Emerson Bristol 2808 Series, Instruction Manual

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1A-2 / GP Transmitters 2808B
1A.2 THEORY OF OPERATION
The Transmitter body is composed of an electronics housing and a sensor module assembly
as shown in the block diagram Figure 1A-3. The electronics housing contains the amplifier
circuitry and the field wiring terminals. The sensor module contains a pressure input
chamber, a fluid chamber, a recessed isolation diaphragm, and a micro diaphragm that
includes electronic sensing circuitry. (Transmitters with flush diaphragms omit the
pressure chamber and have the isolation diaphragm positioned at the very end of the
sensor module.)
Input pressure applied to the pressure chamber is hydraulically transmitted through the
fill fluid contained by the isolation diaphragm. This pressure produces a strain on the
silicon diaphragm.
A micro diaphragm assembly contains four piezo-type, strain gauge resistors that are ion-
implanted on the diaphragm's surface and wired in a bridge configuration. The flexing of
the diaphragm causes changes of resistance in the bridge.
A bridge circuit is powered by a constant current supply and produces a millivolt signal
that corresponds to the measured pressure. Circuitry associated with the bridge circuit
provides measurement stability by compensating for changes of ambient temperature.
The millivolt signal developed by the bridge is applied to a high-gain, linear amplifier and
converted to a two-wire, 4-20 mA current output. Figure 1A-3 shows this output wired to a
typical external loop circuit that uses a 250-ohm load resistor and a 24 Vdc power source.
Figure 1A-3 - Simplified Diagram of GP Transmitters