Computerized compressor speeds pressure-transducer calibration

Rochester, NY--"This is the only portable calibrator that automates pressure generation, control, and testing," says a beaming Dave Peterson, a mechanical engineer at Transmation, Inc. His Model 1292 APC(TM) (Automated Pressure Calibrator) sits beside him, whirring and beeping as it runs a complete exam on a commercial differential pressure transducer in under two minutes. Its speed, 0.05% resolution, 0.01% full-scale accuracy, and data-logging capabilities solve productivity, record-keeping, and maintenance problems in process industries and pipeline operations.

Those problems center around the tedium of calibrating pressure transducers with hand-powered pumps. For many designs, setting the zero position affects the full-scale span and vice versa, so just setting up tests can be frustrating. Because pressurizing air heats it, establishing stable measuring pressures takes time and patience. And many calibration protocols call for measurements at five or more points, trying the technician further. It's not surprising, then, that errors or fudging lead to suspect calibration data.

What the APC does is free the technician from that tedium. Its battery-powered stepper motor drives a compact, 1.5-lb reciprocating compressor. A feedback loop lets it monitor and adjust input pressure in real time. It stores "as found" transducer-output results, assists the technician with graphics and instructions during any required transducer tweaking, and performs a second calibration test, storing "as left" data. According to Peterson, it reduces the time for a typical calibration by an order of magnitude.

The APC project didn't start out to do all those things. Its original goal: A hand-held electric pump to generate and maintain reference pressures for manual calibrations. Physics intervened. "We just wanted to generate 100 psi into fixed volumes typical of industrial transducers," explains Bob Holmes, a senior development engineer who teamed with Peterson on the project. But calculations showed that generating that much compressed air, 10 or 12 times a day, required a battery of about 200 kJ capacity. The battery dictated that the unit would be portable, but not hand-held.

Resigned to the size of the APC, the teammates resolved to optimize the design within that envelope. For example, the original compressor delivered one-part-in-5,000 accuracy. By rebuilding the motor driver, the team boosted accuracy to one in 50,000. "It kept acquiring functionality as it evolved," explains Peterson.

The best example of that evolution is the unit's powerful software engineering. In the two-year design process, the APC went from being a simple compressor to a stand-alone transducer-analysis tool.

"We decided we could do data-logging easily and got the okay for that," Holmes comments. Complete documentation from approximately 40 calibration tests could then be downloaded through a serial port to a printer or PC. With more memory, the APC could store a dozen or more test regimes written for a customer's specific equipment. Calibration then becomes a matter of connecting electrical and pressure leads and pushing the "run test" button.

The stored data became a resource for in-the-field transducer analysis. Graphical display of test results reveals such previously unobtainable information as output hysteresis with ascending or descending pressure.

With real estate still available within the package, additional memory became possible for storing complete operating and instrument-maintenance instructions-all accessible from a 24-key panel. "I could teach just about anyone to operate this thing in 15 minutes," asserts a smiling Holmes.

Human-factors engineering also contributed to the APC's functionality growth. Because it was not to be hand-held, the team made the keyboard/display removable. Technicians could place the 20-lb compressor/electronics unit in a convenient location and keep the controls and display in their hands. Further, the square display snaps into the base in any orientation-even face down so it's protected during transport.

Design-for-manufacturing rules played a large part in the APC's hardware configuration. Assembly requires no precision fixtures; each part locks into place with a minimum number of fasteners. All ports and electrical connections are grouped together on the top of the case. Not only is this arrangement more convenient for users, but it permits use of a one-piece bottom cover. If the APC is set down on end, the cover protects components from as much as two inches of standing water.

Other Applications
  • Laboratory equipment

  • Fluid power

Conversations with its designers revealed more features of the 1292 APC that can fit into a short article. Admittedly, the project grew beyond its original goals. Nevertheless, Peterson says, "We think the performance gains justify it."

Additional details...Contact Mike Walsh, Transmation, Inc., 977 Mt. Read Blvd., Rochester, NY 14606, (716) 254-9000.

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