How to seal (weatherproof) a sensor device... anything better than silicone?


(Mike) #1

Hi guys,

If you use devices outside, what do you use to seal them from moisture or maybe even direct rain or immersion (god forbid)?

I’ve heard silicone mentioned. That’s certainly possible, but it seems a little gunky… thick, and not water-based.

This is for small sensors like the SmartThings Multipurpose or SmartThings Motion Sensor, if it matters.

Is there something a little more runny (less viscous) than silicone? That’s a little thick. If the sealant is more runny, it can seal up seams of the housing better, also without needing blobs of silicone at the seams. But hopefully something not so runny that it’s easy to accidentally gunk up the inside of the device.

If it can dry to be clear or white, that’s always a real plus.

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Silicone is great in many ways, but maybe there’s something even better?

Just asking.

Thanks!


(Andy - United Kingdom) #2

You will laugh - cling film!

Don’t forget you need to open them to change batteries (or reset them, after a hub update)


(Mike) #3

You mean Saran Wrap (plastic wrap)? If that’s what you mean, can’t liquid soak into the layers? Have you tried it a while… even tried immersion as the ultimate test?

Yes, whatever the choice is, it needs to be easily reversible/removable. Silicone is fairly easily to scrape off… it makes a little bit of a mess, but it’s no big deal.


#4

I’ve smeared silicone all over the gaps and seals of a 1st gen ST multisensor. I’ve had it outside for almost a year working great and it looks just fine with clear silicone on it.


(Andy - United Kingdom) #5

In the UK we call it cling film - worked great on a motion sensor for 12months before I implimented a different system
The other thing I have done is to use a food vacuum sealer (don’t know what you call them)
Easy to seal and later open if required


#6

Now I like that idea!


(Mike) #7

I tried shrink wrap; special plastic film that you wave a heat gun over and it shrinks up… in theory it works as advertised, but when you think about it, there is almost always a teeny gap somewhere. I imagine large-scale industrial strength shrink wrap works much better, but I only had some I bought off eBay, and a heat gun for drying e.g. paint. To me, it’s one of those things that sounds good in theory but when you get right down to exactly how much you put where, and how it’s going to shrink (without heating it so much it bursts), I just couldn’t get a complete seal relative to an immersion test.

I can always try it with plastic film / cling film / Saran Wrap … it’s very easy to wrap it around some little thing that’s covered in toilet paper… Then hold it under water a half hour, and see if any of the tissue paper gets the least bit wet.

Andy, it’s not that I totally disagree with you. Obviously plastic wrap or shrink wrap is much better than nothing, and probably more than good enough in many situations. I’m just asking, what ideas do people have? and how good do they work. It is good to hear film works great for you.

John, I am thinking silicone is still in the lead here. It might be slightly gunky but it otherwise ticks every check mark you want. I guess I will give plastic wrap a spin first, to see how well it does. Easy to test.


#8

Batteries outgas, and consequently unless the Device is specifically designed to operate in an airtight container, it’s not a good idea to weatherproof it to that extent.

In addition, there’s an issue of heat dispersion. The casings are almost always designed to be part of the heat dispersion method, and blocking or sealing any part of it may greatly reduce the life of the radio inside.

So rather than trying to weatherproof the sensor itself, unless there is a manufacturer – approved method, just put it in an electronics “project box” approved for your location with the air gap on the bottom (that’s how they’re designed generally anyway). It will be firesafe, it will be easy to change the batteries when needed, it won’t void the warranty, and it will preserve the expected life of the device.

If it’s a PIR motion sensor, you will need to leave a gap over the lens for detection.

Just don’t put a battery operated device in a Ziploc bag, or cling film, or try to seal it with silicon, or anything else which will change the heat dispersion or prevent the battery gases from escaping. :sunglasses:

Your local electronics warehouse should have lots of project boxes to choose from.


If you have sealed devices in silicone for weatherproofing, have they ever ruptured or died due to it?
(Mike) #9

I appreciate what you’re saying JD, but does your argument about battery gas mean that a sealed device would explode from pressure eventually? I am pretty sure a number of people seal with silicone, and I can’t recall hearing of anything like that. Is there a thread on it?

Also, seeing as how these devices have never once been hot to the touch or even warm, I’m sorry but I can’t agree with a heat dispersion problem. Something like the SmartThings Multipurpose or Motion Sensor are, for all practical purposes, sealed against air movement through them, so the designers expect no appreciable heat. (That’s not the same as being completely moisture sealed, though.) Nor would one expect a tiny button battery to last long at all if a device was using energy so fast that it was constantly hot.

Let me ask folks reading this thread: What’s the longest you’ve had something sealed? What kind of sensor was it? Have you seen problems like JD describes?

We’re all just sharing thoughts and experiences here. Thanks, JD!


#10

Some of us are also sharing professional experience. :wink:

Designing for heat dispersion is a huge issue in battery-operated devices, and even more so in networked devices. it doesn’t mean it’s hot enough for you to feel a difference when you touch the device.

By the way, have you had a chance to read any of the threads in the forum on using PIR motion sensors outdoors? That’s an entirely separate issue, but it’s something you should probably be aware of.

Anyway, by all means, write the manufacturer of the specific device you’re considering and ask them if it’s OK to weatherproof it with silicone or anything else.

And for those interested in technical details:

https://books.google.com/books?id=r_QfAgAAQBAJ&pg=PA199&lpg=PA199&dq=designing+a+battery+powered+device+heat+dispersion&source=bl&ots=iiVAerRBxY&sig=x78djJ_FEm0049-4KA0UbR8ZrUg&hl=en&sa=X&ved=0ahUKEwi0mLO7sp7VAhULw1QKHfFeDgwQ6AEIMDAC#v=onepage&q=designing%20a%20battery%20powered%20device%20heat%20dispersion&f=false

with shrinking form factor and increasing functionality and performance, portable battery power equipment is demanding more power than ever before with a limited board space, making thermal management One of the most critical design challenges today.

http://www.batterypoweronline.com/main/news/power-system-design-almost-but-not-quite-plug-and-play/

Unfortunately, sometimes system design engineers fall into the trap of assuming that the IC components they use in their equipment designs are also “plug and play.” It is easy to assume that if you just “connect the dots” in the same way that is shown in the application diagram from the IC’s data sheet, you won’t have to do any additional analysis or validation of your design. But while the guidelines and suggestions provided in modern IC data sheets might simplify system design, they don’t entirely eliminate the work that needs to be done by the product design engineer.
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Designing the power system for battery-operated equipment consists of two major sections. First, we have to choose the right type of battery technology and pack design. Second, we have to develop the electronic circuitry to charge that battery pack and convert the battery’s unregulated “raw” voltage into the regulated output rails needed to operate the actual system electronics. Last month, we talked about the importance of choosing the right type of battery technology as the first step. Once we do this, then the “real work” begins.
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A very common oversight is the lack of thermal analysis. In particular, for very low-power battery-operated equipment, it’s easy to think that the power levels are not high enough to worry about thermal management. However, remember that the power density of these handheld systems can actually be quite high.
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Table 1 shows us that even devices with low absolute power consumption need to be carefully designed, keeping thermal management in mind.
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Remember, we are cramming that low power consumption into a very tiny space. For higher power devices such as ultrabooks, notebooks and computers, heat dissipation is quite noticeable. The device actually feels warm to the touch.
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For lower power devices, with only milliwatts of internal dissipation, the end-user may not feel the unit case getting hot. However, if the dissipation in the low-power circuitry is localized to a small “hot spot” inside the unit, there may be concerns associated with device reliability or usability. This is especially true if the semiconductor components are being operated closer to (or above) the recommended thermal limits.

When you seal it, you inevitably raise the internal temperature of the device. But beyond that, I can pretty much guarantee you that the engineering process for designing that device looked at how heat would dissipate through that specific case. Weatherproofing materials can change the thermal patterns, creating the hot spots mentioned in the article. That’s the main reason that that kind of process almost always voids the warranty on the device.

But again, the best thing is probably to talk to the manufacturer of the specific device you’re intending to modify. They’ll know best what impact, if any, you can expect. :sunglasses:


#11

Try petroleum jelly like vasoline to seal the gap. I used it on my spruce moisture sensor in the grass. Working great so far. Does not dry and keep moisture out of the sensor.


(Mike) #12

Hey wow, that sounds like a truly simple alternative, cuboy! A little less hassle than silicone… not that silicone is a huge hassle, but jars of vaseline sit there for years… silicone is a tube that dries up, doesn’t wash off with water, etc.

Thanks for chiming in! I think we’re working up a range of solutions here. @ady624 over in a related thread I just made, polling about sealing problems, also suggested a simple ziplock bag. A bag won’t work in every situation, but it’s certainly easy and fast, and will work in some.

These are aside from the issue of whether something should even be sealed at all, as @JDRoberts is saying… that’s why I made the poll.

Thanks, everyone! Community at work!


(Jason "The Enabler" as deemed so by @Smart) #13

I’ve got an Iris gen 2 motion sensor in my shower. It’s been there for at least a year and I’ve never had a problem with it. There is no rust on the inside and it’s always very reliable.

It’s placed under a shelf and does not get direct water, that often. My wife likes to spray it for some weird reason.

I’ve also done nothing to seal it in any way at all.


(John C) #14

What @JDRoberts says about batteries & outgassing is very important. There are boxes specifically made to “breathe” without letting water in.

That being said, the vents usually point downward. Other parts of the box are sealed, often with O-rings or similar gasket material. Someone earlier asked for alternatives to common silicone sealant. There used to be a silicone product for sealing around an automotive windshield that was “runnier” than common sealers. Perhaps you could check auto supply stores for that?


(Paul Haskins) #15

Do all batteries outgas? I ask because I have several waterproof flashlights using everything from lithium 123 (same as many sensors) to small button battery powered ones. Since a flashlight would have a much higher draw wouldn’t outgassing be even higher?


#16

electronics failures are not a yes or no thing. Excess heat degrades transistors within electronics over time and causes them to fail sooner than they are designed. Even if it doesn’t feel hot or you don’t notice a problem, that doesn’t mean there isn’t one. Electronics in RF components are vary sensitive and require dissipation and free air movement.


(Paul Haskins) #17

That’s way too much of a generalization. There are millions of electronics devices designed and built with no free air movement. There are multiple ways of achieving design levels without free air.

I highly doubt if there is any requirement for “free air” to keep a ST sensor kool. As long as it’s installed within temperature specs.


( I hate Mondays) #18

The contact sensor in question was designed without free air movement. It’s just not water tight, but it has no venting holes anywhere.


#19

Yeah, what do I know. I just have a Masters in electrical engineering and design semiconductors.

You both misinterpreted me. Heat has to go somewhere. When electronics are enclosed in a box they have to dissipate through the package. The outside of that package needs air flow for the heat to dissipate, hence putting another sealed box around it was not intended by the design team.

Will it work? Sure, you just risk the reliability lifetime of the sensor. The main EFR of semiconductors is related to heat. It’s yours to do with what you want.


(Never Trust @bamarayne) #20

Such as house?

I think all anyone is trying to understand is the limits which is evasive, because the responses are vague and seem a bit on the side of alarmist.

If the sealed box around it is your house you keep at 80 degrees F, you’re okay?

If the batteries are in a waterproof sealed flashlight it’s okay?

Alright then. So let’s talk about the details of what is within operating specs…

<img src="/uploads/default/original/3X/7/a/7a06d9105eb32eb1cfeb044250f247c32787298f.png" width="690"height=“265”>

According to this the ecolink motion sensor can operate in ambient temps up to 120 F. So, battery gassing aside, if this was sealed and in a 80 F, would the dissipation be suitable for a motion event 20 times a day causing a radio signal? Now I doubt anyone can be that specifc, but you get the idea. Still very interested in how all these waterproof devices I have with 18650s and CR123s are humming along… flashlights are just the start of it… lots of diving equipment as well, etc.