Same heater, three numbers
Three suppliers describe what appears to be the same product. One quotes a surface temperature figure measured on the outer shell, one on the heater carrier beneath it, one at the moment the controller cuts power. The three numbers differ by more than any plausible difference between the products, and nobody in the chain has lied. They measured different things, and the specification sheet recorded only the result.
This is worth stating plainly because the usual explanation, that sellers inflate numbers to win listings, is both unprovable and unhelpful. The mechanism is more ordinary: a temperature exists only in relation to a point, a moment and a surrounding condition, and a number copied out of that context keeps none of it. Buyers can neutralise the mechanism without impugning anyone.
Mechanism one: where the probe sits
The outer shell and the heating element are separated by an air gap, an adhesive layer and, in some designs, a carrier film. A thermocouple on the shell reads the surface the hand touches. A thermocouple bonded to the element reads the source. Between them sits the entire thermal design, and the gap exists precisely so that the two numbers are not the same.
The same applies across a single face. The centre of a flat face and the crown of a curved shell have different contact with the probe and different distance from the element. A figure measured at the hottest accessible point describes that point, not the face, and a figure measured at the centre of a dual-side unit may describe only one of the two faces. The measurement point note sets out the vocabulary for stating this properly, and the uniformity map is what turns a single reading into a description of the whole surface.
Mechanism two: the room the test ran in
A warmer tested in a cool room loses more heat to its surroundings and can stabilise lower than the same unit tested in a warm one, depending on where the controller's sensor sits and how the unit is held or insulated during the test. A unit tested while resting on a bench is not the same experiment as a unit held in a hand or wrapped in a cloth.
Ambient conditions are therefore part of the specification, not background detail. A claim that omits the ambient temperature is not wrong; it is incomplete to the point of being unusable for comparison. The same logic applies to the starting state: a unit that begins a test already warm from a previous run will stabilise differently from one that begins at room temperature, and a test that never says which state applied cannot be repeated.
Mechanism three: the peak, not the plateau
Most controllers regulate around a target, and the surface passes through a peak before settling. A figure captured at the peak is higher than the stabilised value, and both are honest readings of different moments. The gap between them depends on the control loop, the thermal mass of the shell and where the sensor sits relative to the element.
A peak figure is not useless. It describes what a user feels when the unit is switched on, which is a real part of the experience. But it cannot be compared with a plateau figure, and a specification that mixes the two across models has stopped being a specification.
Mechanisms and how to neutralise them
| Mechanism | How it lifts the figure | What to ask to neutralise it |
|---|---|---|
| Probe location | Reading taken at the element rather than the shell | State the point: shell surface, element, or battery compartment |
| Probe contact | Poor contact reads low and slow; firm contact reads the local surface | State the attachment method and the instrument |
| Ambient condition | A warm room or an insulated setup retains more heat | State ambient temperature and how the unit was held |
| Moment of reading | Peak is higher than stabilised | State whether peak or stabilised, and the stabilisation criterion |
| Face measured | Hottest point or one face of a dual-side unit | State the position on the face and which faces were measured |
| Load condition | Heating while charging or while output is active | State whether other functions were running |
A verification protocol you can reuse
- Ask for the test record, not the marketing figure, and read the measurement point first.
- Confirm the sample identity: model, revision and build stage.
- Confirm the ambient temperature, the mounting or holding method, and the instrument.
- Confirm whether the reading was peak or stabilised, and what counted as stabilised.
- Repeat the measurement yourself on one unit at the same point, using your own instrument, and compare the trend rather than the absolute value.
- Compare like with like: same point, same moment, same ambient, same level.
- Record the difference and the conditions beside the supplier's figure in your own file.
Step five is the one buyers skip, and it is the only step that produces independent evidence. A single repeat reading will not match a laboratory figure exactly, but a consistent direction of difference across three units tells you something a document cannot.
What a temperature claim does not mean
A higher figure does not mean a better product. A unit that reaches a high number on the shell may be doing so because the air gap is thin, which can also mean the surface is uneven or that the cells are running warmer than intended. A lower figure may indicate a conservative controller or a larger gap, either of which can be a deliberate choice.
A temperature figure is also not a safety statement. Safety depends on the protection chain: the NTC thermistor's placement, the OTP protector, the control board's cutoff logic and the cell's own protection. None of those appear in a surface temperature number, and a unit can hold a modest surface temperature while having a defective protection chain.
Nor does the figure describe runtime. The runtime conditions article explains why a unit that stabilises at a given level for a long time and a unit that reaches the same level briefly can quote similar numbers while behaving very differently in the hand.
Writing a specification that can be re-measured
- [ ] Name the measurement point and the instrument.
- [ ] State peak and stabilised values separately, with the stabilisation criterion.
- [ ] Record ambient temperature and the holding method.
- [ ] List each heat level separately, with the level definition.
- [ ] Note whether other functions were active during the test.
- [ ] Identify the sample: model, revision, build stage.
- [ ] Attach the uniformity map for each heated face.
- [ ] Name who signs the record and when it must be repeated.
A specification written this way is longer by a page and shorter by several arguments. It also survives a supplier change, because the new supplier has to reproduce the method rather than the number.
Next step
Take one current claim and run it through the seven-step protocol, or send a specification sheet to our engineering team for a measurement-point review through contact.
Written for the Emberly heating-element programme. Specifications are illustrative; confirm every figure against a supplier test record.