Resistance Thermometer Sensors
Resistance Thermometer Sensor Measurement Principle
Resistance Thermometer Sensor are based on the characteristic that the electric resistance value of metal, semiconductor, etc. changes with temperature. For metals, platinum, nickel or copper is used and a characteristic where when the temperature increases, the resistance value increases is used. As an industrial usage, platinum resistance thermometer sensors whose use temperature range is wide and the resistance temperature factor is large are used most widely. Figure-1 shows the representative temperature - resistance value characteristic. The current JIS C 1604 adopts R100/R0 = 1.3851, the resistance ratio between 100℃ and 0℃, conformed to the international standard (IEC 60751); however, previous Japanese original standard uses R100/R0 = 1.3916 and currently some products have adopted it.
Figure-1 Platinum Resistance Thermometer Sensor Resistance Value Characteristic
The present latest international standard is IEC60751-2008 whose content greatly differs from conventional standards. It was reflected also on the JIS C 1604 standard in 2013.
Thermistors measure the resistance change as temperature using semiconductors. They cannot be used for a wide range of temperatures since the resistance change per 1℃ is large. They are rarely used for industrial purposes but used for consumer usage in many cases.
Since the resistance change is not output without conversion, to measure the resistance values, the resistance value measurement method which uses a bridge or the potentiometric method where the resistance change is replaced with a voltage change with a constant current source is used. There are three ways to connect conducting wires when the resistance is measured as follows. They have the following features which correspond to the wiring diagrams sequentially starting from top to bottom
2-wire type: Since the conducting wire resistance value is added to the resistance value, the conducting wire resistance must be decreased or must be known in advance. This type is rarely used except for the case of a relatively high resistance.
3-wire type: Since the variation of three conducting wires resistance affects the accuracy, you must be careful with long distance transmission. This type is generally used in many cases.
4-wire type: Since the conducting wire resistance does not affect the accuracy, this type is used for measurement with high accuracy. Generally a constant current is sent and the resistance value is measured with the potential difference.