Selecting an Environmental Test Chamber about Humidity Range

Humidity is probably one of the most misunderstood topics when it comes to environmental test chambers because the relative humidity (RH) percentage is temperature specific. For example, the amount of moisture in the air at 20°C and 50% RH is not the same as 10°C and 50% RH. As the temperature of air is decreased, its capability to hold moisture also is reduced.

The standard temperature/humidity range for most chambers is 7°C to 85°C with 10% to 98% RH, limited by a 5°C dew point. The limitation of a 5°C dew point can be very confusing.

Since the amount of moisture varies at every temperature, the chamber manufacturers use dew point to describe the RH limitation. Inside the chamber, there is a refrigerated coil controlled at 5°C or slightly below. Moisture in the chamber will be attracted to the cold surface and condense. The accumulated water is drained out of the chamber, lowering the relative humidity. The refrigerated coil is never below freezing so frost will not develop.

If you follow the bottom line of the standard range section of the graph, those temperatures and humidities represent the 5°C dew point.

For example, the lowest humidity level achievable at 20°C is 40% RH. At 50°C, the chamber will achieve 10% RH. It must be operated within the limits set by the manufacturer. Damage to the refrigeration system can occur if points outside of the standard range are attempted.

To achieve lower humidity levels, most manufacturers offer a low RH package. It normally includes a dry air purge system and refrigeration valves to allow the refrigerated coil to go below freezing.

The low RH range in Figure 3 is achievable when the dry-air purge and frozen coil are activated. There is a potential for the coil to accumulate frost at these conditions. However, the dry-air purge helps to offset this by maintaining a positive pressure in the chamber and sublimating some of the accumulating frost off the coil.