Instruction Manual
D200124X012
2500 and 2503 Controllers/Transmitters
June 2017
30
loading pressure to the Bourdon tube. This loading pressure moves the Bourdon tube away from the flapper. Also, in
this condition, full loading pressure is on the upper diaphragm of the relay. The loading pressure moves the diaphragm
down, closing the exhaust end and opening the inlet end of the relay valve, allowing full output pressure.
When the process level sufficiently decreases, the flapper pushes in the plunger of the Bourdon tube valve enough to
release the loading pressure and seal the inner Bourdon tube channel (see figure 13). This decrease in the loading
pressure moves the Bourdon tube toward the flapper, producing the snap action. Also, this decrease in loading
pressure allows relay spring pressure to move the large diaphragm up, closing the inlet end and opening the exhaust
end of the relay valve, allowing full exhaust of the output pressure. This control action continues until a sensor level
change moves the flapper away from the plunger, permitting the Bourdon tube valve to close, restoring loading
pressure to the pneumatic circuit. Reverse‐acting controllers produce the opposite effect.
Figure 13. Direct‐Acting Left‐Hand‐Mounted Fisher 2503 Controller
BD4466-A
CD2114-E
A1890-1
INNER BOURDON TUBE
CHANNEL
OUTER BOURDON
TUBE CHANNEL
THREE‐WAY BOURDON TUBE
VALVE (OPEN FOR RELEASE OF
LOADING PRESSURE)
MOVABLE ARM
FLAPPER
FIXED PIVOT
LARGE DIAPHRAGM
EXHAUST END OF
RELAY VALVE
INLET END OF
RELAY VALVE
CAM
SMALL
DIAPHRAGM
SWITCH POINT ADJUSTMENT
TORQUE TUBE
SUPPLY PRESSURE
LOADING PRESSURE
OUTPUT PRESSURE
Maintenance
2500 and 2503 controllers/transmitters work in combination with 249 displacers. Refer to figure 16 for key number
locations, unless otherwise indicated.
WARNING
Always wear protective clothing, gloves, and eyewear when performing any maintenance operations to avoid personal
injury.
