The specification review happened six months before the chamber arrived. The test plan called for damp heat testing to IEC 60068-2-78 — 40°C at 93% relative humidity for 56 days. The chamber had been ordered, installed, and commissioned. Then someone looked at the chamber data sheet carefully enough to notice that the humidity system went down to 40% RH, not up to 93%. The chamber was a thermal chamber. The test required a climatic chamber. The programme lost eight weeks and significant cost.
The actual difference, stated plainly
A thermal chamber controls one variable: temperature. A climatic chamber controls two variables simultaneously: temperature and relative humidity. That's the entire distinction. The question that determines which category a chamber belongs to is whether it has a humidity generation and control system. If it does, it's a climatic chamber. If it doesn't, it's a thermal chamber. Every climatic chamber can operate as a thermal chamber. The reverse is not true.
What a thermal chamber does well
A thermal chamber is the right tool when your test standard specifies temperature only — IEC 60068-2-1 (cold), IEC 60068-2-2 (dry heat), IEC 60068-2-14 Test Nb (temperature cycling). It's also right for burn-in, HALT, thermal shock, and tests where temperature drops below +10°C. Below +10°C, the physics of water vapour make meaningful humidity control impossible. A programme that requires -40°C with simultaneous 85% RH is physically impossible in any standard chamber.
What a climatic chamber does that a thermal chamber cannot
The moment your test standard references relative humidity as a controlled variable, you need a climatic chamber. IEC 60068-2-78 (damp heat, steady state at 40°C/93% RH), IEC 60068-2-30 (damp heat, cyclic), IEC 60068-2-38 (temperature/humidity combined cyclic), the 85/85 test (85°C/85% RH, 1000 hours) — none of these can be run in a thermal chamber.
The spec sheet traps that catch buyers
Temperature range listed without humidity range — a chamber operating from -70°C to +180°C could be thermal-only. Check for a separate humidity specification. Humidity range that doesn't cover your test — a chamber rated to 80% RH cannot run IEC 60068-2-78 at 93% RH. Temperature range for humidity listed separately — most climatic chambers list the full temperature range and the humidity-controlled sub-range separately. Only the humidity-controlled range is relevant for your damp heat tests.
When you genuinely need both
Some programmes require both types — not as alternatives, but as separate instruments. A standard electronics qualification programme might include temperature cycling (thermal chamber), damp heat steady state (climatic chamber), and thermal shock (two-zone thermal shock chamber). The decision sequence that avoids procurement mistakes: read the test standard first, identify every parameter that must be controlled, then specify the chamber. Not the other way around.