Thermocouples: what are their problems in use and prevention?

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Category: Metallurgy

Subject: Thermocouples: what are their problems in use and prevention?

Question

What problems should a user be aware of when selecting a thermocouple for use in a reducing atmosphere?

Answer

Using precious metal thermocouples in a reducing atmosphere or in a vacuum can be successful but does require extra caution.

Problems arise because otherwise inert oxides may be reduced or decomposed to elements that readily alloy with the thermocouple limbs and cause output drift, embrittlement and premature melting.

Also, a vacuum can produce metal vapour from the charge which condenses on the thermocouple and may cause similar problems.

Ideally all low grade or contaminated ceramics and fittings within the furnace would be replaced but this is not generally practical and does not prevent problems arising from the furnace charge.

The practical approach is to prevent contaminants reaching the thermocouple limbs using layers of high purity recrystallised alumina (RA) ceramic or precious metal sheathing.

It is easier to seal metal sheathed couples to prevent loss of the furnace atmosphere but RA forms a more effective if

less flexible diffusion barrier. Unfortunately too many protective layers reduce the accuracy and response rate of the thermocouple.

The problem is not the direct effect of hydrogen but the indirect effect of harmful contamination released in a reducing atmosphere or in a vacuum. The contamination must not reach the thermocouple.

Answer posted October 2004

Submitted by: Ms Susan Ashton

Affiliation: Johnson Matthey

Answered by: Roger Wilkinson

Affiliation: Johnson Matthey Noble Metals