With a choice of ceramic or polysilicon capacitive sensors, Cerabar S pressure transmitters from Endress+Hauser, Greenwood, Indiana, handle a wide range of pressures and environments. Ceramic diaphragms withstand extremely corrosive and abrasive conditions from full vacuum up to 600 psi, while the touch polysilicon sensors measure up to 6000 psi.

The transmitters are available with interchangeable, field-replaceable electronics,and sensing modules for measuring gauge and absolute pressure. Interchangeability means fewer spares are needed, and etted parts and electronics can be exchanged on site. The Cerabar S imposes a digital signal onto its standard 4...20 mA DC pressure output, making it a smart transmitter capable of providing measurement and diagnostic  information or accepting calibration and setup information. The digital signal is available in two versions: HART or Endress+Hauser´" Intensor.ü The ceramic sensor met the needs of Colgate in Piscataway, New Jersey, because it could measure toothpaste pressure without failing, according to Rich Sems project engineer. Sems says toothpaste is a very thick, loose slurry and very abrasive. "The ceramic sensor is almost completely flush, and it resists abrasion very well" says Sems. "We also like it because the ceramic doesn´" get damaged during routine maintenance. With the oil-filled and metal-bellows sensors we had before, the diaphragms were easily damaged by a screwdriver or some other hard object, and we had to send them off for recalibration." Sems says Colgate has about 40 of the transmitters installed in the plant, and he recommend them for applications other than toothpaste.  Ted Royer, operations manage at Western International Gas&Cylinder, Belleville, Texas, also likes the ceramic sensor.  Western supplies acetylene, oxygen, acetone, and other industrial and welding gases. "We use eight transmitters on acetylene lines where they are subjected to extreme pressure pulses", he says.

Each Cerabar S is mounted at the discharge of a single-cylinder  reciprocating compressor that producers a pressure swing of about 50 psi, 350 to 400 psi, twice per second. "This killed the other sensors we had been using. Their diaphragms would fail from metal fatigue in a matter of weeks," says Royer.

The transmitters are subjected to intense electromagnetic and radiofrequency interference (EMI and RFI) from variablespeed drives. but Royer says this has no effect. "These transmitters seem to be completely noise-immune. We run control cables in the same trays as the drives, and there is no drift." Neither Sems nor Royer use the HART or Intensor protocols. Royer says they calibrate the transmitters the old-fashioned way, using a handheld programmer to set zero and span. "The transmitters have no interaction between zero and span, so they are extremely easy to calibrate."

They have about 30 installed, using only two basic models, with 0...30 and 0...500 psi ranges. "Because of their wide rangeability, we used them for everything, including measuring level in an open-top tank," says Royer. The environment at Las Vegas Valley Water District is not nearly as tough, but there are different problems, according to electronic technician Jim Ross. "We have 100 Cerabar S transmitters installed on wells and pump stations for potable water all over the valley," says Ross. Water supply is critical in the Las Vegas summer, so reliability is paramount.

A typical unmanned pumping station has five pumps, with two pressure transmitters on each pump: one on the suction side, and one on the outlet manifold. Ross says maintenance crews calibrated the station  instruments twice per year.

"We use the HART protocol for calibration." says Ross. "We use a pressure calibrator standard and a nitrogen source. We just plug our handheld terminal into the transmitter at any point in the HART

system and read everything right off the display. The transmitters don´" drift, so calibration is fairly easy."ü Ross reports that they have lost one transmitter so far because it froze during a particularly cold winter. "Endress+Hauser replaced it immediately, no questions," he says.

Ted Royer echoes the sentiment. "The transmitters are reliable and Endress+Hauser replaces them immediately should a problem occur again. We had two fail from infant mortality, and they shipped replacements the minute we hung up the phone." In fact, Royer says, "we don´t" keep any spares on hand. With that kind of response, we don´t ""need spares."