M970-07/04
PAGE 16
11.Furnace Operation and Calibration.
Insert required metal equalization block into furnace work tube. Use large 3in. diameter
metal blocks in the main outer work tube and smaller blocks (up to 2.55 ins. Diameter) in
the separately mounted model 970-1 furnace tubes. The large 3 ins. Diameter blocks
are used when large capacity calibration is required or for large head and short probe
designs. For 3 in. diameter block design using ceramic tube insulators (refer to Appendix
5) ensure the block is mounted with ceramic tubes in lower half of block resting on the
work tube.
Position block in the furnace to suit the probe length being calibrated. For long
thermometers / standard probes, position block in centre of furnace (Appendix 5). For
medium length probes (11 to 12 ins.) position block as shown in Appendix 6. For short
or extra short probes (8 / 9.5 ins. or 4.5 / 5.5 ins.) position the block as illustrated close
to one end of furnace.
Prepare thermocouples for calibration (refer to Section 2.2 Calibration Procedures CP10
manual) and insert into correct holes in block for close fitting (refer to Appendix 3 and
4). Connect thermocouples either directly to measuring instrument or via TC
compensating wire. Connect reference thermocouple to instrument channel (AO or BO
for TTI7 instrument) and position in metal block. Using high temperature continuously
rated Kaowool to 1200 deg.C., insulate carefully the ends of the furnace to prevent heat
losses particularly around air gaps where probes protrude.
Switch on laboratory supply to furnace.
Switch on the furnace control instrument circuit breaker, the instrument RED supply lamp
is lit and main RED OFF lamp adjacent to furnace power off button illuminates. All 3
temperature controllers and over temperature control unit front panels are illuminated.
Set “Over-temp” controller “up – down” controls to 50 deg.C. above maximum calibration
temperature to be used. Set centre “furnace control” unit to the first lowest calibration
temperature on lower controller display using the up/down arrow keys.
Press furnace power ON green button on furnace control unit. The furnace will start to
heat up and the upper furnace centre controller display will start to rise. The two end
zone controllers will shortly indicate they are driving power to the furnace end zones
after their differential input increases with rise in centre zone temperature. Their top left
lamps will either flash almost continuously or remain permanently on.
For initial adjustment of end zone controller offsets to correct for end zone losses and
improve horizontal temperature uniformity in the furnace and metal block, adjust
up/down end zone controller arrow keys. For 300 deg.C initial temperature control
adjust up keys for +0.4mV. on each end zone. For control to 500 deg.C set end zones
to +0.6 mv.
