ATEX temperature classes T1 to T6 describe the maximum surface temperature permitted for equipment intended for explosive gas or vapor atmospheres. The class on an inspection camera helps determine whether its surfaces can remain below the ignition threshold required for the location. T4 means a maximum surface temperature of 135°C; it does not mean the camera can operate in a 135°C environment.
That distinction matters before anyone discusses image quality or insertion length. A videoscope may fit the inspection port and carry the right Zone marking, yet still fail the site's temperature-class requirement. Another camera may satisfy the gas requirement but be unsuitable for the heat inside the equipment being inspected.
Quick answer: T1, T2, T3, T4, T5 and T6 limit equipment surface temperature to 450°C, 300°C, 200°C, 135°C, 100°C and 85°C respectively. A higher T number means a lower allowed surface temperature. Select a camera whose temperature class meets the site's documented gas-atmosphere requirement, then verify the full marking and operating limits.
A T-Class Controls Ignition Risk, Not Inspection Heat Tolerance
Flammable gases and vapors can ignite when exposed to a sufficiently hot surface, even without an electrical spark. The T-class limits the equipment surface temperature under its specified approval conditions, based on the ignition characteristics of the atmosphere. The classes form part of the explosion-protection framework used for Group II gas equipment.
A common misunderstanding is to treat the T-class number as a measure of toughness or heat resistance. A camera marked T1 is not approved to work inside a 450°C vessel, and a T6 marking is not proof that its electronics can tolerate an 85°C ambient temperature. These are different thermal questions.
The T-class limits how hot the approved equipment surfaces may become.
The system has a separate allowable operating environment that must be respected.
The inserted camera end has its own specified heat limits and exposure conditions.
The six-class definitions align with established explosion-protection references, including Phoenix Contact's temperature-class comparison. For buyers, the important distinction is between a certified ignition-control characteristic and a camera's practical operating environment.
ATEX Temperature Classes T1 to T6 Compared
The allowable maximum surface temperature falls sharply from T1 to T6. These are equipment surface limits, not typical process temperatures or recommendations for how hot a component should be during inspection.
The site class is derived from the ignition characteristics of the substances that may be present. As an illustrative example, if the governing gas has an autoignition temperature of 180°C, an equipment class with a maximum surface temperature of 135°C (T4), 100°C (T5) or 85°C (T6) meets the temperature comparison. T3, at 200°C, does not. The responsible site team determines the actual requirement using its classified-area assessment.
Important: The atmosphere's autoignition temperature is not the same as the equipment's operating temperature. Do not use an equipment T-class as a substitute for either the site's hazardous-area classification or the camera's environmental specification.
Why T6 Is Cooler, but Not Automatically a Better Camera
A higher class number places a stricter limit on surface temperature. If a site permits T4 equipment, a correctly marked T5 or T6 camera can satisfy the temperature-class portion of that requirement. The reverse does not work: a T4-only camera cannot be treated as T6 equipment.
That does not make T6 a universal upgrade. A T6 camera still needs the correct equipment category, gas group, protection concept, Equipment Protection Level (EPL), ambient rating and approved configuration. The camera also has to see and reach the inspection target.
This is why the useful comparison is not simply "Which number is higher?" It is whether the marked class covers the actual atmosphere at the site, without losing the camera functions the inspection needs.
Three Temperature Ratings Can Govern One Inspection
A hot vessel inside a hazardous area introduces two independent problems: preventing ignition in the surrounding classified atmosphere and protecting the inspection equipment from process heat. The camera's internal electronics and the inserted tip may also face different temperatures.
Consider a refinery inspection where the surrounding area is classified Zone 2 with a T4 requirement, but the equipment interior is still hot. A T4 videoscope may clear the surface-temperature classification and still be unsuitable for insertion until the process temperature falls within its permitted probe range. In the opposite case, a heat-tolerant standard videoscope might handle the interior temperature but lack the necessary hazardous-area approval.
Operating temperature, storage temperature, exposure duration, approved accessories and special conditions of use remain separate from the six T-class numbers. A product should not be selected for a hot inspection from a single T4 or T6 label.
What T4 Means on an Actual Inspection Videoscope
AIT VideoScope (AITVS) provides a practical example because its full ATEX gas marking is II 3 G Ex ic nA IIC T4 Gc. The T4 element identifies the maximum surface-temperature class: 135°C. The other portions define a different part of its hazardous-area suitability.
AIT VideoScope (AITVS)
AITVS is AIT's nonincendive articulating videoscope for applicable ATEX Zone 2 and Class I Division 2 gas or vapor environments. Its ATEX marking includes Group II, Category 3G, IIC gas group, T4 temperature class and Gc Equipment Protection Level.
Probe diameters span 3.2 mm to 8 mm, with insertion lengths selected by configuration. AIT can review the classified area and the requested probe combination for quotation; the facility remains responsible for approving equipment use at its site.
AITVS has separate environmental and probe limits. Its system operating range is specified independently from the T4 surface-temperature classification; use the approval documentation for the exact configuration to establish the permitted conditions.
For the other parts of the code, AIT's ATEX marking explanation and gas-group article cover the Zone, protection concepts and IIC classification without confusing them with T4.
The Right T-Class Does Not Override Zone, Gas Group or Dust Limits
Temperature class is one field in the complete approval. A Zone 2 T4 camera is not automatically a Zone 1 instrument. An IIB-marked T6 camera is not automatically suitable for an IIC gas atmosphere. The acceptable equipment must meet all applicable parts of the documented site classification.
Equipment category and EPL establish the intended hazardous-area protection level. T4 does not change a Category 3G camera into Category 2G.
IIA, IIB and IIC address gas-group compatibility, not maximum equipment surface temperature.
Combustible-dust equipment commonly uses a stated maximum surface temperature in °C, with dust cloud and layer risks assessed separately.
For a dust-classified space, an ATEX marking that specifies G for gas does not automatically authorize use in a dust atmosphere. The class shown for gas cannot replace the dust-specific approval and temperature assessment. AIT's ATEX ratings article explains why the equipment category and gas-or-dust designation must be read together.
Where the Temperature Class Can Change an Inspection Plan
The practical consequence differs by application, even though the same six T-class ceilings apply to equipment for explosive gas atmospheres.
A videoscope can fit a filler neck or access port while lacking the temperature class or other approval required for the atmosphere around the work.
Solvents used elsewhere in the process may have lower autoignition temperatures than the primary product, affecting the site's equipment requirement.
The surrounding gas atmosphere governs the T-class; residual heat inside the asset governs whether the camera and its probe can be used at that moment.
Those scenarios do not justify assuming that any particular plant, tank or vessel is Zone 2 or T4. The site determines its classification. The inspection-camera review then focuses on equipment that is both appropriately approved and physically capable of the work.
The Camera May Fit the Port and Still Never Enter
Imagine two videoscopes with the same insertion diameter, reach and image quality. Both can see the target. One carries the T-class required by the atmosphere; the other does not. For the second camera, the inspection ends at the hazardous-area boundary, regardless of what its camera sensor can capture.
That is the detail often missing when equipment is chosen on optics first. The most useful specification may be a small character on the Ex marking, because it decides whether the camera can be considered for the inspection at all.
