Weatherproof Distance Sensors ?

I need to accurately measure distances of up to one. maybe 1.5 meters in an outdoor environment. Accuracy to 1/2" at
the outside of the detection envelope is required.
I've looked at the Sharp IR distance sensors and they're usable - but I'm worried about condensation building up in humid environments. I've found a number of ultrasonic sensors that are accurate, versatile and weatherproof - but they cost a bleedin' fortune (usually > $250US) and I need two of them for each project.
So - two questions ...
First, does anyone know what that plastic is that they make those condensation-resistant bathroom mirrors out of ? If I put the Sharp sensors behind a thin sheet of that ... no more condensation problems. (power-consumption IS an issue here so heated optical windows are out of the question)
Second, has anyone come across an AFFORDABLE ultrasonic distance sensor roughly fitting my accuracy needs ? 1/2" at 1 meter is adequate but the thing MUST be capable of withstanding 100% humidity pretty much forever.
Any help appreciated. Reply to group.
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B1ackwater wrote:

Anti-fog plastic just contains a form of detergent that keeps the water from beading up. Not a 100% reliable solution, and it'll wear out quickly in constant humidity. The same result can be obtained from a spray-on surface treatment, which needs to be applied often.
Certain important optically-based sensors have been known to mount a miniature mechanical wiper on the front glass for these types of situations.
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wrote:

Hmmm ... I used to own one of those fog-proof mirrors. The effect didn't disappear over time, no treatments were necessary. Likely the stuff was dosed with a halogen or something to alter the surface chemistry and/or charge. Maybe I could buy one and eat the aluminum backing off with HCL ...

Yea, I've considered it - but I also believe in the KISS principle and moving parts are just something else to break down (and consume power). Besides, it might *smear* instead of clean - yuk !
Ultrasonic would be perfect ... but the *cost* :-(
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> Hmmm ... I used to own one of those fog-proof mirrors.

But where is that mirror now? How long did you have it? By quickly I meant something like a year, in almost constant humidity...a shower is going to be dry 23 hours out of 24.
And as far as ultrasonic goes, you could always glance a few threads back. ;) www.maxbotix.com $30.
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cbm5 wrote:

Those look nice, but they're hardly weatherproof.
Mitch
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B1ackwater wrote:

Maybe this stuff is ready. From http://web.mit.edu/newsoffice/2005/fog.html
"Foggy windows and lenses are a nuisance, and in the case of automobile windows, can pose a driving hazard. Now, MIT scientists may have found a permanent solution to the problem. The team has developed a unique polymer coating--made of silica nanoparticles--that they say can create surfaces that never fog. The transparent coating can be applied to eyeglasses, camera lenses, ski goggles ?K even bathroom mirrors, they say. The new coating was described Aug. 29 at the national meeting of the American Chemical Society."
Mitch
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On Mon, 9 Jan 2006 13:46:43 -0500, "Mitch Berkson"

I'd heard about that ... but they don't exactly have tubes of the stuff down at K-Mart yet.
There was another approach, I believe using flourine gas applied to the surface of the plastic. Not really a do-it-yourself kind of thing however. Both techiques creat a hydrophobic nano-roughened surface that H2O doesn't properly 'fit' into.
They were *supposed* to have diamond coatings by now also for eyeglasses and such - created by decomposing methane and H2 at certain temperatures and pressures. Deposited thin, flat diamond 'flakes' on any surface. Might not work on plastic lenses though because the underlying substrate is STILL soft and subject to macroscopic gouges by sand and such. Kinda like putting tinfoil over jello - yea, the metal is a lot harder than the jello but you can still scoop out a chunk of the rasperry/lime right through the foil using a spoon.
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Hello B1ackwater,
Please check out PSR-1 at www.eutectos.com. and let me know what you think.
As in most Sonars, there is a dead zone, which in this case (gain and pulse width dependent) can be as low as 8 inches. Current implementation provides 1" resolution, which could be reduced to perhaps 1/4 inch. Hardware supports it, but firmware would need to be changed. Can you tell me more about your requirement both technically, application wise and what the potential volume may be? We might be able to help, and would like too.
Regards,
Larry
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Larry wrote:

From the title and his original post, isn't it clear that he wants a WEATHERPROOF sensor? Yours isn't and neither are other offerings in the <$100 price range. I think there would be many people thrilled to know about a WEATHERPROOF distance sensor for less than $100. For starters, this would mean that it would work when wet.
Also, the Devantech or the new (see other thread) MaxSonar-EZ1 are about half the price of the PSR-1. Is there an advantage of yours over those offerings?
Mitch
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Providing a waterproof sensor is no problem accept packaging increases cost, size and flexibility. Putting the sensor in a plastic or metal box that fits your specific platform is the best bet. Put a couple of o-rings around the transducer and mount the sensor through a close fitting hole in the box. Then ...
Have you tried putting a latex finger-cott over the sensor head? The thin membrane will be acoustically transparent. One way I've used to protect the transducers is as follows: 1) partially inflate a balloon. 2) apply a thin film of super-glue, silicone RTV or epoxy around the side of the transducer. 3) press the balloon against the face of the transducer so that the balloon deforms over the sides of the transducer. 4) hold until adhesive cures. 5) cut away excess balloon material.
Stay dry,
Larry
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Larry wrote:

There are a lot of good hermetic sealed US transducers, between $5 .. $10, see e.g. Farnell, look for "ultrasonic" or "400ET" or "400EP"
Stef Mientki
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On Mon, 9 Jan 2006 18:27:04 -0500, "Mitch Berkson"

Let's just say my project needs to visit drainpipes ...
Anyway, I wasn't expecting SUB-$100 weatherproof ultrasonics, but $149.99 would have been bearable. Are you aware of anyone who has put an ultrasonic behind a very thin rubber/plastic membrane and got it to halfway work ? If it had a lot of xmit power ...

I think his offer an 'echogram' in addition to a simple distance measurement - so faster offboard hardware can do fancy stuff with the signal. Might be interesting, especially if you used a binocular sensor arrangement and some way to scan the beams around. A low-rez 3-d 'image' with exact distance easily availible ... but you'd need to be better than me with the math because it would be kinda like processing an MRI or CAT scan.
An echogram would also be cool for a robot sub - if the sensor could be seriously waterproofed that is ...
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