Resistance thermometer

A resistance thermometer in which a temperature sensing resistor is connected in parallel with a series combination consisting of a signal output resistor and a compensating resistor. By choosing the resistance ratio between two resistors to be equal to the ratio of the desired resistance value to the error of the temperature sensing resistor, the output voltage developed on the signal output resistor reflects the voltage expected on the temperature sensing resistor without the resistance error thereof. The temperature sensing resistor and the compensating resistor constitute a circuit unit which is detachable from the rest of the resistance thermometer.

BACKGROUND OF THE INVENTION 
The present invention relates to a resistance thermometer, and more 
particularly to a resistance thermometer which does not need any 
adjustment after a replacement of a temperature sensing unit. 
For a temperature sensor for use in a resistance thermometer, a platinum 
resistor is commonly used, since it has a stable resistance value during 
aging. However, it is troublesome to make platinum resistors so as to have 
the same precision resistance, so that the manufacturing yield of platinum 
resistors is low and the price is very high. The wide scattering of the 
resistance value is very inconvenient, especially for the replacement of 
the temperature sensor. In a conventional resistance thermometer it is 
often necessary to adjust the electrical circuit of the thermometer after 
the replacement of a temperature sensor, since the resistance value in 
general varies for each sensor. 
SUMMARY OF THE INVENTION 
An object of the present invention is, therefore, to provide a resistance 
thermometer which does not need any adjustment after the replacement of a 
temperature sensor, even if the resistance of the temperature sensor to be 
replaced is not exactly the same. 
Briefly, a resistance thermometer based on the present invention comprises 
a temperature sensing resistor, a compensating resistor, a signal output 
resistor and a current source. The temperature sensing resistor which is 
to be subject to a temperature to be measured is connected in parallel 
with a series combination of the compensation resistor and the signal 
output resistor, and this entire parallel combination of resistors is 
supplied with a current from the current source. In this circuit 
construction, the resistance ratio of the signal output resistor to the 
compensation resistor is chosen to be equal to the ratio of a desired 
resistance value of the temperature sensing resistor to its positive 
resistance error. The temperature sensing resistor and the compensating 
resistor constitute a temperature sensing circuit unit. But thermally, the 
compensating resistor is isolated from the temperature sensing resistor, 
and is kept at the same room temperature as the signal output resistor. 
This temperature sensing circuit unit is designed so as to be detachable 
from the rest of the resistance thermometer. A voltage developed on the 
signal output resistor is used as an output temperature signal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A circuit is shown in FIG. 1 for illustrating the principle of the present 
invention. A temperature sensing resistor 2 constitutes a circuit unit 1 
together with a variable compensating resistor 3. One end of each resistor 
has a common connection forming a common terminal 4. The other end of the 
resistor 2 is connected to terminals 5 and 6, while the other end of the 
compensating resistor 3 is connected to a terminal 7. Corresponding to the 
above-noted terminals, the main part 8 of the device is provided with 
terminals 5' and 6', 7', and 4'. Between the terminals 6' and 7', a signal 
output resistor 10 is connected, while a constant-current source 9 has 
connections to terminals 5' and 4'. In such a circuit construction, the 
temperature sensing resistor 2 has a resistance value R.sub.0 +NR.sub.0, 
where R.sub.0 is a precise resistance value desired of this resistor, 
wherein NR.sub.0 is a positive error value. (N is, of course, much smaller 
than unity.) Therefore, if the resistance ratio between the signal output 
resistor 10 and the compensation resistor 3 is chosen such that R.sub.0 
:NR.sub.0 =1:N, then the voltage developed on the signal output resistor 
10 is equal to the value expected on the temperature sensing resistor 2 
when it exactly has the desired resistance R.sub.0 without the error 
resistance component NR.sub.0. This is easily shown as follows: 
If the resistor 2 is supplied with a current of I, the voltage E.sub.0 
developed on the resistor 10 is given by 
##EQU1## 
where R.sub.1 and NR.sub.1 are resistance values of the signal output 
resistor 10 and the compensation resistor 3. Therefore, if the current I 
flowing through the sensing resistor 2 is kept constant, an output voltage 
developed on the signal output resistor 10 always reflects a temperature 
measured by the sensing resistor 2, even if the resistance value varies in 
accordance with a variation of the temperature to be measured. In the 
circuit shown in FIG. 1, R.sub.1 +NR.sub.1 must be much larger than the 
resistance of the sensing resistor 2, for keeping I constant irrespective 
of the resistance variation of the sensing resistor 2. Unless the current 
I is kept constant, the output does not reflect the measured temperature, 
although it is exactly equal to the voltage expected on the error-free 
sensing resistor. In case the temperature varies with respect to t, the 
resistance R.sub.0 +NR.sub.0 =R.sub.0 (1+N) varies with respect to R.sub.0 
(1+N) (1+.alpha.t)=R.sub.0 (1+.alpha.t)+NR.sub.0 (1+.alpha.t), where 
.alpha. is a temperature coefficient. The first term R.sub.0 (1+.alpha.t) 
corresponds to the output voltage E.sub.0, while the second terms 
correspond to the voltage appearing on the compensating resistor 3. 
In such a temperature measurement technique, an error which is expected due 
to the resistance of the wire connecting the terminal 4' to the terminal 4 
can be excluded by adjusting the compensating resistor 3 in advance in 
consideration of the wire resistance. 
FIG. 2 shows a circuit construction of a first embodiment of the present 
invention. In the figure, all the components corresponding to those shown 
in FIG. 1 are given the same reference number. In this embodiment, an 
impedance converter 12 is provided between the output terminal 6 (6') and 
the signal output resistor 10, so that a current I supplied to the sensing 
resistor 2 from the constant-current source 9 is kept constant with 
respect to temperature variations, even if the resistance of the resistor 
10 is not much larger than the resistance of the sensing resistor 2. On 
the other hand, the output from the terminal 7 (7') is fed from the output 
point 15 of an impedance converter 14 to the inverting input terminal of a 
differential amplifier 16 through an input resistance 17 so as to be 
subtracted in the amplifier 16 from the output of the signal output 
resistor 10. This output from the signal output resistor 10 is input to 
the non-inverting input terminal of the differential amplifier 16 through 
a resistor 18. The differential amplifier 16 thus outputs only a voltage 
developed across the signal output resistor 10. 
FIG. 3 illustrates a circuit construction of a second embodiment of the 
present invention. This embodiment is an example of applying the present 
invention to a heater control. A temperature sensing resistor 2, subjected 
to the heat produced by a heater (not shown in the figure) to be 
controlled, has its current supplied from a constant-voltage source 28 
having a terminal 22 through a resistor 21. Furthermore, in this 
embodiment, the series combination of a signal output resistor 20 (which 
corresponds to resistor 10 in FIG. 2) and a compensating resistor 3 is 
directly connected in parallel with the sensing resistor 1. Therefore, in 
order to make the output of the sensing resistor 2 independent of these 
resistors 20 and 3, their resistance sum is chosen to be sufficiently high 
so that the current through them can be ignored in comparison with the 
current flowing in the sensing resistor 2. The output voltage of the 
constant-voltage source 28 is thus proportionally distributed between the 
resistor 21 and the sensing resistor 2 according to their resistance 
ratio, resulting in a voltage e.sub.M across the sensing resistor 2. This 
voltage eM, which varies in accordance with the resistance of the sensing 
resistor 2, is supplied to the inverting input terminal of a comparator 
27. On the other hand, the potential difference between the output of the 
voltage source 28 and the output point 24 of an impedance converter 23 is 
divided by resistors 25 and 26 and a variable resistor VR which is in 
series therewith, so as to provide a reference voltage e.sub.S to the 
non-inverting input terminal of the comparator 27. The impedance converter 
23 is supplied with the output of the compensating resistor 3, and the 
output point 24 is therefore kept at a potential equal to the output of 
the compensating resistor 3. 
In such a circuit construction, when the voltage e.sub.M of the sensing 
resistor 2 is lower than the reference voltage e.sub.S, the comparator 
continues to output a high-level signal. Once the voltage e.sub.M exceeds 
the voltage e.sub.S owing to a temperature increase which causes the 
resistance of the sensing resistor 2 to increase, the output of the 
comparator 27 becomes a low level. The output of the comparator 27 is, 
therefore, used as an on-off control signal for controlling the operation 
of a heater. The setting of the temperature is carried out by setting the 
reference voltage e.sub.S to the value corresponding to a predetermined 
temperature through an adjustment of the variable resistor VR. 
FIG. 4 shows a third embodiment of the present invention. This embodiment 
is an example of modifying the first embodiment in FIG. 2 so as to make 
the output approximately linear with respect to temperature variations 
even in the case where the temperature-resistance characteristics of the 
sensing resistor cannot be considered to be linear. 
A well-known method for reforming the non-linear characteristic of a 
temperature-sensing resistor is to provide the temperature-sensing 
resistor with a characteristic-improving resistor in parallel therewith, 
the resistance of which is much higher than the resistance of the sensing 
resistor at the lowest temperature in the range of the temperature 
measurement. In a temperature range nearest the lowest temperature, the 
parallel resistance of the sensing resistor and the 
characteristic-improving resistor is determined mainly by the resistance 
of the sensing resistor, because the characteristic-improving resistor has 
a much higher resistance than the sensing resistor. As the resistance of 
the sensing resistor increases in accordance with a temperature increase, 
the resistance of the characteristic-improving resistor gradually begins 
to contribute to the whole resistance of the parallel combination, causing 
its resultant resistance to decrease, and thus bringing the resultant 
temperature-resistance curve near to a straight line. At very high 
temperature, exceeding a certain temperature range, the resultant 
resistance, of course, converges to the resistance value of the 
characteristic-improving resistor. Therefore, this method for improving 
the linearity has an application limit. The resistance of the 
characteristic-improving resistor is determined in consideration of a 
desired degree of linearity and the temperature range in which the 
linearity is expected. In the present embodiment, a resistor 30 is a 
characteristic-improving resistor. Its value can be chosen as desired 
according to the above-noted condition for determining the resistance 
value irrespective of the signal output resistor 10a (which corresponds to 
resistor 10 in FIG. 2), because the resistor 30 and 10a are isolated with 
respect to their currents by the impedance converter 12. 
FIG. 5 shows a fourth embodiment of the present invention. As is mentioned 
above, the above-noted method of improving the temperature-resistance 
characteristic cannot be applied when the range of the temperature 
measurement is very wide. For eliminating such a disadvantage, this 
embodiment is devised to make the feedback resistance of a differential 
amplifier 31 changeable in accordance with the temperature range in which 
a linear characteristic is desired. By selecting the combination of 
resistors 32, 33, and 34 through the operation of switches 36 and 35, the 
output from a terminal 19 is linearized with respect to temperature 
variations in a stepwise manner in four temperature ranges. 
That is, FIG. 5 is identical to FIG. 2 except for the fact that feedback 
resistor 29 has been replaced by the parallel combination of resistors 
32-34. The two switches 35 and 36 have a total of four combinations of 
operation, resulting in the feedback resistance for amplifier 16 
(analagous to resistor 29 of FIG. 2) having four different values equal 
to: the resistance of resistor 32 above, the resistance of resistor 32 in 
parallel with resistor 33, the resistance of resistor 32 in parallel with 
resistor 34, and the resistance of resistor 32 in parallel with resistor 
33 and resistor 34. 
Thus, since the gain of the amplifier 16 can be varied in a stepwise manner 
so as to have four different values, the corresponding output voltage from 
terminal 19 (which corresponds to the temperature sensed by resistor 2) 
can be varied in a stepwise manner so as to effectively linearize the 
temperature-resistance characteristic of the resistor 2. 
According to the present invention, even if the temperature-sensing 
resistor 2 does not have an exactly proper resistance, the measurement can 
be carried out by adjusting the compensation resistor 3 in the 
temperature-sensing unit 1. The previous adjustment of the compensating 
resistor 3 enables the sensing unit to be replaced, when the sensing 
resistor 2 is damaged, with a new sensing unit without any other 
adjustments of the apparatus.