Archive: Dec 2012

Wyatt Engineering’s PMT: The REAL Lo-Loss® Flow tube

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LO-LOSSIn 1961, the performance of the first and original Lo-Loss flow tubes, which were designed and marketed by the Penn Meter Company out of Philadelphia, was documented in an ASME paper, “Design and Calibration of the Lo-Loss Tube (61-WA-80).” A year or so later, Penn was purchased by Badger Meter out of Milwaukee, and the Lo-Loss tube became the Badger Meter PMT Lo-Loss® flow tube (the “PMT” stands for “Penn Meter Tube”). Over the years, Badger refined and developed the design, continuing to improve the PMT’s performance. In 2001, Wyatt Engineering purchased Badger Meter’s differential-producing flow element division, sometimes called “the Tulsa operations.” All the Badger Meter product designs and data were part of the acquisition. In the next decade, Wyatt continued to refine the PMT Lo-Loss®, leading to an even better definition of the discharge coefficient and lower permanent pressure loss.

The Lo-Loss® was originally designed to generate a stable and high differential pressure (DP) while keeping the energy loss (headloss) low. Classical venturi meters need a small beta ratio to produce a high DP, but the small beta ratio leads to higher headloss. This tradeoff was important because transmitters in the ‘50s and ‘60s had poor resolution and needed a large signal to achieve acceptable performance.

The Lo-Loss® was a radical departure from the traditional venturi design: First, the high pressure tap was located at the transition from the cylindrical inlet section and the radial converging section, not some distance upstream of a converging cone as in the venturi. Second, the Lo-Loss® does not have a long cylindrical throat, like the classical venturi, but a continuously curved transition between the inlet section and the recovery cone. The locations of the pressure taps and the resultant meter factors were to be modified subsequent to publication of the Lo-Loss® paper because Badger CALIBRATION RESULTS REVEALED THAT THE DESIGN GIVEN IN THE ASME PAPER RESULTED IN AN UNSTABLE METER FACTOR FOR HIGHER REYNOLDS NUMBERS. These changes in the PMT design corrected its Reynolds number sensitivity and changed the discharge coefficient, changes that have not been published or otherwise distributed.

Is this why Wyatt Engineering’s PMT Lo-Loss® is the only flow tube you should consider? Putting it bluntly: Yeah, pretty much. Others do not have insight to any of the improvements made to the design in the last 50 years, including off-the-shelf software providers and common flow measurement handbooks. The average error between that published in the ASME paper and the current state of the art is a little over 2%; the error function peaks at close to 7%; our survey of information from handbooks and calculations from flow calculating programs result in large errors relative to Wyatt’s PMT uncertainty band. This doesn’t mean that others who have “knocked-off” the PMT can’t provide a good meter, they just can’t provide the performance you expect – or they expect – from the Lo-Loss. Alchemists may have had “years of experience” trying to turn lead into gold, but at best, it’s fool’s gold.

What’s a poor engineer to do? GET THE DATA. If someone tries to sell you their “Low Loss” flow tube (or any meter, for that matter), simply ask for a summary of more than 20 flow calibration results (a random sampling) that cover a wide range of line sizes and beta ratios and that indicates the predicted coefficient and the coefficient found in the lab. Just for fun, ask for copies of some of those calibrations, they should be from flow labs independent of the manufacturer.

Do you have questions or comments about Wyatt Engineering and the PMT flow tube? Join the conversation here.