Understanding Humidity in Environmental Test Chambers
Humidity is one of the most misunderstood aspects of environmental test chamber operation. This is largely because relative humidity (RH) is temperature-dependent.
For example, the amount of moisture in the air at 20°C (68°F) and 50% RH is not the same as at 10°C (50°F) and 50% RH. As air temperature decreases, its ability to hold moisture is reduced. As a result, for a constant amount of water vapor:
- Lower temperatures result in higher relative humidity
- Higher temperatures result in lower relative humidity
Understanding this relationship is critical for accurate environmental testing.
Standard Temperature and Humidity Range in Environmental Test Chambers
Most environmental test chambers operate within a standard, controllable temperature and humidity range of:
- 5°C (41°F) to 85°C (185°F)
- 10% to 98% relative humidity (RH)
This range is limited by a 7°C (44°F) dew point, which defines the lower humidity boundary across temperatures.
Understanding Dew Point in Environmental Chambers
Dew point is used by environmental test chamber manufacturers to define humidity limitations because moisture content varies at different temperatures.
Inside the chamber:
- A refrigerated coil is maintained near 0°C (32°F)
- Moisture in the air condenses on the coil
- The condensed water is drained from the chamber
- This process reduces relative humidity
The coil temperature is controlled to remain above freezing, preventing frost formation.
Why Dew Point Matters
The 7°C dew point defines the lowest achievable humidity levels at different temperatures.
Examples:
- At 20°C, the lowest achievable humidity is approximately 43% RH
- At 50°C, the chamber can achieve approximately 10% RH
These values follow the lower boundary of the standard operating range.
Operating outside of these limits can result in:
- Reduced performance
- Potential damage to the refrigeration system
Environmental test chambers must always be operated within manufacturer-defined limits.
Low Humidity Operation (Dew Points Below 7°C)
To achieve lower humidity levels down to 5% RH, most environmental test chamber manufacturers offer a low relative humidity (low RH) package.
This system typically includes:
- A dry air purge system
- Refrigeration valves that allow the refrigerated coil to operate below freezing
While frost may form on the coil under these conditions, the dry air purge helps offset this by:
- Maintaining positive pressure within the chamber
- Sublimating a portion of the accumulated frost
Types of Humidity Systems in Environmental Test Chambers
Environmental test chambers use several types of humidity generation systems, each with specific performance characteristics.
Water Pan (Water Bath) Humidity Systems
- Provides very stable humidity control
- Slower response time compared to other systems
Boiler / Steam Generator Systems
- Standard in many environmental test chambers
- Capable of achieving up to 98% relative humidity
- Suitable for a wide range of chamber sizes
Consideration:
- Heat generated by the boiler (live load) can impact performance at lower temperatures, especially when the device under test (DUT) also generates heat
Atomizing Humidity Systems
- Introduces very fine water droplets into the airflow
Advantages:
- Provides effective humidity control under live load conditions
Typical Use:
- Commonly used when the device under test exceeds 500 watts, with some variation depending on chamber size
Humidity Water Requirements for Environmental Test Chambers
De-ionized (DI) water is recommended for use with environmental test chamber humidity systems.
Water should be maintained within a resistivity range of:
Distilled water or reverse osmosis (RO) water outside of these limits may cause corrosion.
Water Filtration and Supply Considerations
For users supplying tap water:
- A demineralizer filtration system is recommended
- This removes impurities and minerals that could damage the chamber
This is not required for facilities that already have a de-ionized water supply.
Recirculating Water Systems
A recirculating humidity water supply system is available for applications without a plumbed water source.
This system:
- Can be filled with tap water and a DI filter, or directly with DI water
- Collects condensate from the chamber
- Filters and stores the water in a reservoir for reuse
Note:
If the device under test emits harmful vapors or contaminants, these may be absorbed into the condensate and reused, which could damage the test sample.
Water Pressure Requirements
If using a city water supply, a pressure regulator is strongly recommended.
Recommended pressure levels:
- 25 PSIG for steam generator/boiler systems
- 10 PSIG for atomizing humidity systems
High water pressure may:
- Crack the top or bottom cap of the water filter
- Cause leaks
- Result in damage to the chamber or surrounding equipment
Additional Filtration Requirements
If the water supply contains particulate matter, additional filtration may be required, such as:
- 5-micron or 25-micron polypropylene pre-filters
Additional filtration may also be necessary if the water contains:
- Organic contaminants
- Free chlorine or chloramines
- Phosphate compounds
In these cases, a roughing filter (such as a carbon absorber system) may be recommended.
Water quality and resistivity should be monitored periodically to ensure proper system performance.
Why Humidity Control Matters in Environmental Testing
Accurate humidity control is essential for maintaining consistent and repeatable test conditions.
Misunderstanding the relationship between:
- Temperature
- Relative humidity
- Dew point
can lead to:
- Incorrect test conditions
- Inconsistent results
- Potential system performance issues
Understanding humidity behavior within an environmental test chamber ensures reliable testing and long-term system performance.
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