Duty temperature decides the construction before anything else does. At 150 °c to 200 °c, ntc thermistor sensors is a candidate; the sections below set out what it will hold, what has to be true for it to hold it, and what fails first when it does not.
What the 150 °C to 200 °C Band Requires
The top of the flexible polymer range. Silicone rubber and polyimide both run continuously at 200 °C; above it both degrade, and no amount of derating changes that.
| Band | 150 °C to 200 °C |
|---|---|
| This construction, continuous | 200 °C |
This band reaches the ceiling of the construction. At 200 °C there is no headroom left, so the control has to hold the set point rather than approach it from above, and the limit device has to be set where it can act before the insulation does. A duty that spends its time at the top of the band is a duty that should be quoted against the next construction up.
Why the Boundary Is Where It Is
200 °C is the top of the flexible polymer range. Silicone rubber and polyimide both run there continuously, and both degrade above it — silicone hardens and loses its tear strength, polyimide embrittles. No derating changes either.
Cross this line and the construction changes completely: fired thick film on stainless or alumina, or a mica-insulated element. Those parts are rigid, clamped and made to a different drawing, so the boundary is a redesign rather than a substitution.
What an NTC Thermistor Sensors Is
The 10K B3950 thermistor is the default sensor on a low voltage heater: cheap, sensitive, and readable by any microcontroller with one resistor.
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 | 200 °C |
The continuous limit of 200 °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.
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.
Other Constructions That Hold 150 °C to 200 °C
Constructions in this catalogue that will hold 150 °c to 200 °c continuously:
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.
NTC Thermistor Sensors — 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 | Negative temperature coefficient thermistor | Resistance falls as temperature rises, steeply |
| Standard part | 10 ku03a9 at 25 u00b0C, B25/50 = 3950 K | The value nearly every controller expects by default |
| Other values | 5 ku03a9, 50 ku03a9, 100 ku03a9; B values 3435 K to 4100 K | Chosen to match the controller, not the heater |
| Tolerance | u00b11% resistance, u00b11% B value | u00b10.5 u00b0C interchangeable parts available |
| Temperature range | -40 u00b0C to +200 u00b0C depending on encapsulation | Epoxy bead to +125 u00b0C, glass bead to +200 u00b0C |
| Response time | 1 to 10 seconds depending on mass and mounting | Mounting dominates: a sensor in air reads air |
| Encapsulation | Epoxy bead, glass bead, or laminated into the heater | Laminated is what makes it read the element |
| Leads | PTFE or silicone insulated, to your length and termination |
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.
