Thermocouples immersion depth

Couples Purposely Insufficiently Immersed-— It is frequently desirable purposely to immerse the couple to an insufficient depth. In many processes the furnace is operated at such a high temperature that a thermocouple or protecting tube cannot withstand the severe conditions to which it may be subjected. In this case the couple may be immersed only part way through the furnace wall, or to a distance flush with the inner wall of the furnace. The temperatures indicated by couples installed in this manner are always lower than those of the furnace interior, but they bear a fairly definite relation to the temperature of the furnace, and hence the method is satisfactory for temperature control and reproduction of furnace conditions from day to day. 

The inability to control precisely the reference junction temperature or to obtain accurate compensation would also affect thermocouple reliability. In the case of an ice bath, factors that can cause the junction temperature to depart from 0°C include non uniformity in the temperature of the ice-water mixture, small depth of immersion, insufficient ice, and large wire sizes (conduction effects). Reference 46 describes various sources of errors in an ice bath. 

Melt thermocouples are also used. These devices are made to contact the actual flowing polymer. This information can be the most useful temperature data from an extruder because it is the polymer temperature that is most important, not the metal temperature. One common type is a flush-mounted probe that has a sensor diaphragm mounted along the inside barrel surface, usually at the head end of the barrel. This type only reads the temperature of the melt in contact with the barrel, usually the same temperature as the barrel itself. Because the temperature of the melt away from the barrel may differ by over 30 °F from the surface temperature, some extruders have a more useful immersion thermocouple that penetrates further into the melt stream. This type can be designed to traverse to various depths. 

Thermocouple, copper-constantan foil type, 0.0005 in. (0.013 mm) thickness, and precision potentiometer. The thermocouple is calibrated by immersing to a depth of 1 in. (25 mm) in a circulating liquid thermostat and comparing with a thermometer of known accuracy. 

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