Transducers are voltage-output devices that can be used with simple signal conditioning but are more sensitive to electromagnetic interference. The electrical resistance of the connecting cable can cause significant errors if the cable is long. They require three or four connecting wires to supply power and deliver the output signal.
Transmitters are current-output devices and may have two or three wires. Where two wires are used to both receive power and transmit an output signal, significant cost savings can be made where long cables are needed. They are frequently scaled to vary from 4 mA to 20 mA as the pressure varies from minimum to maximum. So the on-board electronics has to be capable of operating with a maximum current drain of less than 4 mA.
Being ‘current driven’, the in-built circuitry controls the voltage across the transmitter’s two terminals, to ensure that the appropriate pressure-proportional current is maintained irrespective of line resistance up to a specified limit. These devices are suitable for use with long cables and are much less susceptible to electromagnetic interference than voltage-output transducers.
Sometimes called current loop or serial devices, additional displays at different locations can easily be included in the loop without degrading the output signal. Such devices normally suffer no significant degradation of signal output with distance.
Digital output transmitters normally contain a microprocessor which converts measured pressure values into digital codes which are transmitted to a remote receiver, or ‘host’, via wires, optical fibres or radio.
There are a number of standard systems available, such as Fieldbus (IEC 1158) and HART, the latter having the facility to operate in combination with the more traditional 4 mA to 20 mA current-output systems.
Beyond supplying pressure values, digital transmission can include diagnostic information, status and alarms and can also facilitate remote reconfiguration of transmitters.
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