A thermometer is used to measure the temperature, but a thermometer appropriate for the object to be measured and the temperature should be selected. Types of thermometers mainly include RTD and thermocouples.
To measure temperature correctly, it is important to remove various measurement error factors related to the measurement environment. For example, to prevent the body temperature from being measured immediately when the thermometer is placed aside, it is necessary that the thermometer is in sufficient contact with the object to be measured and the thermal equilibrium is maintained. If the thermal equilibrium is not adequate or there are other error factors, it cannot be said that the correct temperature is measured. When measuring at low temperatures, the thermometer may be pointed higher than the temperature of the object to be measured due to heat flowing from the outside air through the thermometer. This also contributes to the measurement error. To eliminate this error, it is necessary to insert the thermometer sufficiently into the measurement object.
Demand for temperature control of liquid hydrogen related to efficient transportation and storage of hydrogen is also expected to increase in the future in order to realize a hydrogen society. The temperature of the liquid hydrogen is-253°C. In this cryogenic environment, the inflow of heat from the thermometer as described above is also a large error factor, so measures such as making the type of conductor relatively small in heat conduction or reducing the wire diameter are taken.
At cryogenic temperatures, the output value of the thermometer may differ from the value specified in JIS standard, causing errors. It is therefore required to calibrate the thermometer and adjust the instrument prior to use. When calibrating this thermometer, a device called a cryostat must be used to achieve near the temperature of liquid hydrogen. Using the latent heat of helium, which is used as a refrigerant, the temperature in the equipment is reduced to a very low temperature. Insert a reference thermometer called a capsule-type platinum resistance thermometer and a thermometer to be calibrated into the copper block so that the temperature is uniform. Heat anchors are installed near the copper block and conductors are wound there to prevent heat from flowing in from the outside air. This kind of cryogenic environment is created, in which the output value of the thermometer to be calibrated is compared with that of the reference thermometer to perform calibration.
We conduct JCSS calibration at temperatures below 0°C to-196°C. It is necessary to realize a low temperature environment during calibration, but the method varies depending on the temperature. In the temperature range below 0°C and up to-80°C, a tank heating device using ethanol as a heating medium is used. Temperatures lower than-80°C are easily reduced to-196°C using cryostats such as the cryostats described above, and liquid nitrogen in a heat-insulating container. In both methods, a reference thermometer and a thermometer to be calibrated are inserted into a copper block with good heat conduction, and the output values are compared for calibration.
JCSS calibrated thermometers will be certified with JCSS certification symbol. This proves that the calibration is performed by a technically competent calibration provider, and that the calibration values are traceable to the national measurement standards in Japan. Calibrating the thermometer can tell you the characteristics of the thermometer, which leads to the correct temperature.
