Looking for a low cost semi-permeable membrane.

Other Hardware Sensors
1

I'm putting this under the sensors category because this is for a sensor probe I plan to build and I'm looking for some ideas.

I'm planning to build a sensor probe out of about a 12 inch long piece of 2 inch (Maybe smaller) PVC tube. Inside the tube, I want to place a DHT22 temperature and humidity probe. Along the outside I want to install a pair of small diameter titanium rods which will be used as a soil moisture sensor. The idea is that I would push the probe down into the soil, placing the DHT22 at the desired depth and use this to measure sub-surface soil temperature, humidity and soil moisture percentage. My only worry is that when the probe is stuck into the ground, first, it's going to get dirt inside it, so something needs to block the end of the tube, but i need to be able to let ambient humidity from the surrounding soil get into the probe housing without letting water simply flow in.

I do realize there are already sub surface probes available inside of a sintered metal housing to do exactly what I'm describing. Unfortunately, it seems that while some of these are priced around $15 USD, for some reason they all want an additional $21 USD for shipping. I'm hoping to come up with a simple solution that doesn't require getting ripped off by dishonest vendors.

Some ideas I have:
Cut a small disk of cinder block with the same outside diameter as the inside diameter of my PVC tube
and maybe a quarter inch thick.

Cover the end of the probe with fabric similar to what is used to make tents, umbrellas and kites. Maybe also coat that in something hydrophobic.

The main purpose here would be to stop the ingress of water to the probe while measurements are being taken. The plan is to put the probe in place, start measuring and begin watering the area with a garden hose or sprinkler. Then, continue to take measurements over the next several hours, perhaps a full 24 hours between tests. I want to try and come up with some good numbers for best time of the day to water, and for how long based on direct measurements being taken over the course of about a week.

All of my leads will be coming out of the top of the probe through a water resistant housing and connected to an Arduino datalogger I have built, which will be at the end of a shielded 25 foot cable. Really the only concern of water ingress is from water in the soil coming up from underneath the probe and damaging the sensor inside.

So far I like the cinder block idea the best, but have not yet attempted to cut any and it sounds like it will be difficult with limited equipment.

Thanks all for any suggestions, I know this is kind of impractical, but I think we can come up with something.

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wouldn't it be sufficient to simply have the sensor above standing-water level? If your soil is reasonably well-drained at all and doesn't form puddles, then the water shouldn't rise high enough to ingress up the tube to your sensor.

also, take a look at the SHT20 sensor, they're a bit more rugged than the DHT22 and come with that waterproof screen. These units are $12 each with free shipping.

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Goretex stops water while letting water vapour through.

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silly_cone:
wouldn't it be sufficient to simply have the sensor above standing-water level? If your soil is reasonably well-drained at all and doesn't form puddles, then the water shouldn't rise high enough to ingress up the tube to your sensor.

also, take a look at the SHT20 sensor, they're a bit more rugged than the DHT22 and come with that waterproof screen. These units are $12 each with free shipping.

Thanks for helping me find a link! I'd been looking on Amazon, primarily because I have prime and I would normally be able to get things like this in a few days but it seems these are all coming from out of the country or otherwise not fulfilled by Amazon. Like I said, most of them wanted $21 for shipping, not sure why. I'll check out the one at aliexpress though, they have yet to lose my trust.

On the point of not having standing water come up the bottom of the tube, I could probably get away with that, the only problem is IF water manages it's way up the tube, I'm not sure if a DHT22 will survive. Perhaps I'll just give it a try with some paper towel in the tube where the sensor would be placed and just see if the paper ends up wet. The live testing is going to be done at a friend's house who has grass, unlike me, so the soil conditions at his place are going to be a lot better. I think you're probably right though that water is unlikely to come up the tube. I'm also going to be building this probe in such a way that the top end will be sealed against water ingress so it might be watertight enough to keep a little bit of positive pressure against water ingress from underneath.

Guess I'll have to try it.

5

wvmarle:
Goretex stops water while letting water vapour through.

That's kind of along the lines I was thinking too. I have some nice rip-stop nylon like kites and tents and things like that are made from and those are pretty good at keeping water out. Maybe if I further treat the fabric with a hydrophobic chemical treatment it will further protect against water ingress. I'll have to try this as well.

I'm going to scrap the cinder block wafer idea for now, primarily because I don't have the proper tools to reliably manufacture a thin, round wafer of cinder block. I'll probably end up just ordering one of the sintered metal probes as listed above by Silly_Cone.

Thanks for the suggestions so far. I'll be checking back on the thread throughout the week.