Luckily, several of these were cleared up on Feb. 19 when I attended the second of two 90-minute webinars hosted by the ISA and presented by Hunter Vegas, project engineering manager at Wunderlich-Malec, Ned Espy, technical director at Beamex and Roy Tomalino, professional services engineer at Beamex. I liked the second webinar so much that I asked ISA for the YouTube link to the first one held on Oct. 2, 2014, and then had the bright idea to write this issue's "How to Calibrate Pressure Instruments" feature.
Anyway, besides learning about detailed best practices for calibrating pressure devices, I was reminded by the presenters of the underpinnings of not just pressure, but process applications in general. Very refreshing. Lifting my head from all the specialized niches I cover, I could again see how process control fits into the larger world.
"When we're talking about good measurement, we're really talking about good metrology practice and data with demonstrable pedigree that can show traceability back to international standards," says Espy. "We do calibration to bring transmitters that have drifted back to their good-as-new condition."
At its most basic, pressure is defined as force divided by unit area. However, Vegas explains that this simple equation can occur in some unexpected ways. For instance, if a large force is spread over a relatively large area, then the net local force is small, while a small force over a small area can have a high net local force. For instance, a 14,000-lb elephant that always has at least two 314-sq.in. feet on the ground generates 22.3 lbs./sq.in., but a 120-lb. woman with one 0.25-sq.in. heel on the ground produces 240 lbs./sq.in.
[pullquote]Likewise, Vegas added it's important to remember that, when using a standard orifice plate in an air line, differential pressure (dP) is multiplied by four when the flow is doubled, and dP is multiplied by nine when the flow is tripled. "Flow and dP have a squared relationship, so the dP's square root is needed to convert or relate to a given flow," adds Vegas. "This is usually done in the DCS, so if it's done in the field, you need to make sure the DCS doesn't do it again."
Apart from its counterintuitive behavior, pressure also comes in many units that can be hard to sort out. The primary pressure units are atmospheres, pounds per square inch (psi), Newtons per square meter (kPa), bars that are 0.01 kPa, inches of water column (in.H2O), millimeters of mercury (mmHg, Torr) and inches of mercury (in.Hg). "People get confused because there are so many units, and then ambient pressure is also affected by altitude, temperature, humidity and even latitude," adds Vegas. "Depending how your scale is set, at sea level you may see any of these: 0 psig (gauge), 14.7 psia (absolute), 1 atmosphere, 30 in.Hg or 760 mmHg. In.H2O is based on the weight of a 1-in. cube of water, and 27.7 in.H2O equals 1 psi."
Besides using or converting to the right units, pressure measurements also depend on where on the scale those measurements start. Espy adds, while absolute pressure begins with zero in a vacuum and gauge pressure begins with zero at ambient barometric pressure, dP happens in a closed system that looks at the difference between two pressure signals coming from a high leg and a low leg, and zero differential happens when both legs are connected.
After struggling to understand the second webinar, it was a relief to get some grounding from the first. It probably would have helped to not view them bass-ackwards, but my loss can be your gain. Check them out.