[chbot] Wanting real data - How best to mount sensors outside in the elements?

[Stephen Irons]

Waterproofing is hard, really, really hard, for housings fully exposed 
to the weather.
IP-codes (IP-65, etc) don't mean what you think they mean [1].Common 
IP-65 rated housings suck in water that collects in the groove between 
base and lid as the internal pressure changes with temperature.PTFE 
pressure-relief vents prevent this, as long as water does not pool on 
the vent mebrane. PTFE is the magic stuff in outdoor clothing that 
claims to keep the water out, yet lets your skin breathe.PTFE 
pressure-relief vents are permeable to gas, including water-vapour. The 
humidity inside the housing will approach the ambient humidity in about 
a week [2].All polymers (including ABS and PC) are permeable to gases, 
including water-vapour [3]. The humidity inside the housing will 
approach the ambient humidity in about a month [2].If at all possible, 
use a potting compound to exclude gas. This solves the pressure-change 
problem, and many epoxies and polyurethanes are waterproof (polyester 
resins were not quite waterproof enough, as older boats with osmosis 
blisters show).
...but you need to find a compound that will stick to the insulation of 
all of the cables that penetrate the potting. Some cable jackets are 
made of polymers that resins don't likt to stick to.
Otherwise, install the housing inside a cabinet sheltered from sun and 
rain.Those end-user replacable batteries are also in a waterproof 
housing, and have exactly the same problems.It is really tricky to seal 
the battery-compartment lid properly. Too loose, and water gets in. Too 
tight, and the lid bends and water gets in.Waterproof connectors have 
the same issues.Multi-core cables and with braided screens have voids 
between the strands. Capilliary action will draw water into these 
voids, corroding the conductors. If this it not enough, they are 
subject to the same pressure variations as the main housing which helps 
to draw water into the cable.
[1] IP-codes apply to the product as a whole, not to the housing. Water 
might be acceptable inside an IP-67 or IP-68 rated product, as long as 
the product still works. Obviously, it will be easier to make a 
water-resistant product using a 'waterproof' housing.

[2] IP-65 rated ABS and PC housings from Jaycar, with Bluetooth 
environment sensors sealed in them, installed in a 100% humidity 
chamber (a plastic crate with a bowl of water) with its own Bluetooth 
environment sensor to monitor the humidity chamber itself. Temperature, 
pressure and humidity were measured hourly for 2 months. The humidity 
chamber was indoors, but not temperature controlled so it was subject 
to daily temperature variation.

After a few weeks, it is advisable to get the intern to open the 
humdity chamber, preferably somewhere far away from me.

We tested 6 configurations, each with a Bluetooth environment sensor 
sealed inside:
ABS and PC housing screwed shut with the supplied seal and screws.ABS 
and PC housing screwed shut with the supplied seal and screws, with 
PTFE vent installed. PTFE vent on a vertical edge to avoid water 
pooling.ABS and PC housing epoxied shut.
[3] PDF file with some gas permeability of various polymers at 
<https://www.sciencedirect.com/science/article/pii/B9781884207976500024/pdf>

Good luck!

Stephen Irons


On Thu, May 28, 2020 at 21:14, Andrew Sands <andrew at theatrix.org.nz> 
wrote:
> Hey all list dwellers,
> 
> I'm looking for advise, suggestions and general pointers to what 
> other have done in order to get various types of sensors working out 
> in the real world - that is beyond the bench.
> 
> What sensors perform better / marginal, what methods provide the best 
> mechanical / environmental protection.
> 
> Where should I position say an outdoors temperature sensor?
> 
> Mostly I'm after less of the theory and more of the I tried this but 
> it mutated a spider which ate my cat kind of thing. So don't do that.
> 
> Thanks for reading, stay safe. Wash your hands.
> 
> Regards,
> Andrew
> 
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