Even though many Temperature/Humidity Environmental Test Chambers are typically specified with a range of 20% to 98%RH from 20C to 85C (or sometimes 7C to 85C), they in fact will not achieve certain (lower) RH ( ample 20%RH) within this temperature range at certain (lower) temperatures (example 20C).

Chamber manufacturers limit the 20-98%RH range by specifying a Dew Point Temperature (normally 2C to 4C) at which the full 20-98%RH range is possible.  This means that unless the environment where the chamber is located (test laboratory) is dry enough (has less than a 4C dew point), the chamber will not be capable of achieving the lower Humidity levels (20%) at lower temperatures (20C) unless an optional dehumidifier package is added (details below).

For example, as can be observed in the chart below, the lowest humidity achievable in a standard refrigerated Temperature/Humidity chamber situated in an air conditioned test facility with 24C@50%RH at Sea Level Altitude (example given for the chart below) and with the internal chamber temperature set  to 20C, will be about 40% to 50%RH.  As the temperature inside the chamber is increased, lower humidity levels are achievable and visa versa. 

In most  refrigerated Temperature/Humidity Environmental Test Chambers used for cycling (not steady state chambers), dehumidification is accomplished by utilizing a condensate cooling coil.  This coil is located in the conditioning plenum of the chamber.  When the controller calls for dehumidification, the condensate coil is automatically fed with refrigerant, as required, and is controlled by the output from the humidity sensor located in the chamber.  However, the amount of dehumidification is  limited using this technique and lower humidity control on a continuous basis is limited (grey area in chart below).

Steady State Chambers are even more limited, since they use the main cooling coil for dehumidification (similar to commercial air conditioning units).  This is adequate for steady state applications, but not for cycling and lower humidity applications.  Typically Steady State Chambers are used for high heat, high humidity conditions and dehumidification is therefore not necessary.

When humidity’s less than standard are required (20%RH between the range of 20C to 85C), a dry air system can be provided (dehumidifier package).  Typical plant compressed air (clean dry and oil-less at about 100 PSI) is connected to a solenoid valve, which is operated by an event from the controller.  The air then passes through a drier, which is a desiccant drying system.  After this, it passes through a flow control valve and fed into the chamber.  The air is dried to a dew point of -40C, and a low humidity level of 5% at 20C can be achieved (red area in chart below).  This assembly can also be used as an air purge system on temperature only chambers.