Cryogenic measurement equipment

Abstract: A temperature measurement equipment in which the accuracy of temperature measurement is improved by smoothly connecting the characteristic curves measured by two sensors having different characteristics, and dividing the characteristic curve into a plurality of curves, in particular in the vinicity of 4 K, digital processing being performed with a set resolution for each of these divided regions.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to a cryogenic temperature measurement equipment that performs cryogenic temperature measurement. In particular, it relates to a cryogenic temperature measurement equipment adapted for temperature measuring to ensure safe operation of a super-conducting coil cooled by liquid helium. DESCRIPTION OF THE RELATED ART Conventionally, to make measurements from room temperature of 300 K (27°C) to the vicinity of 4 K (-269°C) in a strong magnetic field, measurement was performed by two sensors having different characteristics. Specifically, first of all, from room temperature to 30 K (-243°C), temperature was measured using a platinum temperature measurement resistance (Pt) sensor. Then, from 30 K (-243°C) to 4 K (-269°C), temperature was measured using a carbon glass resistance (CGR). Both of the above types of sensor have a nonlinear characteristics, so a linear approximation is made by a plurality of broken lines. In the vicinity of 30 K, where the sensitivity is poor, accuracy therefore had to be increased by increasing the number of broken lines. However, in the conventional measurement equipment, measurement accuracy was poor, due to temperature drift resulting from measurement using analog amplification circuitry from the sensor input to the final output. Also, since two sensors having different characteristics were employed for the measurement, at the sensor transfer point in the vicinity of 30 K, different measured values were output by the two sensors, resulting in a discontinuity of the characteristic curves at the transfer point. Measurement accuracy was particularly poor in the vicinity of 4 K, between 4 and 30 K. The reader may be further enlightened as to the state of the art by reference to the patent application JP-A-60 165 527. The present invention as claimed is characterised with reference to the disclosure in JP-A-60 165 527. The object of the invention is to provide a cryogenic temperature measurement equipment whereby temperature is measured continuously between room temperature and cryogenic temperature by two sensors having different characteristic and wherein the accuracy of temperature measurement can be improved. According to a first aspect of the invention there is provided a cryogenic temperature measurement equipment according to Claim 1. Further according to the present invention there is provided a cryogenic temperature measurement method according to claim 5. Fig.1 is a schematic diagram of an embodiment of a cryogenic temperature measurement equipment according to this invention. Fig. 2 is a diagram given in explanation of the correction action during falling temperature and during rising temperature of the cryogenic temperature measurement equipment illustrated in Fig.1. Fig.3 is a diagram given in explanation of the detailed operation of the correction action during temperature fall of the very cryogenic temperature measurement equipment illustrated in Fig.2. Fig.4 is a flow chart illustrating the content of processing of a digital circuit employing a microprocessor. Fig.5 is a diagram given in explanation of a correction action of the cryogenic temperature measurement equipment during temperature fall and during temperature rise different from the correction action of the equipment illustrated Fig 2. DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of this invention is described with reference to the drawings. Fig. 1 is a schematic diagram of a cryogenic temperature measurement equipment according to a first embodiment. In a measurement circuit 10 of Fig. 1, temperature is measured from 300 K down to 30 K. This circuit has the following scheme. Constant―current source 2A supplies DC current IA to terminals 3A and 3A' of a cryogenic temperature cooling cryostat 4A. A Pt sensor 1 is thermally isolated from the atmosphere by a double layered cryostat liquid nitrogen (temperature:77 K) and liquid helium (temperature:4.2 K), and generates an output voltage VA between voltage terminals 5A and 5A' in response to DC current IA supplied from the constant-current source 2A. Isolating amplifier circuit 6A receives the output voltage VA generated between voltage terminals 5A and 5A' and outputs an output voltage amplified up to a specified signal level. This output voltage changes in response to change of temperature because the resistance of the Pt sensor 1 changes. Second measurement circuit 20 measures the temperature from 30 K to 4 K. It is constructed as follows. Constant-current source 2B supplies DC current IB to the current terminals 3B and 3B' of a cryogenic temperature cooling cryostat 4B. The CGR sensor 1' is thermally isolated from the atmosphere by a double layered cryostat of liquid nitrogen (temperature:77 K) and liquid helium (temperature:4.2 K), and generates an output voltage VB between voltage terminals 5B and 5B' in response to DC current IB that is supplied from the constant-current source 2B. Isolating amplifier circuit 6B receives this output voltage VB generated across voltage terminals 5B and 5B' and outputs the output voltage amplified up to a specified signal level. This output voltage changes in response to change of temperature because the resistance of the CGR sensor 1' changes. To effect measurements between 300 K and 30 K, a measurement circuit switch 7 transfers toα, so that the first measurement circuit 10 is employed for measurement. To effect measurements between 30 K and 4 K, the measurement circuit switch 7 transfers to β to perform measurements by means of the second measurement circuit 20. The A/D converter 8 converts the analog output voltages from the first measurement circuit 10 and the second measurement circuit 20 to a digital voltage signal. The digital voltage signal from the A/D converter 8 is linearized by means of a CPU 9a in a digital circuit 9 and converted into an output value (0 to 100%) in accordance with the temperature 0 to 300 K. The linearization processing will now be described with reference to Fig. 6, using the characteristic curve of the CGR sensor 1'. The dotted line shown in Fig. 6 illustrates the characteristic curves from 30 K to 4 K of the CGR sensor 1' employed by the second measurement circuit 20. This characteristic curve is linearly approximated by three solid broken lines in the three temperature regions: 4 K to 10 K, 10 K to 20 K, and 20 K to 30 K. As shown in Fig.1 ,the digital output values 11 that are output from the digital circuit 9 are input to a data logger 12, an alarm output13, and are displayed in analog form by means of the D/A converter 14. However, due to the differences in characteristic curve of the Pt sensor 1 provided in the first measurement circuit 10 and the CGR sensor 1' provided in the second measurement circuit 20, the linearized output values generated in response to temperature are different, depending on the different resistance values that are output from the two circuits 10 and 20. The correction action in the vicinity of 30 K at which the measurement circuit switch 7 operates is described below. First of all, the correction action during fall of temperature will be described with reference to Figs. 2 , 3 , 4. The temperature measured by the Pt sensor 1 of the first measurement circuit 10 is output from room temperature to 30 K. At the point when the temperature measured by the Pt sensor 1 reaches 30 K, the measurement circuit switch 7 is transfered from α to β, so that measurement is performed by the CGR sensor 1' of the second measurement circuit 20. First of all, the output voltage VA across these voltage terminals 5A and 5A' of the Pt sensor 1 at temperature 30 K (point A) is read off by the isolating amplifier circuit 6A. Then, output voltage VA is amplified up to a set level by the insulated amplifier circuit 6A, and is converted to a digital voltage signal by means of the measurement circuit switch 7 and the A/D converter 8. This digital voltage signal is input to the digital circuit 9, where it is converted to T30Pt (%), which is an output value corresponding to this digital voltage signal. Likewise, the output value corresponding to the temperature measured by the CGR sensor 1' at this time-point (point B) is designated as T30CGR(%). The difference in the two output values (%) at this point is then d1 = T30Pt-T30CGR. If correction processing is carried out such that this difference d1 becomes 0 over the period from 30 K to 25 K, the amount of change ΔT with respect to the temperature of the converter output is: ΔT = T30Pt-T30CGR 30-25 Since this correction value changes with temperature, it is calculated as follows by the CPU 9a in the digital circuit 9. Expressing the correction value TD (%) at temperature X (K) as: TD = (X-25)(T30Pt-T30CGR) 30-25 and representing the output value measured by the CGR sensor 1' at temperature X (K) as TXCGR (%),the corrected output value TD'(%) is: TD' = TXCGR+ (X-25)(T30Pt-T30CGR) 30-25 Between 30 K and 25 K, the measurement circuit switch 7 of Fig. 1 is alternately transfered between α and β with a sampling period of about 100 ms, so that the values measured by the Pt sensor 1 and the CGR sensor 1' are read. Then, the measured value of the Pt sensor 1 is corrected by means of the correction value TD of the above expression (1) such that it coincides with the measured value of the CGR sensor 1'. Then, when in Fig. 3 temperature of 25 K (point C) at which TD'=T25CGR is reached, by transfering the measurement circuit switch 7 from α to β, subsequently measurement is performed by the CGR sensor 1' up to the vicinity of 4 K.For example, if temperature X(K) is 30 K, TD' = T30CGR+ 5 5 (T30Pt-T30CGR) =T30Pt If temperature X (K) is 25 K, TD' = T25CGR+0 = T25CGR Thus the output of the converter becomes continuous with a fixed width (from 30 K to 25 K)that is predetermined from the set temperature 30 K. Next, the correction action during temperature rise will be described with reference to Fig. 2. In the case of temperature rise, the action is the opposite of that when the temperature is falling, and an output value is output corresponding to the temperature measured by the CGR sensor 1' from 4 K to 30 K. When the output value measured by the CGR sensor 1' reaches a value corresponding to the temperature of 30 K, the measurement circuit switch 7 transfers from β to α so that the measurement is also performed by the Pt sensor 1. First of all, the isolating amplifier circuit 6B reads the output voltage 4B across these voltage terminals 5B and 5B' of CGR sensor 1' at the temperature 30 K (point B). Output voltage VB is then amplified up to a set level by the isolating amplifier circuit 6B, and a digital voltage signal is output by means of the measurement circuit switch 7 and the A/D converter 8. The digital circuit 9 inputs the digital voltage signal of the A/D converter 8 measured by the CGR sensor 1' and converts it to T30CGR (%), which is the output value corresponding to this digital voltage signal. Likewise, let the output value corresponding to the temperature measured by the Pt sensor 1 at this time point (point A) be T30Pt (%). The difference of the two output values is then d2 = T30Pt-T30CGR. The amount of change ΔT of the converter output with respect to temperature, when correction processing is performed such as to make this difference d2 have a value of 0 from 30 K to 35 K is: ΔT = T30Pt-T30CGR 35-30 The correction value changes with change of temperature, so it is calculated as follows by the CPU 9a in the digital circuit 9. The correction value TU (%) at temperature X (K)is expressed by: TU = (X-35) (T30Pt-T30CGR) 35-30 If the output value measured by the Pt sensor 1 at temperature x (K) is represented by TXPT (%),the corrected output value TU (%) is expressed by: TU' = TXPt+ (X-35) (T30Pt-T30CGR) 35-30 In the interval from 30 K to 35 K, the measurement circuit switch 7 of Fig. 1 is alternately transfered between α and β with a sampling period of about 100 ms, so that the values measured by the Pt sensor 1 and the CGR sensor 1' are read. The effect of the correction value TU of equation (1)' above is to make the measured value of the CGR sensor 1' follow i.e. coincide with the measured value of the Pt sensor. Thus, when the temperature 35 K (point D) is reached, at which in Fig.2 TU'=T35Pt, the measurement circuit switch 7 transfers from β to α, and subsequently measurement is effected by the Pt sensor 1 up to room temperature. For example, if temperature X (K) is 30 K, TU' = T30Pt+ -5(T30Pt-T30CGR) 5 =T30CGR If temperature X (K) is 35 K, TU' = T35Pt+0 =T35Pt Fig.4 is a flow chart showing the correction action in the case of falling temperature. The items shown in brackets ( ) refer to the case of rising temperature. As described above, when changing over between different sensors, temperature measurement can be performed continuously by performing a correction action. Next, another method of correction processing will be described with reference to Fig.1 and Fig.5. The difference lies in the time point of changeover of the measurement circuit switch 7 i.e. the starting time point of the correction action. The description will concentrate on the parts which are different from the correction action of the cryogenic temperature measurement equipment of the first embodiment shown in Fig.2. First of all, when the temperature is falling, an output value is output corresponding to the temperature measured by the Pt sensor 1 from room temperature down to 35 K. When the output value corresponding to the temperature measured by the Pt sensor 1 reaches the output value corresponding to 35 K, the measurement circuit switch 7 is transfered from α to β, so that measurement is also performed by means of the CGR sensor 1'. The output voltage corresponding to the temperature measured by the Pt sensor 1 at temperature 35 K (point D) is converted to a digital voltage signal by means of the isolating amplifier circuit 6A, the measurement circuit switch 7 and the A/D converter 8. The digital circuit 9 inputs this digital voltage signal and converts it to T35Pt (%), which is the output value corresponding to this digital voltage signal. As in the case of Fig. 2, the output value corresponding to the temperature measured by the CGR sensor 1' at this time point is designated as T35CGR(%). The difference in the two output values at this point is d3 = T35Pt-T35CGR. If correction action is performed such as to make this difference d3 have a value of 0 from 35 K to 25 K, the amount of change ΔT of the converter output with respect to temperature is: T = T35Pt-T35CGR 35-25 The correction value changes with change in temperature, so it is calculated as follows by the CPU 9a in the digital circuit 9. If the correction value TD2 (%) at temperature X (K) is represented by: TD2 = (X-25) (T35Pt-T35CGR) 35-25 and the output value measured by the CGR sensor 1' at temperature X (K) is TXCGR (%), the corrected output value TD2' (%) is expressed by: TD2' = TXCGR+ (X-25) (T35Pt-T35CGR) 35-25 During the interval from 35 K (point D) to 25 K (point C), the measurement circuit switch 7 of Fig.1 is transfered alternately between α and β with a sampling period of about 100 ms, so that the values measured by the Pt sensor 1 and the CGR sensor 1' are read. The measured value of the Pt sensor 1 is corrected by means of correction value TD2 obtained by the above expression (3) so that it coincides with the measured value of the CGR sensor 1'. When the temperature 25 K (point C) at which TD'=T25CGR in Fig.5 is reached, the measurement circuit switch 7 transfers from α to β, so that subsequently measurement is made by the CGR sensor 1' up to the vicinity of 4 K. For example, if temperature X (K) is 35 K, TD2' = T35CGR+ 10(T35Pt-T35CGR) 10 =T35Pt If temperature X (K) is 25 K, TD2' = T25CGR+0 = T25CGR The correction action on temperature rise will now be described. When the temperature is rising, in the opposite process to what happens when the temperature is falling, an output value is output corresponding to the temperature measured by the CGR sensor 1' from 4 K up to 25 K (point C). When the temperature measured by the CGR sensor 1' reaches a value corresponding to the temperature of 25 K (point C), the measurement circuit switch 7 transfers from β to α, and measurement is performed by the Pt sensor 1 also. First of all, the output voltage corresponding to the temperature measured by the Pt sensor 1 at the temperature of 25 K (point C) is converted to a digital voltage signal by means of the isolating amplifier circuit 6A, the measurement circuit switch 7 and the A/D converter 8. The digital circuit 9 inputs this digital voltage signal, and converts it to T25Pt (%), which is an output value corresponding to this digital voltage signal. In the same way, the output value corresponding to temperature measured by the CGR sensor 1' at this time point is designated as T25CGR (%). The difference between the two output values at this point is d4 = T25Pt-T25CGR. If a correction action is performed such that this difference d4 is 0 between 35 K and 25 K, the amount of change Δ T of the converter output with respect to temperature is: ΔT = T25Pt-T25CGR 35-25 The correction value changes with change in temperature, so it is calculated as follows by the CPU 9a of the digital circuit 9. If the correction value TU2 (%) at temperature X (k) is expressed by: TU2 = (X-35) (T25Pt-T25CGR) 35-25 and the output value measured by the Pt sensor 1 at temperature X (K) is TXPt (%), the corrected output value TU2'(%) is expressed by: TU2' = TXPt+ (X-35) (T25Pt-T25CGR) 35-25 Between 25 K and 35 K, the measurement circuit switch 7 of Fig.1 transfers alternately between α and β with a sampling period of about 100 ms, and the values measured by the Pt sensor 1 and the CGR sensor 1' are read. Thus, the measured value of the CGR sensor 1' is corrected by the means of the correction value TU2 of expression (3)' above such that it coincides with the measured value of the Pt sensor 1. Thus, when the temperature of 35 K (point D), at which TU2'=T35Pt in Fig. 5 is reached, the measurement circuit switch 7 transfers from β to α, so that subsequently measurement is effected by means of the Pt sensor 1 up to room temperature. For example, if temperature X (K) is 25 K, TU2' = T25Pt+ (25-35) (T25Pt-T25CGR) 10 = T25CGR If temperature X (K) is 35 K, TU2' = T35Pt+0 = T35Pt In the cryogenic temperature measurement equipment described above, the Pt sensor and the CGR sensor were employed as the two types of sensor, but it would also be possible to employ for example a carbon resistance sensor, platinum cobalt resistance sensor, or a thermocouple sensor. Also, in the above description, the temperature of changeover of the sensors was set to the above-mentioned temperature, but it could be set to a temperature other than this. Furthermore, it would be possible to scan over the entire temperature range.

Claims: A temperature measurement equipment for measuring temperature over a wide temperature range comprising a first measurement circuit (10) having a first temperature sensor (1) with a first temperature characteristic curve for measuring temperatures above a changeover temperature, a second measurement circuit (20) having a second temperature sensor (1') with a second temperature characteristic curve for measuring temperatures below the changeover temperature, in which different measured characteristic values are output by the two sensors at the changeover temperature, a CPU (9a), an analogue to digital converter (8) adapted to convert the characteristic signal output for processing by the CPU (9a), characterised in that , the temperature measurement equipment measures temperatures between room temperature and cryogenic temperatures, a measurement circuit switch (7) is provided which transfers between a first state (α) where the characteristic curve signals from the first measurement circuit (10) are output, and a second state (β) where characteristic signals from the second measurement circuit (20) are output, when the output is characteristic of the changeover temperature (A), said CPU is adapted to generate a further characteristic curve that connects the characteristic curve of said first temperature sensor (1) to the characteristic curve of the second temperature sensor (1'), the switch (7) outputting signals from the first measurement circuit (10) when the temperature is above the changeover temperature, the measurement circuit switch (7) being set to transfer between the first state (α) and the second state (β) to output characteristic signals from the second measurement circuit (20) in response to the temperature falling below the changeover temperature (A), the CPU calculating a falling correction value (TD) equal to the difference between a temperature (X) determined from the output characteristic signal and a low temperature (C) less than the changeover temperature times the difference in the values of the first and second characteristic curves at the changeover temperature, divided by the difference between the changeover temperature and the low temperature (C), adding the correction value (TD, TD2) to the digital characteristic value read from the second measurement circuit (20) to generate a corrected output value (TD', TD2') found on said further characteristic curve, until the temperature (X) equals the low temperature (C), and as the temperature rises to a changeover temperature equal to or above the low temperature (C), transferring the measurement circuit switch (7) to the first state (α) to output signals from said first measurement circuit (10), calculating a rising correction value (TU, TU2') equal to the difference between the temperature (X) determined from the output first characteristic signal and a high temperature (D) above the changeover temperature times the difference in the values of the first and second characteristics at the changeover temperature divided by the difference between the high temperature (D) and the changeover temperature, and correcting the digital characteristic value read from the first measurement circuit (10) by adding the correction value (TU, TU2) to generate a corrected output value (TU', TU2') found on the further characteristic curve until the temperature (X) equals the high temperature, supplying the corrected output values (TD', TD2', TU', TU2') to a digital to analogue converter (14) for display in analogue form. A cryogenic temperature measurement equipment according to claim 1 wherein the transfer temperature is equal to the high temperature (D) when the temperature is falling from above the high temperature (D) and equal to the low temperature (C) when the temperature is rising from below the low temperature (C) A cryogenic temperature measurement equipment according to any one of the preceding claims wherein the temperature measurement circuit switch (7) is set to alternate between the first state (α) and the second state (β) to alternately output characteristic signals from the first and second measurement circuits (10, 20) in response to the temperature crossing the changeover temperature. A cryogenic temperature measurement equipment according to claim 3 wherein the alternating has a sampling period of 100ms to read the values sensed by a Pt sensor (1) and a CGr sensor (1'). A method of measuring temperatures comprising the steps of: measuring a temperature above a changeover temperature using a first measurement circuit (10) having a temperature sensor (1) with a first temperature characteristic curve, measuring a temperature below the changeover temperature with a second temperature measurement circuit (20) having a temperature sensor with a second temperature characteristic curve, in which different characteristic values are output by the two sensors at the changeover temperature, converting analogue signals emitted from the measurement circuits to digital signals in an analogue to digital converter (8) for processing in a CPU (9a), characterised by the method measuring temperatures in the range between room temperature and cryogenic temperature, transfering by means of a measurement circuit switch (7) between a first state (α) where the characteristic signals from the first measurement circuit (10) are output and a second state (β) where the characteristic signals from the second circuit (20) are output to the CPU (9a) when the output is characteristic of the changeover temperature (A), generating a further characteristic curve (AC, BD) in the CPU that connects the characteristic curve of the first temperature sensor (1) to the characteristic curve of the second temperature sensor (1'), setting the switch(7) to the first state (α) ) when the temperature is above the changeover temperature, and to the second state (β) when the temperature is falling below the changeover temperature, calculating a falling correction value (TD) equal to the difference between a temperature (X) determined from the output characteristic signal and a low temperature (C) less than the changeover temperature times the difference in the values of the first and second characteristic curves at the changeover temperature, divided by the difference between the changeover temperature and the low temperature (C), adding the correction value (TD, TD2) to the digital characteristic value read from the second measurement circuit (20) to generate a corrected output value (TD', TD2') found on said further characteristic curve, until the temperature (X) equals the low temperature (C), and as the temperature rises to a changeover temperature equal to or above the low temperature (C), transferring the measurement circuit switch (7) to the first state (α) to output signals from said first measurement circuit (10), calculating a rising correction value (TU, TU2') equal to the difference between the temperature (X) determined from the output first characteristic signal and a high temperature (D) above the changeover temperature times the difference in the values of the first and second characteristics at the changeover temperature divided by the difference between the high temperature (D) and the changeover temperature, and correcting the digital characteristic value read from the first measurement circuit (10) by adding the correction value (TU, TU2) to generate a corrected output value (TU', TU2') found on the further characteristic curve until the temperature (X) equals the high temperature, supplying the corrected output values (TD', TD2', TU', TU2') to a digital to analogue converter (14) for display in analogue form.

Inventors: HAYASHI SOICHIRO, ISHII KATSUTO, OKAMURA HIROYASU, SHIMIZU MASAMI, TANAKA YOSHIO, HAYASHI, SOICHIRO, ISHII, KATSUTO, OKAMURA, HIROYASU, SHIMIZU, MASAMI, TANAKA, YOSHIO, Hayashi, Soichiro, c/o Intell. Property Division, Ishii, Katsuto, c/o Intell. Property Division, Okamura, Hiroyasu, c/o Intell. Property Division, Shimizu, Masami, c/o Intell. Property Division, Tanaka, Yoshio, c/o Intell. Property Division
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1992
Main classification: G01K7
IPC codes: G01K7