Thermister electrical resistance values can range from 100 to 1,000,000 ohms at 25°C. The resistance change is approximately -4.4% per °C at 25°C. The large resistance change limits thermistors to narrow temperature ranges and low temperatures. However, in these situations, thermistors have greater accuracy and ambient temperature stability than RTD's or thermocouples. Lead wire can be as long as 1,000 feet without significant error.
Platinum RTD's electrical resistance values change approximately .385% per °C. The DIN standard element is 100 ohms at 0°C, showing a nearly linear change of +38.5 ohms between 0 and 100°C. Platinum RTD's provide very good system accuracy at very low temperatures to 1000°F. Two wire RTD's have significant errors when over 10 feet long. However, these errors can be eliminated by using 4 wire RTD's.
Thermocouples are constructed of two wires of dissimilar metals that generate .05 MV per °C at the "hot" junction. The "cold" junction must be maintained temperature constant by electronic means to avoid signal errors. Low signals and linear complexities make system accuracy and integrity difficult. However, thermocouples respond to temperature changes very quickly, are virtually impervious to vibration or other mechanical conditions, have a wide range and high temperature capability.
FOOD, DAIRY, BEVERAGE & PHARMACEUTICAL SENSORS INDEX
INDUSTRIAL THERMOCOUPLES INDEX
