High-temperature strain gages for use in sodium environments

Abstract

High-temperature long-term static strain measurements have, for many years, been a problem to the experimental stress analyst. Strain gages operating in environments from 600–1000° F over extended periods of time have varying characteristics as to temperature compensation, gage-factor shifts and, also, drift. The weldable-type high-temperature gage offers a great deal of promise, considering the ease of application to nonlaboratory-type structures. In addition, the Microdot-type weldable gage offers additional protection against most environmental conditions.

In the high-temperature range, the Microdot half bridge containing a platinum-tungsten alloy encased in magnesium oxide and protected by an outer shell of Inconel-X was evaluated. One of two major difficulties encountered with this gage is the silver-gold alloy used for brazing all of the connections that provide a sealed gage element. This alloy was a difficulty because the gages were to be used in a sodium environment which easily dissolves the silver-gold elements. The other difficulty is the highly individual characteristics of the gages. This paper describes two significant developments made in adapting this gage for use in sodium environments at high temperatures. These are:

• 1. A calibrating technique whereby the bonded-gage characteristics of each gage can be obtained prior to actually bonding the gage to the structure under test

• 2. Techniques for protecting the gage elements for varying periods of time from the sodium environment

Included in the paper is not only the description of the techniques used, but also the various tests conducted and the results that were obtained.