We’ve been getting more and more customers asking advice on how to design for the great outdoors. In our first article, we covered water and dust resistant designs. This second article is focused on how to design that products can survive the outdoor environment. This is no small feat considering the hottest temperature ever recorded was 57.8 °C (136 °F rec. 1922, Libya). And the coldest was -89.2 °C (-128.6 °F rec. 1983, Antartica) .
The Catch 22 for product designers is that you need to have a product that can survive the extremes in order test that the product can survive extremes. This gets even more complicated when you consider that survival of the full product depends on the survival of all the electronic components used inside.
The goal of this article is to provide a practical way to break that Catch 22. We will do this in three steps: estimate the operating temperature range for the electronic components; building and test a real sample; and adjusting based on considerations for other temperature factors.
Determine the Ambient (Outdoor) Temperature Range.
First is considering the outdoor temperature ranges that the product will be subjected to. Table 1 shows the common temperature ranges based on common application classification. Most outdoor products fall in the “Mixed Use” range. Table 1 is just the starting point.
Table 1: Common Ambient Temperature Ranges based on Application
|Consumer||0°C||70°C||[32°F to 158°F ]|
|Mixed Use||-20°C||70°C||[-4°F to 158°F ]|
|Commercial||0°C||85°C||[32°F to 185°F ]|
|Industrial||-40°C||100°C||[-40°F to 212°F ]|
|Automotive / Industrial Extended||-40°C||125°C||[-40°F to 257°F ]|
|Military||-55°C||125°C||[-67°F to 257°F ]|
Second, designers need to adjust the classification range based on actual geographical deployments. Nexsun had a customer who wanted their solar powered outdoor meter to be able to survive a winter in Alaska. The coldest day in Alaska was recorded at Prospect Creek Camp at -80°F or -62°C. For practical design, however – it made more sense for our client to set the low side to ?40°C.
Based on the Table 1 and geographical assessment; we now have the “Ambient Temperature” or outside temperature extremes. These will be denoted as TLOW and THIGH.
Estimate Operating Temperature Requirement for Components.
The process of estimating Operating Temperature is a two-step process. First, we need to measure the heat the printed circuit board (PCB) and electronics generates within the enclosure environment. The three measurements are:
- Q – the average heat generated by the PCB/Electronics
- TIS – the average enclosure wall surface temperature
- TI the average air temperature inside the enclosure
This can be done in several ways:
- build up a sample with standard components
- testing a product of similar design and build
- thermal analysis simulation – Ex. SOLIDWORKS simulation, Autodesk CFD, and ANSYS Icepack
For methods 1 and 2, conduct tests at a ambient temperature closest to overall average. Simulations (aka method 3) can provide good backup reference – but do watch out for default values and assumptions made by the software that can dramatically swing the results.
Second, we need to determine the thermal heat transfer that can happen based on the enclosure material and design. In other layman’s term – how much does the Ambient Temperature effect the internal electronics. Below is a quick way to determine the worst-case temperature ranges
- L, W, H. Approximate geometry of the real enclosure with a rectangle.
- t. Measure/estimate the thickness of the enclosure.
- k. Determine the thermal conductivity of the material used by the enclosure. It is best to use the datasheet for the material used specific to the enclosure. However, if you need a quick value, there are several sources online – here is a source from Professional Plastics for plastic enclosures.
- ?. Determine the emissivity of the material used by the enclosure. It is best to use the datasheet for the material used specific to the enclosure. Also, there are several sources online – here is a source from Mikron that covers a broad range.
- Follow the calculations provided by this article from Heatsinkcalculator . Keep two things in mind as you do the calculations:
- Measured Internal Temperature TI and TIS is used in the first half covering convection. However, we are going to solve for the TI in the section “Radiation to Internal Surfaces” – equations 10 on; by setting Tamb = TLOW and Tamb = THIGH
- Keep in mind that thermal energy moves from High to Low. As you get your results, make sure to rationalize if calculated TI for the extreme temperatures make sense versus the measured TI at ambient.
Trust but Verify
Using the calculations above, you now have an operating temperature range that all your internal electronic components will need to meet. This list of parts is the hardened Build of Material (BOM). Nexsun has a rich set of suppliers that make wide-temperature parts. Some examples include:
- Wide Temperature TFT LCD displays by Winstar goes from -30°C to 85°C
- Chip Carrier Surface Mount and Through-Hole by On-Shore Technology goes from -55°C to 105°C
- DVI connectors by Amtek goes from -55°C to 85°C
- EMI Line filters by Yuan Dean goes from -30°C to 130°C
- Surface Mount Polymer ESD Surge Protectors by Surge Components goes from -40°C to 90°C
Additionally, check the spec sheet of each part to see if there are performance differences at the extreme ends of the temp range. Performance will vary by product category. Some parts, like LEDs, will have a derating curve at high temperatures. Other parts, like batteries, will perform poorly at low temperatures and include a similar curve.
The final verification is actually building a sample made from the hardened BOM and then testing it in a temperature controlled chamber. Depending on the product, operational stress can be added to simulate running at long periods of time. Only through live testing can product survival at TLOW andTHIGH be truly verified.
We tried to keep the sections above to a level that get product designers off to a start with a worst-case approach. Product enclosures are almost never fully sealed rectangles. The required Operating Temperatures for components can be drastically effected by enclosure design and heat dissipation strategies. Consider:
- Will adding heat dissipation (ex. a $1.00 fan and vent holes) reduce the cost of using hardened components?
- But…will the heat dissipating method violate the waterproof / dust proof requirements from the prior article?
- And…is this article good enough for exposures to fire, blizzards, and other temperature related natural disasters? [Hint: The answer is no. We will cover “Outdoor exposure and durability” in the next and final article].
Contact us to discuss your design.
Sources:  Wikipedia  “How to calculate the temperature rise in a sealed enclosure” by Hint Sink Calculator