MIL-STD-810 Humidity Testing Overview [Method 507.6]

Graphic: During their life cycle, rugged servers, workstations and mini PCs are sometimes exposed to technologically inhospitable environments, such as a warm, humid jungle. MIL-STD-810 humidity testing can help prevent damage and impairment from such exposure.

Note: Trenton Systems is not a compliance testing facility. We manufacture rugged servers and workstations that conform to military and industrial standards, such as MIL-STD-810 and DO-160, and we can ensure that our systems comply with these standards using our in-house testing equipment, or by sending our systems to a third-party compliance testing laboratory for validation, but our facility does not offer compliance testing services for products manufactured outside of Trenton Systems. For a list of laboratories that can assist you with your testing needs, please read this blog post, which lists the best compliance testing laboratories in the world.

This is Part 5 in a series of the different test methods in MIL-STD-810H, the successor to MIL-STD-810G. View Part 4: Shock Testing and Part 3: Sand and Dust Testing.

Brian, software engineer: Hey, John, is it moist in here, or is it just me?

John, test engineer: Um. It's just you.

Amanda, engineering supervisor: John! You left the door to the humidity test chamber open again. Look at that. How are we supposed to conduct MIL-STD-810 humidity testing properly with the chamber door open?!

MIL-STD-810 humidity testing is a common test method used in the ruggedization of military computers.

In this blog post, we'll tell you exactly what the Department of Defense's trusty MIL-STD-810 standard says about it.

What is MIL-STD-810 humidity testing?

The purpose of MIL-STD-810 humidity testing, or Test Method 507.6, is to determine the resistance of a system to the effects of warm, muggy environments, according to the standard’s latest revision, MIL-STD-810H.

The method applies to systems that will be stored or deployed in these types of environments at some point during their life cycle.

High-humidity conditions occur seasonally or year-round in the tropics. Central America, The Caribbean, South America, Africa, India, Singapore, Vietnam, Indonesia, Malaysia, the Philippines, Vietnam and Australia are just a few of the places that experience particularly high levels of humidity throughout the year.

Systems enclosed in non-operating vehicles within these environments may also experience high humidity levels.

As such, humidity testing for rugged computers is essential if they’re expected to survive and function properly in these environments.

Photo: Trenton Systems' Thermotron temperature and humidity test chamber, which is used for MIL-STD-810 temperature testing as well

How do you test for humidity?

Testing humidity involves the use of humidity testing equipment, such as a humidity test chamber. Steam or water injection is used to create desired relative humidity conditions.

At Trenton Systems, humidity is tested during our storage temperature test.

We stress the system thermally in packaging material for a period of time before removing it from the temperature and humidity testing chamber and verifying that the system still works.

We put the board under test in the chamber and run the storage program. This program will first bring the temperature of the Thermotron down to a –40°C (-40°F) for three days. 

After the three days have been completed, the program will raise the temperature to 70°C (158°F) for three more days.  At 70°C, humidity will run at 5 percent the first day, 50 percent the second day and 90 percent the last day.

- Iain Adams, test engineer at Trenton Systems  

Relative humidity, expressed as a percentage, is dependent on two factors: water vapor and air temperature.

Relative humidity increases if the amount of water vapor in the air increases, and vice versa. Relative humidity also increases if the amount of water vapor in the air remains the same and temperature decreases.

On the other hand, relative humidity decreases if water vapor content remains the same and temperature increases.

During humidity chamber testing, systems are inserted into the chamber and exposed to high levels of relative humidity and temperature variations for prolonged periods to mimic natural, storage and transit conditions. The conditions vary based on a system’s anticipated shipping, storage and deployment environments.

Trenton Systems offers in-house humidity testing services for its made-in-the-USA rugged computers.

We test a few times per year. These requests typically come from our military, aerospace and defense customers.

Usually, the server will be used in a humid environment, and the customer wants to verify that our server will perform, and not fail, in a comparable or more excessively humid environment.

- Gary Ziadeh, compliance coordinator at Trenton Systems

Relative humidity testing is proudly performed at our Lawrenceville, Georgia facility, and if we can’t accommodate a specific test, we partner with local compliance testing labs who can easily stress-test your system to the humidity testing standards your program or application needs.

What are the systemic effects of warm, humid environments?

Humidity and temperature variations can cause internal condensation and other physical or chemical effects, including:

• Oxidation 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, producing a corrosive film
• Changes in friction coefficients, resulting in binding or sticking
• Swelling of materials due to sorption effects
• Loss of physical strength
• Electrical and thermal insulating characteristics
• De-lamination of composite materials
• Change in elasticity or plasticity
• Degradation of hygroscopic materials
• Degradation of explosives and propellants by absorption
• Degradation of optical element image transmission quality
• Degradation of lubricants
• Electrical short circuits
• Fogging of optical surfaces
• Changes in thermal transfer characteristics

What are the different humidity testing procedures?

Method 507.6 involves only two humidity testing procedures: Procedure I – Induced and Natural Cycles, and Procedure II – Aggravated.

Procedure I – Induced and Natural Cycles

Procedure I - Induced and Natural Cycles involves three cycles that represent storage or transit conditions and three cycles performed on systems that are open to the environment.

The natural cycles are constant high humidity, cyclic high humidity and hot-humid.

Constant high humidity is found most often in tropical environments. It represents conditions in heavily forested areas where nearly constant temperature and humidity prevail. These areas have little to no exposure to solar radiation.

Areas of the world where the constant high humidity cycle is observed include the Congo and Amazon Basins, the jungles of Central America, Southeast Asia, the north and east coasts of Australia, the east coast of Madagascar and the Caribbean Islands. In the most humid areas of the tropics, consistently high humidity levels occur 25-30 days each month.

Cyclic high humidity conditions are found in tropical areas where solar radiation is a factor. As a result, systems in these environments often endure alternate wetting and drying.

MIL-STD-810 permits the usage of simulated solar radiation when testing for the effects of this cycle if the system is expected to endure actual solar radiation in the field.

Cyclic high humidity conditions occur in the same geographical areas as constant high humidity conditions, but the latter usually occurs beneath a sunlight-obscured jungle canopy.

Hot-humid or high dewpoint conditions occur 10-15 times annually along the coastal strips of the Persian Gulf and Red Sea. Otherwise, these environments experience hot, arid conditions most of the year.

The induced cycles are induced constant high humidity, induced variable – high humidity and induced hot-humid.

Induced constant high humidity is defined as relative humidity above 95 percent with nearly constant 27 °C (80 °F) temperatures occurring for periods of a day or more.

Induced variable – high humidity and induced hot-humid occurs when systems in each of these categories receive heat from solar radiation with little to no cooling air.

Overall, testing can take quite a while – between 15 and 180 days based on the system category, the humidity cycle and whether conditions are natural or induced.

Table: The durations for hazardous and non-hazardous test items expressed in days. MIL-STD-810H defines hazardous test items as those in which any unknown physical deterioration sustained during testing could ultimately result in damage to material or injury or death to personnel when the test item is used.

Procedure II – Aggravated

Procedure II – Aggravated exposes a system to more extreme temperature and humidity levels than those found in nature.

MIL-STD-810 cites one advantage and disadvantage of this procedure: It produces results quickly, proving to be faster than the induced and natural procedure, but the observed effects may not accurately represent application-specific conditions.

Therefore, the document urges test engineers to exercise caution when interpreting the results and use the procedure as more a targeted identifier of systemic problem areas.

The minimum number of 24-hour cycles for this test is 10. This number of cycles can be increased, however, to provide a higher degree of operational confidence in the system and its overall survivability in consistently high-temperature, high-humidity conditions.

Should you ask about humidity testing for your rugged computer?

If your rugged server or workstation is expected to function in the tropics or induced high-humidity environments, ask your rugged computer manufacturer about a MIL-STD-810 humidity test today.

The expressly warm, humid environments native to the tropics are not at all hospitable to computers and can be detrimental to their functionality.

Mission-critical applications deserve durable, reliable and secure rugged computers that are MIL-STD-810-certified.

All day, every day, they’re the difference between success and failure.