Stainless Steel-Cased Fixed-Point Cells and Their Applications

Fused silica has been used as an envelope material for fixed-point cells for many years. It has been proven to be an excellent material because no contamination to the pure metal of fixed-point cells has ever been found from fused silica. In the past, fixed-points were mainly used in national laboratories. Now the situation has changed greatly, and more and more calibration laboratories have adopted the use of fixed-points in their work. These laboratories desire a stronger material, such as stainless steel (SS), instead of fused silica, for the outer case material of fixed-point cells. Several SS–cased fixed-point cells for the freezing points of indium, tin, and zinc were built and tested. New SS-cased fixed-point cells were compared against traditional fused silica–cased cells. The long-term stabilities of the new cells were checked, and no obvious drift was detected. The expanded uncertainties were estimated, which are small enough for the calibration of most standard platinum resistance thermometers. Applications of the new SS-cased fixed-point cells for secondary level and industrial calibrations are discussed.