Outdoor electronics and condensation - Page 1
Forum section: electronics / projects

author: e100
author_rank: Frequent Contributor
timestamp: February 13, 2017, 01:51:27 pm
content: Is it normal for outdoor electronics to have heaters or humidity sensors inside the enclosure?I'm thinking of things like traffic light controllers and the various other electrical cabinets that you see scattered around the street scape.Are humidity sensors reliable enough to be used to control a heater?Would a big bag of desiccant be good enough if the enclosure was reasonably air tight with foam seals?

author: nctnico
author_rank: Super Contributor
timestamp: February 13, 2017, 04:17:48 pm
content: From what I've seen conformal coating and a way to let the water out is usually enough for on-land applications. Electronics always gets a bit warm so condensation on the PCB is unlikely.

author: Ian.M
author_rank: Super Contributor
timestamp: February 13, 2017, 04:41:39 pm
content: An outdoor hermetically sealed enclosure is possible, but it has to be very sturdy with well engineered seals to prevent pressure changes due to diurnal temperature variations + heating from direct insolation causing the seals to leak. If any air leaks out, there will be a partial vacuum when the enclosure cools, which can suck in any condensation that is on or near the seal line. Assume a hermetic enclosure may be opened and resealed in sub-zero winter conditions, and design it to withstand the pressure rise (by Boyle's Law) caused by the maximum possible temperature increase possible due to 100 year summer peak temperatures + direct insolation + internal dissipation at max possible power dissipation.Better yet, provide port(s) to purge with dry inert gas and pressurise it, + an indicator that it has maintained interior pressure, and specify the fill pressure in terms of the ambient temperature at time of filling.

author: DBecker
author_rank: Frequent Contributor
timestamp: February 13, 2017, 05:49:28 pm
content: One issue with sealing electronics is that sudden cooling and immersion go hand-in-hand. Or hand-to-lake/bay/ocean/toilet.

author: Gyro
author_rank: Super Contributor
timestamp: February 13, 2017, 10:12:12 pm
content: One of my previous jobs was designing traffic controllers. Yes it is a challenging environment, sometimes in ways that you might not have thought of too. For instance the corrosive effect of dogs and road salt. Ants are very fond of warm places to nest too! In addition you have the effect of moisture coming up through the ground.Street cabinets are mounted on an open galvanised steel frame below ground, this is secured underground with a substantial amount of concrete (you'd be surprised how many inebriated bunches of people try to push them over and to resist low speed collisions with white van man parking on the pavement too. The bottom of the cabinet is open, gasket sealed to the chassis frame to prevent water ingress at pavement level. You then have a mass of armored cables coming in underneath from the signal heads, loops and other sensors and of course the 30A mains feed. These are also mechanically and ground clamped to the frame. The cables are bedded in a thick layer of sand to near ground level, followed by a good layer of poured molten pitch. Any defect in this layer and you'll have an ants' nest by the end of the week.The main source of heat in the cabinet is the dimming autotransformer, a large lump of iron mouted on cross bars on top of the frame, at the bottom of the cabinet. At 50W per halogen lamp in each signal head (mains to 12V transformers are mounted in the signal heads, cable resistive losses would be too high at 12V) the power consumption of a large intersection soon builds up. A lights controlled motorway junction roundabout even more so (UK speak). The transformer is used to dim the signal heads at night and it keeps everything nice and toasty.The front cabinet door is another area of water potential water ingress (did I mention the dogs?), it is large and even with screw down cabinet latches and full length hinge in addition to the lock it's a very long length to seal. Flanges on both the door and cabinet opening add strength and help to create a labrynth to protect and assist the the rubber seal but they are again subject to some abuse from the public. In addition there is normally a smaller operator panel door on the main door - more seals.As a result, sealing is impossible, some water moisture ingress is inevitable, so the volume of the cabinet needs to breath and it needs to do it as easily as possible to avoid pressure differentials across the door seals and the ground interface. This is achieved by a water jet shielded baffled vent near the top of the cabinet, either on the back wall or concealed in the joint between the cabinet and lid.So it's really a combination of warmth and venting that keeps the humidity level under control. The dimming transformer keeps the cabinet air volume warm and the PCBs tend to generate enough warmth to stay dry. Conformal coating is rare, again it is better to let PCBs breath (as long as they have been carefully washed in production to remove any salts). The main thing is to get the controller powered up and commisioned as soon as it is installed - the last thing after all of the junction cabling signal heads installation etc. and keeping it that way, it's really bad to leave one out and unpowered.The slow advent of LED signal heads is likely to make things more diffcult. Lower current loading means less heat, at some point additional cabinet heating is likely to be required.

author: e100
author_rank: Frequent Contributor
timestamp: February 14, 2017, 03:12:16 am
content: Quote from: nctnico on February 13, 2017, 04:17:48 pmRegarding coatings, is it common practice to use silicone grease to protect things like connector pins and edge connectors?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 14, 2017, 03:49:22 am
content: There is no universal answer to your question. It depends on a lot of things, including what the outdoor environment is (a tropical coastal environment is a far different beast than a northern mountain top), the maintenance required/allowed, the access required, the service life you are trying to achieve, cost goals and many other things. In some cases a hermetically sealed enclosure is the answer. In others it is annual replacement. There are a wide variety of other answers, some of which have been mentioned above.

author: james_s
author_rank: Super Contributor
timestamp: February 15, 2017, 04:40:11 am
content: Quote from: Gyro on February 13, 2017, 10:12:12 pmI'm reasonably familiar with US signals since my uncle spent a good deal of his career as a signal electrician and I accumulated a bunch of related hardware over the years. Ours don't dim at night aside from the 3M "Programmable Visibility" heads that used relatively complex optics to cut off light from the signal outside of the specific area from which they're meant to be seen. Our traditional signals all used 120V medium base lamps although at least in my region the red and green lights have all been LED for about 15 years and in the last 5-6 years the amber ones have gone LED as well. The cabinets sound fairly similar though, the ones I've seen inside all have a 24V power supply in the top, then a microcontroller, and below that are the banks of load switches (3 channel SSRs), hardware conflict monitor, 4-way flash relay, inductive loop detectors and other various hardware. The cabinet is sealed and the equipment is the only source of heat. It's sufficiently warm inside that condensation is not an issue. The boards inside the the hardware I have are not coated with anything and the individual devices are not sealed.