|Mechanical side of a pressure gauge showing the Bourdon tube.|
In the March issue of Flow Control magazine, we will be launching a new feature called “Who, What, Where, When, Why?” This item will highlight notable people, technology, and applications throughout the history and evolution of fluid movement, measurement and containment. We will debut this item with a “What?” focus on the Bourdon tube. The Bourdon tube, named after its inventor, Mr. Eugene Bourdon, is the elastic element in most pressure gauges produced today.
In preparation for this article, I had an interesting conversation with John Carissimi, media manager at Ashcroft Inc. Being from Ashcroft, John is “in-the-know” when it comes to Bourdon tubes and all things pressure measurement, and he had some interesting information to share. Among the items we discussed was how repeatability and hysteresis dictate the quality of Bourdon tube-based pressure gauges.
Repeatability in the world of Bourdon tube pressure gauges means essentially the same thing it means within the realm of flowmeters or other measurement devices. It is a measure of how consistently the pressure gauge is in accurately repeating the same measurement if all conditions remain the same. So, for example, if exposed to the same pressure on two separate occassions, the gauge would show the same PSI if all other ambient factors (pressure, temperature, etc.) remained the same.
Hysteresis, while similar to repeatability, is a little more nuanced, yet equally and independently important in determining the quality of a Bourdon tube pressure gauge. Hysteresis is a measure of how effectively a pressure gauge is in repeating the upscale reading on the down scale cycle if all conditions remain the same. So, for example, while a pressure gauge may read correctly at zero, midpoint and full scale during the “pressure-up” cycle, it made read too high at the midpoint check during the “pressure-down” cycle, while still correctly returning to zero. This “stretch” that is visible toward the middle of the scale during depressurization is called hysteresis; an inherent trait of an elastic element that hinders its ability to repeat the upscale performance on the way back down.
“If the hysteresis is bad and outside the specification of the gauge, then the gauge is no good, even if the repeatability is good,” said Carissimi.
One of the key factors that can affect the repeatability and hysteresis of a pressure gauge is temperature. “The nemesis of any pressure device is temperature,” says Carissimi.
A letter from C. Des Abbayes, president and general manager of Bourdon pressure gauge company in 1965, to Otto Heise, inventor of the Heise Bourdon pressure gauge, in recognition of the high quality of the Heise gauge. Heise gauges have earned a reputation over the years for their superior accuracy and quality. (Click on the image to download a PDF.)
Courtesy of Ashcroft Inc.
Look for our profile of the Bourdon tube to appear in the March issue of Flow Control magazine. If you are not yet a subscriber to Flow Control magazine, click here to register for a FREE subscription.