1. 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.
2. Description of the Related Art
Conventionally, to make measurements from room temperature of 300 K. (27.degree. C.) to the vicinity of 4 K. (-269.degree. 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.degree. C.), temperature was measured using a platinum temperature measurement resistance (Pt) sensor. Then, from 30 K. (-243.degree. C.) to 4 K. (-269.degree. C.), temperature was measured using a carbon glass resistance (CGR). Both of the above types of sensors have nonlinear resistance versus temperature 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.