By Roger Hornberger, General Manager, Tempil, an Illinois Tool Works Company, South Plainfield, N.J.

Marks from phase-change materials can be made on workpieces, and change color when exact temperatures are reached.

Phase change materials can be used to indicate temperature changes on a variety of parts and metals.

Simple methods often are the best way to measure surface temperatures, and the use of temperature indicating products based on the absolute, predictable phase-change (solid to liquid) of a pure chemical compound presents one of the most straightforward and trouble-free methods to confirm that the desired – and correct – temperatures are reached in industrial applications.

The melting point of a chemical compound – the temperature at which phase-change occurs – is a physical property. Nothing affects it, and it occurs, simply and reliably, when the exact temperature is reached, and it never needs recalibration.

Phase-change temperature indicators are not influenced by external factors such as static electricity, ionized air, time, and humidity, or by being dropped on the floor. Those external influences could cause electrical and electronic instrumentation to function erratically.

Phase-change temperature indicators are available to indicate more than 100 exact temperatures that range from 100-degrees to 2500-degrees F.

Such indicators are available in a range of identifying colors, and are designed so that the dry, opaque mark changes to a distinct melted appearance to indicate that the indicator’s temperature rating has been reached.

Other phase-change temperature indicators include pellets that are placed on or inside a work piece, liquids that are painted on the work piece. Pellets and liquids indicate their temperature rating by changing from a solid or dry appearance to a melted or wet appearance .Temperature indicating liquids are made of phase-change materials that are suspended in an inert, volatile, non-flammable vehicle, and are used to mark polished metals, glass and other smooth or shiny surfaces that are otherwise difficult to mark..

Temperature indicating labels also can be used to monitor temperature exposure and to provide the means for rapid data acquisition. These labels are coated with temperature indicating material or several temperature indicating materials, and are are affixed to a component to indicate exposure to varying temperatures. The use of multiple temperature indicating materials enable the user to recognize an increases in temperature by observing a white “bulls eye” that turns black if a specific temperature is reached.

The labels do not require wires or readout boxes, and provide the user with temperature measuring accuracies to ± 1 percent. They are particularly useful for measuring temperatures in isolated systems, on moving or rotating assemblies, or where other recording instruments are impractical. Applications for such labels include monitoring operating conditions of electronic parts, transformers, resistors and circuit board components.

The accuracy of phase-change materials for surface temperature indication usually is within 1 percent. This accuracy is available because the stick, label or liquid is in direct contact with the work piece, giving the phase-change indicator thermal equilibrium with the surface that is being monitored.

A phase-change indicator does not tell the exact temperature as a thermometer does, but it exactly determine when a pre-set minimum or maximum temperature has been reached.

A variety of applications
Phase-change materials are used to measure temperatures that equipment is exposed to on the job in applications that vary from racecar engines to electronic components to space shuttles.

These indicators are particularly useful in applications in which the critical surface is moving, rotating, or inaccessible, and they are used to determine temperatures for diagnostic work and warranty information.

They also can be used to measure the temperature of preheat and interpass temperatures in welding, and post-weld heat treatment temperatures, cool-down temperatures and annealing temperatures. They can be used in manual and semi-automatic welding applications.

Accurate determination of preheat temperature
It almost always required or recommended that base metals be preheated for welding, and temperature indicators are used to ensure that the recommended level of preheat is achieved. Bringing the base metal up to heat before welding reduces the danger of crack formation after welding, and promotes ductility, reduces brittleness, increases toughness of the metal. It also improves controllability of the weld, reduces the heat-affected zones next to the weld area, and reduces distortion of surrounding surfaces. Preheating also increases diffusion of hydrogen from the metal to limit weld contamination, and vaporizes moisture that may be present.

The most common method used in welding is automatic, electric resistance preheating. However, the time required to preheat work is not exact, and depends on such factors as the thickness of the metal, making it necessary for welders to check from time to time to see how preheat is proceeding.

A calibrated phase-change indicator can be used to mark the work piece before heating begins, or it can be stroked on the piece as pre-heating proceeds. When the indicator’s rated temperature is reached, the mark undergoes a phase change to a distinct appearance.

Temperature indicating pellets that are be placed on the surface of the work piece are used occasionally for preheat indication, but they are used more typically in applications that require higher-temperatures or prolonged heat-treating.

Because the temperature indicator is in immediate contact with the surface to be tested, the phase change is instantaneous when the desired temperature is reached.

These indicators do not require set-up time, and they do not require extensive operator training or experience. It is only necessary for the operator to check that the dry mark has melted.

Obtaining correct annealing temperature
Work pieces may need to be stress-relieved to an appropriate ductile condition by annealing – a process of controlled reheating that returns metal that is over-hardened by the welding process to the approximate hardness of the rest of the material.

To do this, the piece is repositioned, and the welding machine is used in what is, essentially, a manual operation. The heat in-put to the part is varied until the correct annealing temperature is reached.

Correct temperature is extremely important in annealing. If the temperature is too high, it could compromise the strength of the weld. If the temperature is not high enough, annealing does not occur, and the welded part remains over-hardened and brittle.

Less obvious applications
Besides such conventional applications, engineers and mechanics have found dozens of less obvious uses for temperature indicating sticks and liquids. Some examples are :

A large-scale distributor of heavy construction equipment uses temperature indicating sticks in diesel engine diagnosis and troubleshooting. The indicators are used to diagnose performance and problems in components such as the exhaust manifold, cylinder injectors, turbocharger and individual cylinder heads. They also are used for warranty diagnosis.
Similar diagnostic work – some of it actually performed in the pit – is done on engines for racecars and motorcycles. Many airlines equip aircraft engines with temperature indicating labels to collect maintenance and diagnostic data about brake temperatures.
NASA uses phase-change labels as backups to their high-tech instrumentation for monitoring inside skin temperature on orbiting vehicles. The labels are used to confirm thermocouple readings because phase-change is not affected by outside interference:

For additional information, contact Tempil, an Illinois Tool Works Company, 2901 Hamilton Blvd., South Plainfield, NJ 07080. Telephone: 1-800-757-8301, e-mail: tempil@tempil.com; or visit www.tempil.com.