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6 The probe plate supplied with the FB-08C may be machined to allow the sensors being calibrated
access to the bath. This probe plate should always be positioned above the surface of the bath in the
probe plate carrier so as to inhibit the loss of fine alumina particles from the surface of the bath.
Dead Bed Calibration
1 Temperature stability during calibration may be further improved by using a dead bed calibration
technique. This technique involves heating the bath to the required calibration point, then allowing
the temperature of the bath to stabilise for a period of approximately 20 minutes, dependent upon
the actual calibration temperature and the thermal mass of the sensors being calibrated. At this point
switching OFF the fluidising air collapses the fluidised bed so that a solid bed of alumina which acts as a
thermal insulator surrounds the sensors being calibrated. After a period of approximately 2 minutes at
set temperature a stability of 0.01°C can be achieved for a further period of approximately 8 minutes,
dependent upon heat loss along the stem of the sensors being calibrated.
2 Ideal results are achieved when the sensors are mounted within a correctly designed custom build probe
holder which ensures good temperature uniformity between each of the sensors being calibrated.
3 Dead bed conditions may be obtained by manually switching OFF the power switch after the temperature
of the fluidised bath has stabilised at the set point.
Alternatively this dead bed condition may be activated via the RS232 interface by changing the state of
Alarm 1 in the temperature controller. The value of Alarm 1 should be set so that it is below the actual
temperature of the fluidised bath when dead bed conditions are required. The alarm output is used to isolate
power to the heaters and switch OFF the fluidisation and dust extraction.
4 To end a dead bed period, reset the value of Alarm 1 so that it is above the temperature of the fluidised
bath. For details of how to adjust the value of the alarm, see the attached Eurotherm 818P communications
protocol manual.
Automatic Air System
To allow the operating temperature of the fluidised bath to be adjusted from a remote source while the bath is
unsupervised, an automatic fluidising air control system is fitted which alters the air flow rate according to the
actual temperature of the fluidised medium.
The unit measures the temperature of the fluidised medium and operates a bank of solenoid valves which adjust
the fluidising air flow rate to that appropriate to the measured temperature. The valves are operated in a binary
coding sequence and the circuit provides a non-linear transfer function such that the air flow rate changes more
rapidly at lower temperatures than at higher temperatures.
The system consists of the following:
A thermocouple amplifier, IC1, which gives an output of 10 mV/°C. The output of the IC1 is fed to a shaping
amplifier, IC2. IC2 is an inverting amplifier which has feed back break points which are determined by a resistor/
diode network. The potentiometers VR1 is fed from a stable 10 volt reference source and is used to set the zero
of IC2.
IC3 is a shift amplifier, the output of which is fed to the 8 bit A-D converter, IC4. The 5 most significant bits of
IC4 are used as outputs. These outputs are buffered through IC5 and fed to IC6. The open collector outputs of
IC6 are used to drive the mercury wetted relays. LEDs are connected in series with each of the relay coils to
indicate operation, the relay being used to switch the solenoid valves.
The A-D converter is used in the clocked mode; it is updated every 1.2 seconds, the clock being derived from the
mains frequency via IC5 and divider ICP2.
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