810G covers humidity in Method 507.5. This method comprises 21 pages.
The purpose of Method 507.5 is to determine the resistance of material to the effects of a warm, humid environment. It is applicable to material that is likely to be stored or deployed in areas in which high levels of humidity occur. The essence of 507.5 is hot and humid. Effects on electronics can include break down of insulators as they absorb moisture or condensation when a cold object is brought into contact with a warmer humid environment.
The method may not reproduce all the humidity effects associated with the natural environment nor is this method applicable to low humidity situations.
There is not specific method that addresses low humidity. However, for completeness, low humidity exposure should be considered. The usual side effect of low humidity is static electricity build-up and subsequent discharge causing spurious operation or material damage to sensitive electronic devices. MIL-HDBK-263B is the reference for Electrostatic Discharge Control.
Warm, humid conditions can occur year-round in tropical areas, seasonally in mid-latitude areas and in material subject to changes in pressure, temperature, and relative humidity. Material enclosed in non-operating vehicles can experience high internal temperature and humidity conditions.
Specifically, this method does not address:
- Condensation resulting from changes of pressure and temperature for airborne or ground materiel.
- Condensation resulting from black-body radiation (e.g., night sky effects).
- Synergistic effects of solar radiation, humidity, or condensation combined with biological and chemical contaminants.
- Liquid water trapped within materiel or packages and retained for significant periods.
- This method is not intended for evaluating the internal elements of a hermetically sealed assembly since such materiel is air-tight.
Method 520.3 should be considered in conjunction with Method 507.5 to explore the synergistic effects of temperature, humidity and altitude.
The effects of high humidity can include:
a. Surface effects, such as:
- Oxidation and/or galvanic corrosion of metals.
- Increased chemical reactions.
- Chemical or electrochemical breakdown of organic and inorganic surface coatings.
- Interaction of surface moisture with deposits from external sources to produce a corrosive film.
- Changes in friction coefficients, resulting in binding or sticking.
b. Changes in material properties, such as:
- Swelling of materials due to sorption effects.
- Other changes in properties.
(a) Loss of physical strength.
(b) Electrical and thermal insulating characteristics.
(c) De-lamination of composite materials.
(d) Change in elasticity or plasticity.
(e) Degradation of hygroscopic materials.
(f) Degradation of explosives and propellants by absorption.
(g) Degradation of optical element image transmission quality.
(h) Degradation of lubricants.
c. Condensation and free water, such as:
- Electrical short circuits.
- Fogging of optical surfaces
- Changes in thermal transfer characteristics.
In addition to “Natural”, Method 507.5 provides for two Procedures: Induced and Aggravated. Natural simulates a natural environment. Induced simulates a natural environment for storage and transit. Aggravated exposes the test item to more extreme temperature and humidity levels than those found in nature but for shorter durations. The Natural test criteria was selected to mimic Majuro, Marshall Islands with a temperature range of 88 to 105 degrees F. and relative humidity (RH) of 59% to 88% for the Hot Humid test. Cycle B2 (High RH) provides for lower temperatures but RH up to 100%
The test duration is recommended to be a minimum of 45 cycles for non-hazardous materials to 180 days for Induced testing for hazardous material. Hazardous materials are those in which a failure may cause damage to adjacent material or injury or death to a user. The purpose of the higher cycle counts is to establish confidence in the testing.