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Temperature Class Calculator — T1 to T6 Selection Guide
Use this reference to select the correct temperature class for Ex equipment. The equipment T-class maximum surface temperature must be less than the auto-ignition temperature (AIT) of the flammable gas present in the hazardous area.
Temperature Class Reference
| T-Class | Max Surface Temp | Common Applications |
|---|---|---|
| T1 | 450°C | Ammonia, most gases where AIT is high |
| T2 | 300°C | Hydrogen sulphide (AIT 270°C needs T1 or T2) |
| T3 | 200°C | Petrol/gasoline (AIT ~280°C — T3 acceptable) |
| T4 | 135°C | Most process plant — the most common T-class |
| T5 | 100°C | Carbon disulphide environments (AIT 90°C — needs T6) |
| T6 | 85°C | Carbon disulphide (AIT 90°C — only T6 acceptable) |
Auto-Ignition Temperature Reference
| Gas / Vapour | Auto-Ignition Temp | Required T-Class |
|---|---|---|
| Carbon disulphide | 90°C | T6 only |
| Hydrogen sulphide | 270°C | T2 or lower (higher protection) |
| Petrol/gasoline | 280°C | T3 or lower |
| Methane | 537°C | T1 or lower (any T-class acceptable) |
| Hydrogen | 500°C | T1 acceptable (gas group IIC is the critical factor) |
| Propane | 470°C | T1 or lower (T4 common on process plant) |
| Ethylene | 425°C | T1 acceptable — T4 common |
T-Class Selection Rule
The equipment surface temperature must ALWAYS be less than the gas AIT. T4 (135°C max) is suitable for gases with AIT above 135°C — which covers most common hydrocarbons. For hydrogen sulphide (AIT 270°C), T2 (300°C max) is technically acceptable but T1 provides more margin. For carbon disulphide (AIT 90°C), only T6 (85°C max) is safe.
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