Clay tubes are sometimes used in large annealing furnaces where they are cemented into place, forming a sort of well for the insertion of the thermo-couple. They are also used with portable thermo-couples for obtaining the temperatures of molten iron and steel in ladles. Used in this way they are naturally short-lived, but seem the best for this purpose.
FIG. 135.—Straight thermocouple and guard.
Corundite tubes are used as an outer protection for both the Marquardt mass and the silica tubes for kilns and for glass furnaces. Graphite tubes are also used in some cases for outer protections.
Calorized tubes are wrought-iron pipe treated with aluminum vapor which often doubles or even triples the life of the tube at high temperature.
These tubes come in different sizes and lengths depending on the uses for which they are intended. Heavy protecting outer tubes may be only 1 in. in inside diameter and as much as 3 in. outside diameter, while the inner tubes, such as the Marquardt mass and silica tubes are usually about ¾ in. outside and 3/8 in. inside diameter. The length varies from 12 to 48 in. in most cases.
Special terminal heads are provided, with brass binding posts for electrical connections, and with provisions for water cooling when necessary.
APPENDIX
TABLE 32.—Temperature Conversion Tables.
TABLE 33.—Comparison Between Degrees Centigrade and Degrees Fahrenheit.