A new type porcelain protection tube

The choice of a proper protection tube for a thermocouple is nearly as important as the selection of the material for the couple. One of the most important properties of such a tube is low porosity to gases, since furnace gases usually attack the couple.1 There are three general methods of attaining this low porosity: (a) By burning a refractory tube to a very high temperature (3000 deg. fahr.); (b) by adding a flux (such as feldspar) to a refractory body causing it to vitrify at a considerably lower temperature (2550 deg. to 2700 deg. fahr.); and (c) by coating a refractory body, which is not burned at a temperature sufficiently high to vitrify it, with an impervious glaze.