MRF
16/22
MF 55-GB 1.21
45
13.6 Thermocouple
replacement
Disconnect the furnace from the electrical supply, and remove the furnace back panel.
The control and over temperature thermocouples are mounted together in a single ceramic
tube sheath, which is wrapped with foil. The outer end of the foil wrapped sheath is clamped
in a brass bulkhead fitting through a stainless steel box with a flange and 4 fastenings to the
back of the brickbox. Two terminal blocks are fastened to part of the flange.
Make a note of the thermocouple connections.
The negative legs of the thermocouples are
marked blue. Disconnect the thermocouple wires from their terminal blocks and carefully
withdraw from the sheath. The two sets of thermocouple wires, which are in a series of
ceramic sleeves, will come out as a single assembly.
If any broken pieces of thermocouple or sleeve remain in the sheath, try to hook them out
with a piece of wire or remove the sheath
[see section 13.7]
to shake out the pieces.
Re-assemble with new thermocouples observing the colour coding and ensuring that the
thermocouples are not twisted as they are being inserted.
To check that the thermocouples are rewired correctly: replace the furnace back panel,
switch on the controller, turn up the set point temperature so the elements begin to apply
heat. The measured temperature of the controller and PV of the over temperature should
begin to rise. If the measured temperature begins to fall then the thermocouple is wired
incorrectly. This is a dangerous condition for the furnace because the elements will be on
continuously at full power. Isolate the furnace from the power supply and re-wire correctly.
13.7 Thermocouple sheath replacement
Do not attempt this until the furnace chamber is cold.
Disconnect the furnace from the electrical supply and remove the furnace back panel.
Disconnect and remove the thermocouples as described in sections 13.6. Slacken the
sheath fixing nut, remove the 4 screws from the flange of the thermocouple and sheath
assembly, (ref fig 13-3) and remove from MRF 16/22.
Undo and remove the sheath fixing nut. Note the distance from the sheath end to the split
compression rings then remove the split compression rings from the sheath using a flat
Foil wrapped sheath
Stainless steel
box and flange
Sheath fixing nut
Terminal blocks
Fig 14.3 Thermocouple and sheath assembly
Thermocouple tails.
Compensating cables.
Note number labels
+1 -1
+2 -2
Red markers on positive tails
Blue markers on negative tails
Sheath protrudes
20mm
Control temperature TC
Over temperature TC
