Emerson Bristol 2808 Series, Instruction Manual

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2808B DP Transmitters / 1B-1
Section 1B
DIFFERENTIAL PRESSURE TRANSMITTERS
Models 2808-35B
1B.1 PRODUCT DESCRIPTION
Model 2808-35B Differential Pressure (DP) Transmitters measure the pressure differential
existing across an orifice plate or similar type device and converts it into a proportional 4-
20 mA or a 1-5 Vdc signal that can be applied to the input of a device such as a flow
computer, controller, recorder, etc. This Transmitter is typically used with gas, water and
chemical processes to provide accurate measurements under extreme environmental
conditions.
Model 2808-35B DP Transmitters are offered in ranges from 0-100 inH
2
O to 0-300 psi. A
listing of ranges for the Model 2808-35B is given in Table 1B-A.
2808-35B Transmitters can be installed on DP pressure manifolds or they may be specified
with a Universal Mounting Bracket. Universal Mounting Brackets permit the unit to be
clamped to a two-inch pipe or secured to a support structure.
Model 2808-35B Transmitter electronics enclosures are constructed of cast aluminum.
Diaphragm, flanges, flange bolts and the manifold are offered in two materials; stainless
steel, and Hasteloy C.
1B.2 THEORY OF OPERATION
Main assemblies of the Model 2808-35B DP Transmitter are the electronics housing, sensor
module and process flanges as noted in Figure 1B-2. An electronics housing encloses the
amplifier board and the field wiring terminals as shown in the schematic of Figure 1B-3. A
sensor module contains the pressure sensor, two sealed fluid systems, an overpressure
diaphragm, and two isolation diaphragms. Flanges provide the HI and LO port connections
and also function as the outer wall of the pressure input chambers.
An electronic pressure sensor located at the upper part of the sensor module is mounted on
a micro diaphragm that serves as a divider between the two fluid systems. One fluid system
corresponds to the HI pressure input, and the other to the LO pressure input. An isolation
diaphragm in each system isolates the fluid system from the input pressure.
When a differential pressure is applied across the HI and LO ports, both isolation
diaphragms will compress or retract in response to the change of differential. Movement of
these diaphragms causes similar pressure changes in each of the sealed fluid systems that
are detected by the sensor.
If the differential pressure applied to the HI-LO ports accidentally exceeds the upper limits
of the Transmitter, an Overpressure Diaphragm Mechanism takes control of the situation.
Action of the Overpreassure Diaphragm Mechanism prevents the overpressure from
reaching the sensor, thereby minimizing the risk of damage.