This is Part 4 of a series delving into the intricacies of Mil-Std-810G. 810G covers temperature in 501.5 (High Temperature) which was discussed in a previous blog, 502.5 (Low Temperature) and 503.5 (Temperature Shock). Method 520.3 details testing which combines temperature, humidity, vibration and altitude interactively. Methods 520.3 will be examined in future blogs. Method 502.5 covers 6 pages of Mil-Std-810G.
This method is not intended for testing materiel to be installed in and operated in unpressurized aircraft, since such materiel would usually be tested according to Method 520.3.
Low temperature effects on computers and LCDs include:
- Failure of the system to boot
- Drive failure
- Drive won’t spin due to internal frost
- Stiffening of shock mounting devices
- Change in vibration and shock dynamic response
- Cracking of surface coatings
- Cracking from differential expansion (contraction) of dissimilar materials
- Change in electrical properties of electronic components
- Integrated Circuit (IC) bond failure
- LCD will not operate due to sluggish or frozen liquid crystal material
- Delamination of the LCD display
- Excessive static electricity
As with Method 501.5 for High Temperature, with a few exceptions, the Methods for low temperature do not place absolute limits on the devices to be tested. There is no specification delineating a minimum temperature of 0 deg C, for example. Thus, a manufacturer stating their device has been tested per Mil-Std-810G for operation between 0 and 50 deg C has written their own test method. The limitations for 502.5 specify the Method is appropriate for short term (months as opposed to years), even distributions of cold throughout the test item. For the purposes of testing, time is allowed for the test item to stabilize in temperature. Within 502.5 are three procedures:
- Procedure I – Storage
- Procedure II – Operation
- Procedure III – Manipulation
In general, companies will publish both storage and operating temperature specifications but rarely, if ever, Manipulation for computers or LCDs. Use Procedure III to investigate the ease with which the equipment can be set up and disassembled by personnel wearing heavy, cold-weather clothing. In the case of computers, the use of gloves might be an impediment to operating the systems. Testing for cold simulates conditions in selected regions or for worldwide use. Note that Antarctica is excluded from consideration by international treaty restrictions for military equipment. On the other hand, testing for civilian use in Antarctica is appropriate. Solar loading is not considered an issue so testing at constant temperatures is allowed. There are three cold test environments: Basic Cold covers most of Europe; Northern contiguous US; Coastal Canada; High-latitude coasts (e.g., southern coast of Alaska); High elevations in lower latitudes and ranges from -21 to -31 deg C for ambient air temperature and -25 to -33 deg C for induced environment (storage and transit). Cold covers Canada, Alaska (excluding the interior); Greenland (excluding the “cold pole”); Northern Scandinavia; Northern Asia (some areas), High Elevations (Northern and Southern Hemispheres); Alps; Himalayas; Andes and ranges from -37 to -46 deg C for ambient and -25 to -33 deg C for induced environment (storage and transit). Severe Cold covers the Interior of Alaska; Yukon (Canada); Interior of Northern Islands; Greenland ice cap; Northern Asia and specifies a single temperature of -51 deg C. Note that there are frequencies of occurrence that certain low temperatures may be experienced. For example, there is a 20 percent chance of going to -51 deg C and a 1 percent chance of going all the way down to -61 deg C. For extended storage such as years in areas that experience extreme cold, there is a chance the material may experience temperatures approaching -65 deg C. Testing time for storage of 4 hours at a stabilized temperature is generally considered adequate for nonhazardous or non-safety-related equipment. Some items such as explosives may continue to degrade over time at low temperatures and should be tested further as appropriate. Note that Method 502.5 does not mention humidity. Because the humidity is so low, essentially non-existent at these low temperatures, static electric generation does become an issue and must be considered. As a side note, it is much more important to practice safe static electric remediation procedures at low temperatures including use of wrist straps, grounding points, non-static generating packaging and wiping cloths, etc. Users may have a Styrofoam coffee cup handy trying to keep warm and Styrofoam can generate very high static fields. The test itself is rather anti-climatic. The device under test is placed in a chamber, powered down, and the temperature lowered to the desired value. Generally the rate of change in the chamber is limited to 3 deg C per minute to prevent thermal shock. Soak at the storage temperature for 4 hours, then raise the chamber temperature to the operating temperature, let the item stabilize internally at that temperature, then power it on and check operation. CP Technologies is versed in low temperature operation of rugged military grade computers and LCDs and can provide remediation such as extended temperature components or heaters as required. There are multiple options for products such as rugged LCDs to enable low temperature operation.