Temperature and Humidity Test Chamber: What the Standard Requires of the Chamber — Not Just the Test

The test plan cited IEC 60068-2-78. The chamber specification said "temperature and humidity chamber, 40°C / 93% RH capability." The chamber was procured, installed, and placed into service. The test programme began.

At the first audit, the assessor asked for the chamber performance verification report. Specifically, she asked whether the humidity stability had been verified at 40°C / 93% RH at the DUT level — not at the chamber controller sensor. The laboratory produced the factory calibration certificate. It showed performance at the controller sensor position, under empty conditions. The assessor noted the gap: IEC 60068-3-6 requires verification at the specimen level, under conditions representative of use. The factory certificate did not satisfy this requirement.

Two standards govern every temperature and humidity test. The first — IEC 60068-2-78 — specifies what the test must do to the specimen. The second — IEC 60068-3-6 — specifies what the chamber must demonstrate it can deliver. Most test plans cite the first. Most specifications satisfy the second only by assumption. This guide maps both.

01Two standards, one chamber

Every temperature and humidity test programme involves at least two distinct standards that apply simultaneously, with different subjects:

The gap between these two standards is where most compliance problems originate. A chamber that can reach 40°C / 93% RH does not automatically deliver 40°C / 93% RH at the specimen level with the required stability. Demonstrating that it does requires verification per IEC 60068-3-6.

02What IEC 60068-2-78 requires of the test

(cite index="21-1">IEC 60068-2-78:2025 — the current edition — establishes a test method for determining the ability of components or equipment to withstand transportation, storage and use under conditions of high humidity. The standard investigates the effect of high humidity at constant temperature without condensation on a specimen over a specified period.

The standard defines preferred severities — combinations of temperature, humidity, and duration. The most commonly used conditions in electronics qualification:

03What IEC 60068-3-6 requires of the chamber

(cite index="23-1">IEC 60068-3-6:2018 provides a uniform and reproducible method of confirming that temperature and humidity test chambers, without specimens, conform to the requirements specified in climatic test procedures of IEC 60068-2.

The standard defines the verification methodology for the chamber itself — independent of any specific test being run. It specifies sensor positions, stabilisation criteria, measurement periods, and the data that must appear in the verification report. Key requirements:

Sensor positions. Minimum nine positions distributed across the working volume — corners, mid-planes, and centre. Sensors must be in the air, not in contact with surfaces, and positioned to measure conditions at the specimen level.

Stabilisation period. The chamber must reach stable conditions at the setpoint before the measurement period begins. A minimum 30-minute stabilisation period is standard; longer stabilisation may be required for challenging setpoints.

Measurement period. Continuous recording at defined intervals (typically 1 minute) for a minimum measurement period at each setpoint condition. The period must be long enough to demonstrate stability, not just a momentary achievement of the setpoint.

Verification at specimen level. The critical point: the compliance parameter is the condition at the specimen level — not at the chamber controller sensor. The controller sensor is calibrated to control the chamber. It does not necessarily represent conditions at the DUT position, particularly at challenging setpoints or under load.

04The humidity control range problem

Temperature and humidity chambers have a defined humidity control range — the combinations of temperature and relative humidity that the chamber can achieve and maintain simultaneously. This range is not a rectangle. It has boundaries imposed by the physics of water vapour and the capabilities of the humidity generation and dehumidification systems.

At low temperatures, the maximum achievable humidity may be limited by the chamber's dehumidification capacity — maintaining 93% RH at 5°C is technically demanding. At high temperatures, the steam generator must work against elevated vapour pressure — maintaining 85% RH at 85°C requires significantly more humidity generation capacity than at 40°C. Some chambers rated for a given humidity range cannot achieve that humidity at the extremes of their temperature range.

05Stability vs uniformity — the distinction that matters

Two performance parameters define whether a temperature and humidity chamber is suitable for a specific test programme. They are related but distinct:

Stability is temporal — how much the temperature or humidity at a given point in the chamber varies over time during steady-state operation. A stability of ±1% RH means that at any sensor position, the humidity reading does not deviate more than ±1% RH from the setpoint over the measurement period. This is the parameter that determines whether the test conditions are maintained throughout the soak duration.

Uniformity is spatial — how much the temperature or humidity varies between different positions in the chamber volume at the same time. A uniformity of ±3% RH means that the maximum difference between any two sensor positions at the same moment does not exceed ±3% RH. This is the parameter that determines whether specimens placed at different positions in the chamber are exposed to equivalent conditions.

Most chamber specifications state one or the other — usually stability. Both are required for a complete specification. A chamber with excellent stability but poor uniformity will expose specimens at different positions to meaningfully different conditions. In a multi-specimen test, this produces results that are not directly comparable.

06The 85/85 condition and what it demands

The 85°C / 85% RH condition — used in JESD22-A101 steady-state temperature-humidity-bias testing and as a reference condition in several semiconductor qualification standards — is one of the most demanding conditions for a temperature and humidity chamber. It sits near the physical limits of many standard climatic chambers for two reasons:

First, 85°C is near the upper temperature limit of many standard climatic chambers. The chamber's heating system is working at or near full capacity to maintain 85°C against the heat rejection from the refrigeration system and the ambient environment.

Second, 85% RH at 85°C requires a substantial steam injection rate — the chamber must introduce significantly more water vapour than at lower temperatures to maintain 85% RH against the elevated vapour pressure. Not all chambers rated for "85% RH" are rated for 85% RH at 85°C simultaneously.

Before specifying a chamber for 85/85 testing, confirm from the manufacturer that 85% RH is achievable and maintainable at 85°C — not just that the chamber is rated to 85°C and 85% RH independently. These are different specifications.

07How to specify a T/H chamber correctly

A complete temperature and humidity chamber specification must include:

Operating conditions — stated as performance requirements, not equipment ranges. Temperature range at load. Humidity range at operating temperature (not absolute range). Temperature stability ±°C at specimen level. Humidity stability ±% RH at specimen level. Temperature uniformity ±°C across workspace. Humidity uniformity ±% RH across workspace.

The test conditions the chamber must satisfy. State the IEC 60068-2-78 severity, or the specific temperature/humidity/duration combination required. The chamber must be capable of maintaining the required conditions continuously for the full test duration under the specified specimen load.

Humidity control range boundary. The specific temperature at which the humidity requirement must be met. "85% RH at 85°C" is a different requirement from "up to 85% RH."

Refrigerant compliance. Under Regulation (EU) 2024/573, specify R-449A or CO₂ (R-744). Exclude R-404A for any equipment delivered from 2025.

Acceptance test conditions. The chamber must be verified to IEC 60068-3-6 at installation, with results documented in a signed commissioning report.

08Performance verification after installation

After delivery and installation, a temperature and humidity chamber must be verified to confirm it performs to specification in its installation environment. The verification procedure follows IEC 60068-3-6:2018 — not the manufacturer's factory test data, which was obtained at the factory under factory conditions, before transport.

The verification must demonstrate temperature and humidity stability and uniformity at the specific setpoints required by the test programme. For a chamber used for 40/93 testing, verification at 40°C / 93% RH. For a chamber used for 85/85 testing, verification at 85°C / 85% RH. A verification conducted only at the easier setpoints does not confirm performance at the demanding ones.

Re-verification is required after any modification that could affect performance — refrigerant change, humidity generator replacement, sensor replacement, or relocation. A signed verification report before the chamber enters service, and after any significant change, is the minimum documentation standard for regulated applications.

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