Here's a question most product engineers can't fully answer about equipment they use every day: what's actually happening inside the box? You set a temperature. You set a humidity. You press run. But between "press run" and "test complete," there are three separate mechanical systems, a control loop running dozens of times per second, and a set of physical tradeoffs that directly affect whether your test results mean anything.
The four systems inside every temperature and humidity chamber
1. The refrigeration system
Getting cold is harder than getting hot. A standard single-stage mechanical compression cycle works like a household refrigerator, scaled up and pushed harder. A compressor pressurises refrigerant gas, which flows to a condenser, then through an expansion valve, then through an evaporator inside the chamber where it absorbs heat from the air, and back to the compressor.
Single-stage systems can typically reach around -40°C. Below that, two-stage cascade systems chain two separate refrigerant circuits in series — pushing chamber temperatures to -70°C or lower. That's how chambers achieve the extreme cold required for aerospace and automotive cold-soak testing.
2. The heating system
By comparison, heating is simple. Electric resistance heaters mounted in the airflow path heat the circulating air. The controller modulates power to the heaters to hit and hold target temperatures. One thing that surprises engineers new to chamber work: heating and cooling can run simultaneously. During a slow temperature ramp, the controller may apply partial cooling while heating, using the opposition between the two systems to achieve more precise control.
3. The humidity system
Humidity control requires both a source of moisture and a means of removal. Most chambers use a boiler or ultrasonic humidifier to inject moisture, and the refrigeration system handles dehumidification by condensing moisture on cold surfaces. The controller balances injection and removal to hold a target relative humidity — which sounds simple but isn't, because relative humidity is temperature-dependent. As temperature changes, the controller must compensate continuously.
4. The air circulation system
A blower circulates air continuously through the chamber workspace. Airflow velocity and pattern directly determine how uniform conditions are across the workspace. A gradient of 2°C from one corner to another is invisible to the controller but very real to a DUT mounted in the cold corner. This is why temperature uniformity mapping is a required step in chamber qualification, not an optional one.
The control system
Modern chamber controllers run a PID control loop — Proportional, Integral, Derivative. The PID algorithm reads sensor values, compares them to setpoints, and adjusts heating, cooling, and humidity outputs to close the error. A well-tuned controller holds temperature within ±0.5°C of setpoint continuously. A poorly tuned one overshoots, oscillates, and produces test conditions that look stable on a dashboard but aren't.
Where specs meet reality
Ramp rate is measured in an empty chamber. Load your DUT and it drops — sometimes significantly. Temperature range is measured at 23°C ambient. Run your chamber in a room that's 35°C in summer and low-temperature performance degrades. Humidity range has a temperature dependency — the spec sheet is a reference condition, not a guarantee for your specific setup.
The bottom line
A test chamber is a system of controlled tradeoffs. Knowing how those systems work helps you read results correctly — and know when a data point is telling you something about your product versus something about the box it was tested in.