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issue: June 2009 APPLIANCE Magazine

Case Study: Testing Equipment
High Accuracy in Low-Pressure Transducers


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The low-pressure transducers that heating and comfort conditioning OEM NORDYNE uses may not be very big, but they do perform a vital testing function.

The Model 869 ultralow-pressure documenting calibrator is used exclusively to calibrate Nordyne’s low-pressure transducers.

NORDYNE has 24 low-pressure transducers. There are four in each of its six psychrometric test rooms, in which the physical and thermodynamic properties of gas-vapor mixtures are determined. The transducers all have one primary function, measuring the airflow of the company’s air-conditioning systems, and they must do so with precision.

“If you have inaccurate air measurements, then you have no idea of the actual cooling capacity of an individual air-conditioning system,” says Justin Hampton, a calibration technician at the OEM. “And if you don’t know what your cooling capacity is, you cannot make any kind of accurate statement on the overall efficiency of the system.”

Nordyne makes products under Maytag, Frigidaire, Westinghouse, and Kelvinator brand names. The OEM has also developed one of the most efficient air-conditioners available—a 23 SEER (Seasonal Energy Efficiency Rating) unit—and produces many other green products.

NORDYNE is also unique among HVAC OEMs in that it is certified in Demand Flow Technology (DFT), which is used to assure high-quality production. Through the process, everyone touching a product is triple-checking the workmanship. Each product is also 100% computer-automated tested to remove human error in the final analysis.


The portable, battery-powered unit provides a minimum of eight hours of use on one charge.

Quality Concerns

Nordyne did, however, develop concerns about reliability, stemming not from the transducers themselves, but from the device employed in testing the transducers.

“We were using a little static pressure source, basically a hand-driven pump with fine adjustment, and a micromanometer,” says Hampton, who maintains the company’s calibration records as well as writing and updating standard operating procedures for testing. “But that system had definite shortcomings. The most glaring is that if the system had a microleak—which it frequently did—you could not be assured that you were getting a truly accurate reading.” He explains that there are a number of brazed and soldered joints on the copper lines from the unit under test to the calibrator. “This allows for the possibility of microleaks that will not be detected during testing because the source of the measured pressure is a constant source, such as a blower moving air. However, these leaks would affect calibration with the static pressure source being used; this effect would be significant because the measurements are 5 in. of water or lower. What’s more, it meant that we could not maintain the calibration point, so recalibration was often necessary.”

Using the old device was also time-consuming and labor-intensive. “Most times, it ended up being a two-man job, one controlling the pump and the other reading the instrumentation,” Hampton adds. “Plus, it would take 30–35 minutes to run through three test points.”

Nordyne saw a compelling need to rectify the issue. At the time, the company was in the process of replacing its existing low-pressure transducers with transducers from Setra Systems (www.setra.com), a designer and manufacturer of pressure measurement devices located in Boxborough, MA, U.S.

“We were updating our transducers from Setra a few at a time,” recalls Hampton. “The process had actually started about a year or two before we pinpointed our calibration problem. We were very satisfied with the new transducers, so we decided to see what other products the company offered. They were a known quantity to us.”

Web Awareness

Nordyne spotted a Web banner for Setra’s line of calibrators and felt it was well worth checking out. In fact, NORDYNE performed a thorough review of the product line and its capabilities, and ended up choosing Setra’s Micro-Cal Model 869 ultralow-pressure documenting calibrator. The device is used exclusively to regularly calibrate the 30+ low-pressure transducers (half of them are now from Setra, with plans in process to replace them all).

The calibrator is specifically designed for air-handling processes needing portability, high accuracy, and low-pressure documenting calibration to certify pressure needs. It does so by performing calibration checks on sensors with accuracies as high as 0.00025 in.W.C. It is a battery-powered unit providing a minimum of eight hours of operation, while giving a pressure reading accuracy close to 0.0001 in. W.C., with fast, stable, and repeatable pressure generation better than 0.0003 in. W.C. With recorded calibration times as fast as 5 minutes per unit, users see time-saving benefits, as well as substantial cost savings for service and installation time.

The calibrator supplier says its accuracy is far higher than the results achievable in traditional laboratory benchtop calibration units that are accurate only to 0.0004 in. W.C. and require at least one full hour to run calibration checks on one unit.

Nordyne did look at calibrators from other suppliers and was intrigued by one unit in particular.

“The other calibrator caught our attention because it was about half the price of the one offered from Setra,” says Hampton. “It had a nice low-pressure source, and we actually got a model to experiment with. But it turned out to be a source only; it provided no onboard control or data log capability. Despite the lower cost, it was not going to be sufficient for our operation.”

The ease of control and simplicity have been real benefits, according to Hampton. “I conduct virtually all of the calibrations in addition to my other duties,” he says. “So I needed something that was quick and easy, and would not cut into the time I’ve allotted for my other responsibilities. Typically, the Setra 869 allows me to do a calibration in about five minutes, versus the 30 or so minutes with the old device.”

He says that 25-minute saving per procedure may not sound like much by itself, but after several procedures it adds up to real time saving. Another important resource saving comes from the ability to perform calibration unassisted. Hampton no longer needs to involve a second employee in a calibration he is performing.

He did train another staffer on the system, however. “After just five minutes of training, he was already proficient in how to register calibration points,” Hampton notes.

The company is also currently supporting a test station in a gas furnace lab that makes use of 11 low-pressure transducers.

“Some of these readings can be as low as 0.25 in. of water, such as the heat exchanger intake pressure,” says Hampton. “This makes the calibration method even more critical than that of the psychrometric rooms.”

Each of the transducers is calibrated yearly to ensure optimal performance. By doing so, NORDYNE can be confident that the transducers used to ensure equipment performance are providing the most accurate readings possible.


 

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