In many process plants, keeping the operation running smoothly requires stringent calibration of hundreds of process control instruments. Consider all the different types of instrumentation that control vacuum and pressure, for instance. To maintain the highest level of accuracy and integrity, which has a direct correlation to quality of output, the pressure devices are calibrated as needed to ensure they are operating according to specifications.
Calibrating process instrumentation and documenting the results is vital to industries such as oil and gas, water, wastewater, chemical processing, pharmaceutical, food and beverage and even government scientific research labs.
Accuracy is vital
“Accuracy is vital for successful operation,” a process control technician maintaining a prominent U.S. government lab said recently. “Our neutron beam facility, which consists of the linear accelerator, cryogenic plant and target facility, is operated by various software and process controls.”
The facility has a variety of pressure devices to calibrate.
“We calibrate a number of pressure transducers, strain gauges, pressure switches and signal conditioning boards as part of new installations, preventive maintenance and troubleshooting,” the lab technician said.
The lab, like many other facilities, has calibrated those devices using a documenting process calibrator. That calibrator worked well but involved using hand pumps to generate pressure, then finely adjusting and compensating for leaks. All that adjusting, when multiple calibrations are considered, ate up a lot of technician time.
To be sure, instrumentation technicians for years have talked about challenges with proper pressure calibrations. Generating the pressure, for one, required hand pumping. Leaks, in particular, are a challenge, requiring both dependable equipment and connectors.
Coping with leaks
A leaking pressure source makes it difficult to keep the pressure stable at a calibration point long enough to take a reliable reading. Technicians try to fine-tune and adjust pressure applied from a pump when there are slow leaks. Even after the desired pressure setpoint is reached, it is recommended that systems settle for several seconds or even minutes prior to testing so a more accurate and repeatable test result can be performed.
Steps a technician can take to ensure a better calibration experience include:
- Test and debug pressure test systems before entering the field to reduce unnecessary trips back to the shop.
- Try to decrease the number of pressure connections by using the correct hose length and removing extra fittings.
- Ensure test equipment is properly mounted.
- Mitigate leaks by using special-purpose test hoses.
Miniaturized pump technology
Engineers have worked to solve many of these issues over the years. For example, recently an electric pump was developed that was small, powerful and accurate enough to fit inside a handheld calibrator.
This portable automatic pressure calibrator with an internal electrical pump allows the end user to type in a target pressure and the calibrator pumps to the desired setpoint. Adjustment controls automatically stabilize the pressure at the requested value.
The federal lab technician saw this calibrator demonstrated at a trade show and eventually became one of the initial users of the product. “It’s like a one-stop shop to accomplish our calibrations, versus having to carry multiple components like a hand pump and a vacuum pump,” he said.
He estimated he has reduced calibration time by about 40 percent. He said the calibrator also saves wear and tear on his hands because he no longer has to pump up the pressure.
Enter pressure calibration and go
“When we check our pressure transducers we have to pump it up as high as 300 psi. I just enter the pressure required and the calibrator automatically pumps up to that level.”
Additional capabilities of the calibrator include the ability to measure, source and simulate 4-20 mA signals. The technician said he could take voltage readings on pressure transmitter signal conditioners at the same time as measuring pressure.
Each incremental unit of time savings adds up with hundreds of control devices that need calibrating to support dozens of research projects running at a time in the facility. “We don’t have a calibration production quota, we just calibrate the instruments as needed; but the [automatic pressure calibrator] definitely allows us to do more in a day than we would normally be able to do,” the technician said.
Fluke-729 Automatic Pressure calibrator update
Two years from introduction, the Fluke-729 Automatic pressure calibrator has been very popular with pressure test professionals in virtually all the process industries. The ability to automatically test pressure transmitters and automatically document calibration results has been a game changer. As always, these users have given Fluke feedback and have requested performance enhancements.
Fluke has been listening to these customers. Many were asking for better accuracy and resolution for low pressure automatic testing. Fluke investigated this need and shipping today is an updated version of the product with new on-board software that enables pressure control based on reading of an external pressure module as a measurement reference.
With an external 750P series pressure module (50 ranges) with a lower range than the internal sensor in the 729, the calibrator will use the external module reading for the AS FOUND / AS LEFT test result while controlling the pressure automatically with the internal pump. This upgrade enables pressure control and enhanced accuracies down to 10% of the 729’s internal range. This results in pressure measurement accuracy improvements of up to 10 times for low pressure calibrations.
About the Author
Jim Shields is a product manager for the Process Calibration group for the Fluke Corporation. He has worked in the field of electrical, temperature and pressure measurement for more than 25 years.