# T4, T130°C and Ta: The Temperature Ratings That Catch People Out
An ATEX radio label carries two temperature figures.
They mean different things. Most buyers check the first and ignore the second.
The second one is where compliance quietly fails.
## The first figure: temperature class
**T4** on a gas marking means the radio's maximum surface temperature will not exceed 135°C.
The full scale:
| Class | Maximum surface temperature |
|---|---|
| T1 | 450°C |
| T2 | 300°C |
| T3 | 200°C |
| T4 | 135°C |
| T5 | 100°C |
| T6 | 85°C |
Why does it matter? Because every flammable gas has an auto-ignition temperature — the temperature at which it ignites with no spark or flame at all, purely from contact with a hot surface.
The equipment's surface must stay below the auto-ignition temperature of everything in your atmosphere.
Carbon disulphide auto-ignites around 90°C. Diethyl ether around 160°C. Petrol vapour around 280°C. Methane around 537°C.
T4 at 135°C sits below the auto-ignition point of the large majority of industrial gases, which is why it is the common rating on handheld radios.
Note that a lower T number is more demanding, not less. T6 is stricter than T1. It runs the opposite way to the category numbers, which is exactly the sort of thing that trips people up.
For dust, the figure is given directly instead of as a class. **T130°C** means 130°C. Dust layers insulate, so the standard is written around the surface temperature under a defined layer.
## The second figure: ambient range
Look for **Ta** on the label. Sometimes written as Tamb.
Something like:
**-20°C ≤ Ta ≤ +50°C**
This is the ambient air temperature range across which the certification is valid.
Outside it, the certification does not apply. Not "probably still fine". Does not apply.
## Why this catches people out
Because the ambient range is not the same on every model, and it is not the same as the radio's general operating range.
Look at the Hytera HP715Ex. The specification lists **-30°C to +60°C in a non-hazardous area**, but **-25°C to +60°C in a hazardous area**.
Two different numbers for the same radio. The tighter one is the certified one.
The Motorola R7Ex is more restrictive still on the IIA version: **-25°C to +60°C** in digital mode but **-20°C to +60°C** in analogue. The IIC version manages -30°C to +60°C.
The Entel DTEx range is certified **-20°C to +40°C**.
Forty degrees. That is not a high ceiling.
## Where this bites
**Cold.** A tank farm in Aberdeenshire in February. An exposed jetty on a January night shift. A cold store or a chilled process area. A radio rated to -20°C is outside its certification at -22°C.
**Heat.** This is the one people miss. A radio rated to +40°C ambient, worn on a belt, under PPE, next to a hot process line, in a boiler house in August. The air temperature around that radio is not the shade temperature on the weather forecast.
Direct sunlight on a dark surface in a Middle East export project is another obvious case. If you are specifying for overseas sites, check this before anything else.
## What to do about it
Three practical steps.
**Know your actual extremes.** Not the annual average. The coldest morning and the hottest enclosed space where the radio will be carried. If you have process temperature data, use it.
**Compare the hazardous-area figure, not the general one.** Manufacturers publish both. The certified range is the one on the ATEX label and in the certificate, and it is usually the narrower of the two.
**Check the battery separately.** The battery has its own certified range, and it is frequently tighter than the radio's. It is part of the certified assembly, so the narrower figure governs. Charging ranges are tighter again, though charging happens outside the zone in any case.
## The one-line test
Take the label. Find Ta. Ask yourself, honestly, whether your site ever sits outside that band.
If it does, you need a different model — not a policy note saying people should be careful in cold weather.
Certification is binary. It applies, or it does not.