There are many instances where Phidgets might get stuck outdoors: weather stations, RC vehicles (quadcopters, underwater vehicles, robots, etc), outdoor installations, and applications we haven’t imagined yet. Some specific examples of Phidgets in the outdoors are a giant drum machine, a sky temperature scanner and precision agricultural monitors.
In this series, we explore how to protect Phidgets from outdoor conditions, like rain and dust. In this post, we’re going to talk about insulating Phidgets and keeping them at the right temperature. This does not apply to Phidgets that need exposure to air: infrared Phidgets, light sensors, and RFID. This guide to insulation is intended for sensors that do not require interaction with the environment, like interface kits, SBCs, temperature sensor inputs (i.e for thermocouples), current/voltage sensors, spatial sensors, bridges and the like.
Operating temperatures for each device are given on the product pages. Interface boards (like the SBC3 and 8/8/8) operate within 0˚C and 70˚C, and sensors operate in both wider and narrower ranges.
In the previous post, we discussed different kinds of encasements and connectors for Phidgets. When you’re not worried about temperature, you can use pretty much any weatherproof case. However, if your project will be sitting outdoors for an extended period of time, chances are temperatures will go outside of the Phidgets operating temperatures, and you’ll need insulation.
The simplest and most effective solution is to use a foam insulator. Pelican cases, as we suggested in the first post of this series, come lined with polyurethane foam, which is a decent insulator. If you plan on installing the Phidgets in more extreme temperatures (i.e. above 70˚C and below -15˚C) for more than 8 hours, the temperature in the Pelican case will begin to equal the temperature outside the case and you may need to look into different solutions (including, possibly, ditching the Pelican case altogether in favour of an insulating case, like a cooler).
Find a product with thermal conductivity under 0.035W/(m*K), like cotton, styrofoam/polystyrene, polyurethane/urethane silica, or plastic insulation foam. Some of these materials have limited temperature ranges. Polystyrene’s range is -50˚C to 75˚C and Polyurethane’s range is -210˚C and 120˚C (source: engineeringtoolbox.com), but I suspect that you won’t be using Phidgets in such extreme conditions anyway. The bigger concern is how thick the insulation will need to be in the environment you’re working in. If you’re expecting temperatures to get up to 70˚C, you’ll need really serious insulation, and a cooling system, to keep the Phidgets operating.
Calculating Insulation Needs
We can use Fourier’s law to calculate the thickness of insulation that we need:
Where s is the thickness of the foam (in m or ft)
k is the thermal conductivity of the material (found here, in W/m⋅K or W/m oC or Btu/(hr oF ft2/ft))
A is the surface area of the case (in m2 or ft2)
dT is the estimated temperature difference between the inside and outside (in K or ˚C or ˚F)
q is the heat transfer (in W or J/s or Btu/s)
While we could use integrals and look at varying temperatures over the entire surface of the case and SBC, let’s keep it simple and one-dimensional. In this example, we’ll also be ignoring the thermal properties of the case itself. You might want to do more involved calculations yourself, but this should give you an idea of where to start.
If we’re using styrofoam the k = 0.033 W/m⋅k. If we’re using an SBC3, the dimensions are about 10cm x 8cm, which gives a surface area of A = 0.008 m2, if we pack it nice and tight. We can expect the outdoor temperatures to reach as low as -20˚C, and we want to make sure the SBC stays above 0˚C. That makes dT = 20˚C, which equals 20 K. For heat transfer, we can allow the same amount of heat that the Phidget is generating to be transferred through the insulation. The SBC will generate about 1W of heat, while less powerful Phidgets will be lower. Since it’s better to low-ball and do a bit more insulation than not enough, we’ll use q = 0.1W. That gives us:
So, we need 5cm of foam around the SBC. Obviously, we’ve made a lot of assumptions here and took some shortcuts, but hopefully that gets you started on your calculations.
We ran some tests with 2.5cm of styrofoam around an SBC3 in a -20˚C environment. Without taking too many precautions around making a really airtight seal, and using a metal case that wouldn’t provide too much additional insulation, the foam was able to keep the SBC3 temperature above 0˚C for outdoor temperatures above -16˚C. Even when the SBC’s temperature dipped below 0˚C, it still functioned. Styrofoam is a readily accessible material, and although you may need to make it really thick in extreme temperatures, it should meet the needs of most people. There are also other options for insulation, mentioned earlier, that you can explore.
Other Methods of Temperature Control
You can use the colour of the case to work for you as well. Dark materials absorb heat while light materials reflect. Therefore, choose a dark case if you’re working in cold conditions and a bright white case for hot conditions.
You will also want to consider the location. The most significant influence will be direct sunlight, which will cause the case to heat up much faster.
In extreme conditions, you may need to install the Phidgets in a heated or cooled enclosure, whose temperature control would run off electricity, and probably need to be monitored by thermostat. We don’t know of any Phidgets customers having to do this though, so it would be a very special case.
Even better than insulation, and a solution that some customers have come up with, is to house whatever you can indoors (like the interface kit) and run cables to the sensors that need to be outside. This only works if you’re setting up near a building. Whatever you do set up outside, check its product page for operating temperature.
Another option, if you think the Phidget may be exposed to temperatures outside of that range, is to make it mobile, so you can move it indoors when conditions call for it. Overall though, some thick insulating material should do the trick. If you’ve tried something, let us know in the comments.
If you missed it, check out last week’s post on enclosures and connectors. Next week we’ll talk about encasing Phidgets that need to exposed to air. We’ll look at how to keep humidity and gas pressure sensors safe from the elements, without interfering too much with the sensing.