Chemistry Reference
In-Depth Information
autotransformer connected to a reversible electric motor. The methods described in
[4, 5] are based on a null control principle (astatic system). Voltage variations in a
Pt thermistor corresponding to the changes in the thermostat temperature are com-
pared (using the galvanometers and photorelays) with the scheduled voltage on the
heating rate sensor. In [5], a thyristor-based circuit was used instead of standard
photorelays.
In our experiments, a temperature programmer for linear heating (a follow-up
system with proportional-plus-floating control and an electronic heating rate sensor)
designed based on previous experience was used (Fig. 7.7, Table 7.1).
The computer-assisted heating rate sensor (CHS) generates a varying voltage
which corresponds to a change in the chromel-alumel thermocouple voltage during
linear temperature growth at a preset rate,
. This voltage is added to the thermo-
electric voltage of the reference thermocouple (of the opposite sign) installed near
the thermostat heater. The resulting voltage is received by an amplifier (null device).
If an imbalance occurs, the current in the heater circuit is adjusted by a reversible
electric motor connected to an autotransformer cursor until the thermoelectric volt-
age becomes equal to the CHS voltage. Thus the reference thermocouple voltage is
constantly compared with the CHS voltage, which results in the linear growth of the
heater temperature for a wide range of temperatures and heating rates (this method
can also be applied to the linear cooling of a sample).
ω
1
2
3
6
Fig. 7.7
Scheme of
temperature programmer:
1
,
computer-assisted heating
rate sensor (CHS);
2
, control
thermocouple;
3
,dc
amplifier;
4
, reversible motor;
5
, autotransformer;
6
, heater
4
5
Table 7.1
Technical characteristics of the temperature programmer
Working temperature
40-600
◦
C
0.1-20 deg min
−
1
Heating rate
Maximum heating nonlinearity for chromel-alumel
±
1%
Maximum heating power
1 kW
Supply voltage
220 V
