The electronic pressure transmitter is used to transmit a signal to a remote electronic receiver. The receiver may be a controller in a remote building, a remote marshalling cabinet for numerous signals, or a remote terminal unit (RTU). Transmitters should be compatible with the remote control house instrumentation. Indicating transmitters should not be specified; they generally use Bourdon tube pressure elements and motion-balance mechanisms, and are not as reliable as forcebalance transmitters.
An electronic pressure transmitter may be specified with a suppressed range to allow for range and span adjustments without changing electronic or mechanical components. Transmitters should include adjustable damping to smooth out the noise from high frequency pressure transients. The accuracy of electronic transmitters has drastically improved since the introduction of “smart” transmitters. Accuracy should be within 0.25% of calibrated span for standard electronic transmitters and 0.1% of calibrated span for “smart” transmitters. It is advisable to consult individual manufacturer’s specifications; for example, Rosemount gage-pressure transmitters have a span accuracy of 0.05% of span.
The transmitter signal may be digital or 4 to 20 milliamps DC. The transmitter should be loop powered, which means that the operating voltage is carried on the same two wires that carry the output signal. “Smart” transmitters that operate on a 24-volt DC power supply are preferred. Some advantages of “smart” transmitters include lower cost of ownership, improved diagnostic capabilities, ease of calibration and commissioning. The electronics enclosures for newer transmitters minimize the effects of radio frequency interference (RFI). In addition, much of the RFI in the signal wiring can be eliminated by metal conduit.
Electronic pressure transmitters must be specified to meet the appropriate electrical classification. Intrinsically safe transmitters must be specified as such and so labeled by the manufacturer. Transmitters should be supplied with linear output meters and with accessories for mounting on a 2-inch pipe.