Earlier this week, I took a call from a customer having problems with his pressure transmitter. He’d figured out that his last transmitter, installed in an area with nearly continuous wash-downs, had failed because of water intrusion. The bigger problem was that it wasn’t coming from a loose cover, a conduit pipe, or a leaky conduit connection. In his case, the water came into the transmitter through a vent hole and messed up the measurement cell electronics.
So he asked me to recommend a 3000 PSI transmitter that could survive the washdown conditions.
Gauge pressure measurement that a common pressure gauge displays, is referenced to atmospheric pressure (whatever surrounds the outside of the gauge’s bourdon tube). It’s not “nothing” around the outside of the tube, there’s pressure in the atmosphere.
In an electronic transmitter, this atmospheric reference pressure is usually sensed through a vent hole. In comparison, absolute pressure measurement references a hard vacuum — as close to absolute zero pressure as can practically be achieved.
I was skeptical about finding an alternative. All gauge pressure transmitters have a vent, don’t they?
As it turns out: NO, they don’t. Some are designed without a vent, and others that do have vents are designed to limit water ingress through the vents.
In my research, I found a cryptic statement about gauge pressure transmitters in the Siemens catalog: “The pressure transmitters with spans ≤36 bar measure the input pressure compared to atmosphere. Transmitters with spans ≥160 bar compared to vacuum.”
Huh? That made no sense to me: Gauge pressure measurements below 63 bar (900 PSI) but only absolute measurement above 160 bar (2300 PSI)?
So I checked into it, and was amazed by the result: Of the seven available gauge pressure ranges of Siemens SITRANS P DS III transmitters, the top three highest ranged models use an absolute pressure sensor capsule whose reference side is a hard vacuum.
As it turns out, the reason for using an absolute sensor is simple. In Siemens’ production process, the pressure capsules are assembled and totally sealed in a vacuum. Drilling a vent hole for an atmospheric measurement is an extra production step. If it’s not necessary, it doesn’t get done.
In the long run, it’s the customer who gets the benefit of the absolute pressure capsule – very high integrity – because stuff from the outside world can’t get into the sealed sensor capsule.
The sealed absolute pressure sensor capsule is more secure, more durable, and inherently offers protection from water, moisture, and airborne contaminants that can get through (or block) an atmospheric vent hole.
So far, so good!
But what about the difference between gauge and absolute pressure measurements? Both differ by one “standard” atmosphere (14.7 psi), and hour-by-hour barometric pressure changes.
The factory guys who calibrate the transmitters assume one “standard” atmosphere of pressure, and use that in calculating the transmitter’s output value. So, the factory calibration accounts for the bulk of the “one atmosphere” difference between gauge and absolute.
That just leaves the barometric pressure variance. Barometric pressure changes covering a magnitude of ±25mmHg or ±0.5 psi are small enough to be incorporated into the transmitter’s accuracy spec. A barometric change on the order of ±0.5 PSI are just a fraction of that total uncertainty. (The uncertainty of a 0-3000 PSI transmitter with ±0.075% uncertainty is ±2.2 PSI.)
Siemens’ lower range gauge pressure transmitters do have atmospheric vents for true gauge pressure measurement with reference to atmospheric pressure. For these models, you can add a “molecular sieve” option (D12) that allows air to pass through, but blocks water molecules. (It sounds like industrial Gortex to me, but it doesn’t come in Hunters’ Orange.) The molecular sieve provides an IP68 level of protection.
The bottom line:
It took some digging, but a 3000 PSI gauge pressure transmitter that’s protected from water intrusion is definitely doable. I let the customer know, and he’s going to try it out.
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Lesman Instrument Company is the authorized Siemens representative in Illinois, Indiana, Missouri, Paducah KY, Wisconsin, and Michigan’s Upper Peninsula. If you are located outside that area, you can find your local sales office or get technical assistance by calling 800-365-8766.