Experimental results of EMF stability research are presented for MIMS thermocouples of types K and N of external diameters 1.5; 3.0 and 5.0 mm. All thermocouples had heat resistant sheath and were not used before the tests. They were subjected to every-day cyclic heating and cooling. During each cycle there was 6 h annealing in air at the temperature 1200 ºɋ. Results shows, that stability of a cable thermocouple in air at the temperature 1200 ºɋ depends, mainly, on its diameter, instead of on thermocouple type or metal sheath material. Acceptable time of operation in a thermal cycling mode for 1.5 mm cable thermocouples is limited by (15, 16) h, and for 3 mm thermocouples it is equal to (40, 45) h. Besides, the effect of signal shunting is shown in the work for a thermocouple under heating of its average part to the temperature 1200 ºɋ. Experiment has shown that use of long cable thermocouple of 1.5 mm diameter at the temperatures above 1000ºɋ can lead to serious errors in temperature measurement. Heating of 400 mm thermocouple length to 1100 ºɋ causes positive EMF in the 1.5 mm cable thermocouple equivalent to +3.4 ºC and +18 ºC at 1200ºC. For 3 mm cable thermocouple EMF at 1200 ºɋ reaches the value equivalent to +2.3 ºC, but double increase in heating length makes the value greater to +13.5 ºC. The given facts are necessary to consider while using of thin (1, 3) mm cable thermocouples at the temperatures (1100, 1200) ºɋ.
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