What are the major factors in Conventional and Smart Transmitters used in industrial Instrumentation.
A conventional 4-20 mA transmitter regulates the loop current and provides only a process variable signal.
The zero and span adjustments are analog adjustments. In the case of fixed range transmitters the zero and span adjustments are limited to about plus/minus 2-3% of the fixed zero or span values.
Some conventional transmitters have an adjustable turndown range, but calibrating a turndown range requires a simulated process variable (an applied calibration pressure or a simulated temperature (RTD or thermocouple) electrical signal).
Turndown is generally limited to about 5:1 or 6:1.
A smart transmitter provides a process signal but can also send/receive information about the instrument itself, e.g. zero and span info, tag (instrument loop identification), diagnostics, or even multiple process variables using a digital protocol.
Configuration changes are done either with a keypad and display or through a protocol like HART, which is superimposed on the 4-20mA signal, so no additional wiring is needed for smart communication.
HART communication is accomplished with a handheld communicator or a modem and software. Turndown rangeability can be as high as 100:1 (with reduced accuracy at the low end); 15:1 or 20:1 with stated accuracy and can be accomplished without applying an external calibration signal, reference or supply.
Smart transmitters can communicate via a Fieldbus, Foundation Fieldbus or Profibus.
Author - Carl Ellis