810G covers Solar Radiation in 505.5 and serves two purposes:
- To determine heating effects from sunshine impinging directly on equipment (Procedure I).
- To help identify material degradation from sunshine (Procedure II).
Mil-Std-810G, Method 505.5, is a rather complicated section at 15 pages with three Annexes (A – Detailed Guidance on Solar Radiation Testing), (B – Instrumentation Installation, Placement and Guidance), (C – Guidance on Tables and Figures) at 15 additional pages combined.
Of primary concern to users of computers, the heating effects of solar radiation are more important than material degradation. Computers are generally manufactured with metal enclosures. On the other hand, LCDs may suffer from both heating effects and material degradation. Coatings may degrade somewhat with color changes but the impact of plastic becoming brittle, for example, does not apply to a computer. A computer painted black sitting outside will become very hot with the subsequent impact on keeping the internal components within operating temperature specifications.
The maximum surface and internal temperatures attained by materiel will depend on:
- the temperature of the ambient air.
- the intensity of radiation.
- the air velocity.
- the duration of exposure.
- the thermal properties of the materiel itself, e.g., surface reflectance, size and shape, thermal conductance, and specific heat.
Materiel can attain temperatures in excess of 60°C if fully exposed to solar radiation in an ambient temperature as low as 35 to 40°C. Paint color and composition can have a major impact on surface temperature.
810G Method 501.5 (High Temperature) mentions Method 505.5 as a factor to consider (Aggravated solar) when determining effects of high temperature. In addition, Method 503.5 (Temperature Shock) also references 505.5 in section 2.3.1 for ‘Climatic Conditions’.
As you can imagine, 505.5 specifies “Use this Method to evaluate material likely to be exposed to solar radiation during its life cycle in the open in hot climates”.
The impact of solar radiation heating effects include:
- Jamming or loosening of moving parts.
- Weakening of solder joints and glued parts.
- Changes in strength and elasticity.
- Loss of calibration or malfunction of linkage devices.
- Loss of seal integrity.
- Changes in electrical or electronic components.
- Premature actuation of electrical contacts.
- Changes in characteristics of elastomers and polymers.
- Blistering, peeling, and de-lamination of paints, composites, and surface laminates applied with adhesives such as radar absorbent material (RAM).
- Softening of potting compounds.
- Pressure variations.
- Sweating of composite materials and explosives.
- Difficulty in handling.
Material effects of solar radiation, primarily from UV exposure, include:
- Fading of fabric and plastic color.
- Checking, chalking, and fading of paints.
- Deterioration of natural and synthetic elastomers and polymers through photochemical reactions initiated by shorter wavelength radiation. (High strength polymers such as Kevlar are noticeably affected by the visible spectrum. Deterioration and loss of strength can be driven by breakage of high-order bonds (such as pi and sigma bonds existing in carbon chain polymers) by radiation exposure.)
Testing is performed in a chamber with a bank of full-spectrum lamps mimicking the sun’s light and heat output. A maximum irradiance intensity of 1120W/m2 is provided and uniform across the top surface within 10 percent of the desired value. The Method outlines several scenarios for lamp selection and operation to give the desired results.
The ability to vary the lamp output to mimic diurnal variation in solar radiation should be provided for non-static testing. Where only thermal effects are considered, infrared lamps may be used but realize that coatings and filters on the test item may respond differently to those wavelengths versus sunlight. As a side note, infrared account for 42.1% (471.5 W/m2) of the sun’s total irradiance
For Procedure I (temperature), for worldwide deployment, a peak chamber temperature of 120° F is provided along with airflow of 300 to 600 ft/min to mimic naturally occurring winds. Generally, an airflow of as little as 200 ft/min can cause a reduction in temperature rise of over 20 percent as compared to still air. If the item is shielded from the wind in the operating environment, then no airflow would be provided during test. Maintaining the proper chamber temperature can be challenging as the lamps themselves will generate considerable heat and the unit under test will also be warming the air. Thus, cooling the chamber may be more problematic versus heating it.
Humidity is generally not a concern unless the material under test is known to be sensitive to moisture.
The test item should be clean while being tested. That being said, in many parts of the world, dust and dirt are prevalent and should be considered when planning the testing. Dust and other surface contamination may significantly change the absorption characteristics of irradiated surfaces.
Testing for thermal effects should be performed with the test item in a mode that generates the most heat.
Spectral distribution changes with the anticipated operational altitude. There is more damaging UV radiation at higher altitudes which should be considered. For example, a long duration high altitude UAV manufactured with composite wings would be tested for that environment looking for material degradation in the wings which may cause structure failure.
As with other 810G Methods, Method 505.5 is a general outline and it is left to the end user to create a test plan to align the test with the anticipated environment. An item in the middle of an asphalt parking lot in Phoenix would be tested differently than an item on a car dash in Anchorage. The tests should replicate the intended environment.
The tests can be performed mimicking the diurnal cycle (24 hours with variable lamp output and variable chamber temperature) or can be steady state (20 hours with the lamps on and 4 hours off). Repeat the cycle the number of times outlined in the test plan.
Chassis Plans has engineered rugged industrial computers for deployment in exposed locations in high-temperature environments and can assist with your project.
by David Lippincott Chassis Plans www.chassis-plans.com