc. To effectively communicate with the transmitter, a 250-ohm resistor is required in the series with the loop. Sometimes tighter error tolerances are called for when calibrating new instruments on a bench.
How often do HART Smart Pressure Transmitters need to be calibrated?
Modern Smart Pressure Transmitters just don’t drift like they used to. But still, the process and ambient conditions such as temperature and vibration will influence the transmitter’s reliability and repeatability. Some manufacturers might recommend calibration frequencies, for example, Rosemount has a Technical Note on how to calculate pressure transmitter calibration intervals. But it’s up to the end user. Things to consider when determining the calibration interval:
- The criticality of the service.
- How harsh are the process and ambient conditions?
- Regulatory requirements of the industry.
- Does the transmitter form part of a safety system? If so, calibration intervals will be set to meet SIL requirements.
- Stability over time. If a pressure transmitter’s calibration history shows it to be very stable, calibration intervals can be extended.
- Commercial or contractual obligations, is the pressure transmitter being used in a custody transfer application?
Should I verify the readings back to the DCS/SCADA?
It’s good practice to verify each calibration point pack to the screen or display where the operator can view it from. This confirms that the whole loop is accurate and that there are no issues with the Analogue input card. It’s generally not required to record this. Keep in mind, we are verifying the calibration of the transmitter against a reference standard. Comparing a 4-20mA HART Smart Pressure Transmitter’s output on a HART 475 to the DCS or SCADA display is not calibration.
What other checks should I do whilst calibrating the pressure transmitter?
Perform some visual checks on the condition of the transmitter and its mounting, the isolation manifold, and impulse tubing. If you are working in a newly commissioned plant, it’s a good idea to check the transmitter’s parameters against your documentation for discrepancies. If discrepancies are found, do not make changes to the transmitter’s parameters to match the documentation without due diligence. Check first with engineering, Supervision, Technical Authorities, etc. to confirm. Changes could have been made during commissioning that was warranted, however, updates to the documentation could be lagging or redlines could have been lost during the handover process. If your pressure transmitter is installed in a hazardous area, you could nest your calibration scope with completing a hazardous area inspection check sheet.
Summary
This post presented a very basic procedure for calibrating a 4-20 mA HART smart pressure transmitter in the field. Different sites and industries are likely to have slight variations depending on their requirements and the calibration equipment available. The steps listed only consider the act of performing the calibration and it is assumed that safe systems of work are followed.