Material Selection for High-Temperature Sensors and Thermocouple Protection Tubes

Whether the temperature measurement point was chosen correctly is often appreciated only during an incident review. The accuracy of the thermocouple is only one link in the chain: if the protection tube softens at high temperature, cracks or is attacked by the melt, the reading will drift and may even lead the control loop to take the wrong action.

Service Ranges of Three Common Tube Materials

Alumina, molybdenum and tungsten are the tube materials most often mentioned for high-temperature measurement, and each suits a different atmosphere and temperature range.

Inspection and judgment basis for the above requirements: electrode products are controlled under YS/T 1312-2019 Molybdenum Electrodes, molybdenum rod follows ASTM B387 / GB/T 17792, molybdenum plate and discs follow ASTM B386 / GB/T 3876, and a certificate of quality is supplied with each shipment, with the lot traceable.

Choosing for Vacuum and Inert Atmospheres

In the high-temperature zone of vacuum and atmosphere furnaces, molybdenum tubes are used because of their high melting point and low vapor pressure; at higher temperatures or where space allows, tungsten tubes are another option. Alumina tubes may change in gas permeability in vacuum, so the heating rate must be controlled. When selecting a material, the residual gas in the furnace, the heating curve and the cooling method after shutdown must be considered together.

The installation method is often overlooked. If the seal between the protection tube and the furnace body relies only on asbestos rope, it may loosen after heating, outside air enters the tube and the molybdenum tube begins to oxidize from a local point. The materials of the flange, thread and seal ring must be selected by temperature, and the tightening sequence should be written into the work instruction.

Corrosion Resistance in Melts

When measuring temperature in a metal melt or glass melt, corrosion resistance becomes the main issue. Molybdenum has relatively low solubility in some metal melts and can be used for intermittent measurement of specific melts, but it oxidizes rapidly in oxygen-containing environments. Before use, confirm the melt composition, temperature, immersion time and protective gas; experience from intermittent service cannot be carried directly into continuous operation.

The machining quality of the tube affects service life. Uneven wall thickness creates local hot spots at high temperature, and burrs at the tube mouth may damage the insulation of the thermocouple wires. On arrival, beyond dimensions, check whether the inner wall is smooth and whether longitudinal cracks or excessive ovality are present.

Response Speed and Wall Thickness

The thicker the wall, the better the mechanical strength and the slower the thermal response. Where fast temperature control is required, a balance must be struck between the two, using thin-walled tubes, a shorter insertion depth or an exposed thermocouple for short-duration measurement where necessary. Response speed also depends on the gap between the protection tube and the thermocouple wires and on the filler material. The match between tube length and furnace depth must also be confirmed: too long and it hits the furnace bottom, too short and it cannot measure the true temperature.

Judging a failed part also requires a method. A brittle fracture, local wall thinning and discoloration of the inner wall do not have the same cause; keeping the replaced tube and recording the service conditions builds more experience than simply fitting a new one.

Selection Checklist

• Does the temperature limit and fluctuation range leave sufficient margin;
• Is the atmosphere composition compatible with the tube material, and are there alternating exposures;
• Are the insertion depth, support method and vibration conditions clearly defined;
• Do the replacement cycle and spare-part stock match the maintenance schedule;
• Is there a reference measurement or a failure case for comparison.

The value of a protection tube is to keep measurement continuously credible, not to chase a single high-temperature number. State the service conditions clearly first, then discuss the tube material and structure; the judgment will be far more dependable.