Answer / carl_ellis

Smart vs Conventional transmitters

Conventional

A conventional 4-20ma transmitter regulates the loop
current and provides only a process variable signal.

The zero and span adjustments are analog adjustments. n
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.

Smart

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.

Carl Ellis
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