How to Select Thermocouple Type and Placement for a Heating Plate Subjected to Rapid Thermal Cycling?

A heating plate that goes from 50°C to 250°C in seconds and back again needs a sensor that can keep up. The type of thermocouple and where exactly it is in the plate decide whether the controller is ahead or behind the thermal process. Slow temperature sensors result in overshoot, undershoot and long settling times in applications where rapid thermal cycling is required such as semiconductor wafer testing, response injection moulding or accelerated materials ageing. Proper thermocouple selection and placement can turn an unpredictable operation into a tightly controlled one.

Why Response Time Is Important in Rapid Cycling
whether a heating plate is directed to change temperature rapidly the control loop uses the measured temperature to decide whether to cut the power or apply cooling. If the thermocouple reads a temperature significantly out of phase with the actual working surface temperature, the controller will continue heating after the setpoint is reached (overshoot) or begin cooling prematurely (undershoot). In the worst situation the plate temperature can exhibit an oscillation about the setpoint without ever settling.

The important parameter is the sensor time constant - the time it takes the sensor to reach 63.2% of a step change in temperature. If the plate is ramping fast (say 10 to 20 °C per second) then a sensor with a 3 second time constant will lag by tens of degrees. A sensor with a time constant of 0.3 seconds follows the actual surface considerably closer.

The junction type is as crucial as the kind of thermocouple in press-instrumentation technique. The selection of a thermocouple type quick cycling heating plate placement should be based on fast thermal response, mechanical robustness and electrical isolation, if needed.

Types of Thermocouple Junctions: Ungrounded, Grounded and Exposed
There are three common junction styles for sheathed thermocouples. They each respond differently to heat and electricity.

Junction StyleDescriptionTime Constant (standard 6 mm probe)Electrical Isolation Best For Grounded Wires welded to inside of sheath tip; junction in direct contact with sheath 0.1 – 0.5 sNone (sheath at signal common)Fastest responsiveness, where electrical noise is not an issue
Ungrounded wires welded together but insulated from sheath by MgO; junction imbedded in compacted powder 2 – 4 times slower than grounded (0.5 – 2 seconds)very good (ground sheath can be separated)Slow cycling required with electrical isolation
Exposed The junction is outside the sheath and is directly exposed to the medium <0.1 secPoor (can short exposed wire)Gas temperature measurement ; not used inside metal plates
For a heating plate that is exposed to rapid thermal cycling the proper choice is nearly always a grounded junction thermocouple. The direct metal-to-metal contact between the thermocouple wires and the sheath assures the fastest possible heat transmission from the plate material to the sensor junction. But if the sheath touches the plate (as it is supposed to), the thermocouple signal is at the same ground potential as the plate, if there is no electrical isolation. This can cause ground loops if the input to the controller is not well separated. Grounded junctions are safe and effective with most modern PID controllers and data collecting systems that feature isolated inputs.

If the junction is not grounded, it will be much slower, typically 2 to 4 times the time constant of a grounded probe of the same diameter. This additional delay can destabilise the control loop for fast cycling. Ungrounded junctions are best reserved for situations where electrical isolation is a must (e.g., high-frequency induction heating) and where cycling speeds are moderate.

Kind of Thermocouple: K, J or Other?
kind K (Chromel-Alumel) is the most commonly used kind to heat plates in the range of 50°C to 400°C. It has good precision (±2.2°C or ±0.75%), great oxidation resistance and cheap cost. Available are grounded junction type K thermocouple assemblies with tiny diameter sheaths.

Another possibility is Type J (Iron-Constantan). It has a greater thermoelectric voltage (about 5% more output per degree than Type K at 200°C), which may allow a somewhat faster electrical response in some controllers due of a better signal-to-noise ratio. However, Type J has a lower maximum temperature (approximately 750°C compared to 1200°C for Type K) and the iron leg is prone to oxidation, thus it is not as long-lived in cycling applications. Type K is suggested for the majority of rapid-cycling plates because of its reliability and extensive availability.

Type T (Copper–Constantan) can be considered for cryogenic cycles (from -50°C up to high temperatures) but has an upper limit of about 350°C.

Placement: Close to the Working Surface Is Everything
The greatest thermocouple in the world is useless if you stick it too far away from the surface that actually touches the workpiece. The heat is to be transferred from the heating elements through the thickness of the plate to the working surface and finally to the part. The sensor should be placed as close as feasible to that working surface to sense the temperature the part sees.

Placement Suggested method:

Drill a blind hole, starting from the back of the heating plate, to within 1-2 mm of the working face.

Insert the thermocouple (grounded junction, small diameter - normally 1.5 mm to 3 mm) into the hole so that the tip touches the bottom of the hole.

Fill the remaining annular space with a high temperature thermal conductive compound (e.g., boron nitride paste, silver filled grease, or thermally conductive epoxy rated for the maximum working temperature). This reduces air gaps which are otherwise thermal insulators.

Hold the thermocouple in place with a spring loaded compression fitting or a small retention clip to keep it in touch when the plate expands and contracts.

If the sensor is placed too deep, say near the heating components or half way through the thickness of the plate, then there is a thermal lag introduced that is proportional to the distance from the surface. In quick heating a temperature gradient of 5–10°C can be observed from the side of the heater to the working side of a 20 mm thick plate. If a sensor were placed 5 mm from the heaters it would sense a much hotter temperature than the working surface and the controller would turn off the heat too early and leave a cold working face.

A sensor that takes 0.3 seconds to respond instead of 3 seconds makes all the difference." The controller reads the true surface temperature almost quickly, as a fast grounded junction is located 1.5 mm away from the working surface. Overshoot can be controlled to 1-2°C A sluggish ungrounded sensor at 10 mm depth typically exhibits overshoot of 15-20°C during a quick ramp.

Additional Considerations for Reliable Operation 
Thermocouple Size
Smaller diameter probes have less thermal mass and respond faster. A 1.5 mm diameter grounded thermocouple has a time constant around one-third that of a 6 mm probe. But little probes are mechanically brittle. A 3 mm diameter probe is a reasonable compromise of speed and durability for a heating plate that expands and contracts thermally often.

Guarding Against Shear and vibration
The thermocouple lead wire comes out the back of the plate and needs to be strain alleviated. Rapid temperature cycling causes the plate to expand and compress which can flex the lead wire at the entry location.