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2500 and 2503 Controllers/Transmitters
June 2017
All 2500 and 2503 controller/transmitters use the same basic pressure‐balanced relay assembly. The following
descriptions explain how the relay action produces the output signal with the various controller/transmitter
constructions.
2500 Controller or 2500T Transmitter
Full supply pressure comes to the relay from the regulator, as shown in figure 12. The relay has a fixed restriction
through which supply pressure bleeds before entering the large diaphragm area and the inner Bourdon tube channel.
In a steady‐state condition, the process level holds the torque tube and attached flapper steady in relation to the
nozzle. This allows pressure to escape between the nozzle and flapper at the same rate it bleeds into the large
diaphragm area. The large diaphragm holds the inlet end of the relay valve slightly open to compensate for the venting
of output pressure through the proportional valve as it maintains a steady‐state position of the final control element.
The output pressure, through the three‐way proportional valve, affects the Bourdon tube outer channel, holding the
Bourdon tube in a steady‐state position.
A process level change raises or lowers the displacer, moving the flapper with respect to the nozzle. If the process level
change increases nozzle pressure, the large diaphragm moves down; this closes the exhaust end and opens the inlet
end of the relay valve (see figure 12). This action of the relay valve increases the output pressure to the final control
element. Since the area ratio of the large diaphragm to the small diaphragm is three‐to‐one, the small diaphragm
action amplifies the output pressure change. The three‐way proportional valve lets the increase in output pressure
apply to the Bourdon tube outer channel. The expansion of the Bourdon tube moves the nozzle away from the flapper
slowing the response of the pneumatic circuit.
If the process level change decreases the nozzle pressure, the large diaphragm moves up. This action closes the inlet
end and opens the exhaust end of the relay valve which allows output pressure to exhaust. This relay valve action
reduces the output pressure to the final control element and is the reverse of the previous explanation.
The proportional valve varies the reaction of the Bourdon tube to changes in the output pressure. For additional
information on the proportional valve action, refer to the following proportional valve subsection.
Proportional Valve
The three‐way proportional valve is adjustable to allow some or all of the output pressure change to feed back to the
Bourdon tube outer channel, providing proportional band control (see figure 12). The Bourdon tube moves to
counteract the pressure changes in the nozzle, equaling the relay diaphragm pressure differential. The relay valve
maintains a new output pressure according to the change in the process variable.
A wide‐open proportional valve (fully counterclockwise) permits feedback of the output pressure signal change and
produces 100 percent proportional response. A closed (fully clockwise) proportional valve produces smaller
proportional responses, because part of the output pressure change vents through the proportional valve exhaust.
2500S Controller
This construction has the same flapper, relay, and proportional valve as the 2500 controller. However, the nozzle is
connected (figure 12) in such a way that output pressure feedback (from the movement of the Bourdon tube) moves
the nozzle in the opposite direction of the flapper. This action completely opens the relay valve for full output pressure
or completely closes the relay valve for full exhaust of the output pressure, allowing no in‐between throttling.
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Instruction Manual
D200124X012
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