HYSTERESIS, HYSTERIA OR HISTORY FOR TYPE K THERMOCOUPLES

摘要

Type K thermocouples have for many years been told to have hysteresis in the middle temperature range 300 - 600°C. The Thermometry Laboratory at Riso National Laboratory (RNL) has for many years calibrated thermometers for Danish Industry. During these calibrations, special for customers in the heat treatment industry [1] and power plants RNL has in the latest years seen better and better type K thermocouples without the former hysteresis problems. RNL therefore set up a programme for detecting this hysteresis in new thermocouples. Some manufactures have since the beginning of the 1980's improved their type K thermocouples by different annealing processes before the thermocouples leave the factories. Riso National Laboratory has observed this annealing process as a way to minimise the hysteresis and to get better product. Hysteresis still exists, it is not history, but it is minimised at new thermocouples and can nearly be eliminated by extra annealing before calibrating. No reason to be hysteria. Former hysteresis up to 8°C in the 400 - 500°C temperature range is now reduced for some manufactures to be at the same size as the normal uncertainty for measurements for the best product. The test set up at Riso National Laboratory contains both bare wire and mineral-insulated thermocouples from several manufactures.