The resistance value of metal materials are changing with temperature, but to be used to make resistance thermometer, the material has certain requirements:.
1) the need for a high resistance temperature coefficient in the temperature range, that is, when the temperature changes 1 ℃, the change in resistance value is large, can make the sensitivity higher, and the best resistance value of the relationship with the temperature is linear.
2) high chemical stability, easy to purify and replicate.
3) at a certain temperature, the unit length, unit cross-section of the wire resistance value is larger (i.e., larger than the resistance), in order to make the thermometer volume can be smaller.
4) cheap. Practice has proved: suitable for making RTD elements of materials such as platinum, copper, iron, nickel, etc..
Platinum is a more ideal material, it is chemically stable, easy to get high-purity platinum wire, larger than the resistance, high accuracy of temperature measurement, resistance and temperature in the range of -200 ~ 0 ℃ into an approximate linear relationship. Although copper has the advantages of cheap, high resistance temperature coefficient, but the temperature measurement range than platinum is small, generally between -50 ~ 150 ℃, the specific resistance is smaller, large volume, low sensitivity. Although iron, nickel and other high resistance temperature coefficient, but the relationship between resistance and temperature is not linear, and difficult to purify, generally rarely used. Therefore, the measurement of low temperature commonly used platinum resistance thermometer. Compared with thermocouples, it has a higher sensitivity, accuracy at low temperatures, does not require cold-end temperature compensation, easy to transmit far, centralized detection, so in the small air separation unit in the cold box almost all use platinum resistance thermometer.
