The 80 °c to 120 °c band is a material boundary, not a convention. Battery operating windows, incubators, fluid warming and display heating. The upper limit of PET film, and the band where a low-cost printed heater stops being a candidate. This page covers what that means for type k and type j thermocouples, which is rated to 400 °C continuous.
What the 80 °C to 120 °C Band Requires
Battery operating windows, incubators, fluid warming and display heating. The upper limit of PET film, and the band where a low-cost printed heater stops being a candidate.
| Band | 80 °C to 120 °C |
|---|---|
| This construction, continuous | 400 °C |
The band sits comfortably inside the 400 °C ceiling, which is where a heater should be specified: the margin between the working temperature and the insulation limit is what absorbs a poor thermal path, a warm ambient and a control overshoot without shortening the life of the part.
Why the Boundary Is Where It Is
120 °C is where PET film stops. The polyester softens from about 130 °C and loses its dielectric strength as it softens, so the margin between working temperature and failure is thin — and a failure here is an electrical one, not a cosmetic one.
This is therefore the last band in which the cheapest thin heater is a candidate. Above it, the construction changes to silicone or polyimide, and the cost per square metre changes with it.
What a Type K and Type J Thermocouples Is
For temperatures beyond the range of any resistance sensor, and for the fast, robust measurement a thick film or mica heater needs.
A sensing component, specified by its tolerance class, its usable range and its response time. It has no watt density and no outline; it is fitted to a heater or to the surface the heater is warming, and where it is fitted decides what the control loop is actually controlling.
The heat leaves through whatever the component is clamped or bonded to; a sensing component adds no heat of its own and simply has to be in thermal contact with the thing it is reporting on.
| Total thickness | Component |
|---|---|
| Maximum size, one piece | Component |
| Continuous working temperature | 400 °C |
The continuous limit of 400 °C is set by the insulation system. The element alloy would run far hotter; it is never the element that decides.
Which Temperature Is Being Held
Three temperatures matter and they are not the same number: the element temperature, the temperature of the surface being heated, and the ambient. A heater rated for a continuous working temperature is rated at the element — the hottest point in the assembly, and the one nothing on the outside is measuring.
The gap between element and surface is the thermal path. Bonded to metal it may be a few degrees; across an air gap, a void in the adhesive or a dry clamped joint it can be tens of degrees, and every one of them appears at the element rather than at the load.
Attachment and Thermal Contact
A sensing component is fitted where it will read the temperature that matters: against the element for a limit device, against the load for a control device. The two positions give different numbers, and choosing the wrong one is the commonest reason a correctly built loop holds the wrong temperature.
A sensor reads its own junction, not the surface it is near. Bed it in thermal compound, clamp it, or pot it into the assembly: an air gap of a tenth of a millimetre between a thermistor and the plate it is supposed to be measuring is worth several degrees of error, and it is an error the controller cannot see.
Control and Protection
This heater has no way of limiting its own temperature. Resistance heating draws roughly the same power whatever the temperature, so with the load removed, the fluid gone, the fan stopped or the sensor open circuit, the element keeps producing watts until something stops it.
The design must therefore contain a control device and, separately, a protective one: a controller with a sensor to hold the set point, and a bimetal thermostat or a one-shot thermal cutoff wired in series with the element to open the circuit if the loop ever fails. The two are not interchangeable — a controller that has lost its sensor cannot protect anything, which is exactly the fault the cutoff exists for.
How This Construction Fails
- Loss of the control loop. An open-circuit sensor, a welded relay contact or a controller in manual leaves the element at full power with nothing watching it — the case the series thermal cutoff exists for, and the reason it must not be omitted to save a part.
- Measuring the wrong thing. A sensor in the air near a heated plate reports the air, and the loop holds the air where the specification wanted the plate. Nothing in the instrument can detect this.
Where This Construction Is the Wrong Answer
- Do not plan on running above 400 °C. That ceiling belongs to the insulation system, not to the element alloy, and it is not a figure that derating extends — past it the insulation degrades and the dielectric strength goes with it.
- Do not rate this component as though it were the heater. It carries a contact rating or a tolerance class; the heater next to it decides the power, the watt density and the temperature the assembly reaches.
How to Specify One
This is a made-to-drawing part, so there is no catalogue number to quote. A quotation is built from the following, and the more of it that arrives with the enquiry the fewer rounds it takes:
- The temperature to be sensed or switched, and the tolerance the application actually needs.
- Where the component sits — against the element, in the potting, on the load, or in the airflow.
- The electrical duty: for a switching device, the voltage, the current and whether the load is AC or DC.
- Lead length, insulation and termination, and whether the component is fitted by us into a heater assembly or supplied loose.
- The standard the end product is certified to, so the component chosen is one that fits inside that approval.
Lead time: Samples 7-10 days from approved drawing; production 15-20 days.
Type K and Type J Thermocouples — Capability Table
These are process limits for the construction, not the specification of one stocked item. Every line is what the factory can hold; what your part holds is fixed on your drawing.
| Parameter | Specification | Limit / note |
|---|---|---|
| Type K | Nickel-chromium / nickel-alumel, -200 u00b0C to +1200 u00b0C | The general purpose thermocouple |
| Type J | Iron / constantan, 0 u00b0C to +750 u00b0C | Higher output than K, but the iron leg corrodes in moist air |
| Standard | IEC 60584 class 1 or class 2 | Class 1 is u00b11.5 u00b0C or u00b10.004|t| for type K |
| Junction | Grounded, ungrounded or exposed | Ungrounded avoids earth loops; exposed is fastest and least protected |
| Response time | Under 1 second exposed, 2 to 10 seconds sheathed | |
| Cold junction | Compensation required at the instrument | A thermocouple measures a difference, not a temperature u2014 this is where most errors come from |
| Extension cable | Matching type, correct polarity | Copper extension wire silently destroys the measurement |
| Sheath | 304 or 316 stainless, 1 mm to 6 mm diameter |
Figures are the published tolerance and rating bands of the standard sensor and control components fitted to these heaters. Sensor tolerance classes follow IEC 60751 for RTDs and IEC 60584 for thermocouples; thermostat and thermal cutoff ratings are the component manufacturers' own.
