Haven’t had the pleasure of trying any yet, but these guys have screaming military folks on their website, so it must be good 
Or, well, use @bko’ suggestion first 
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I don’t know how I could physically do that. I drilled two 1/2" holes i the top of my tank, and inserted the two transducer elements through those holes (with silicone caulk to seal them), so I don’t have access to the front end of the sensor. Also, in the summer, the air in the tank is already quite hot and humid, I don’t know how hot I would have to heat it above that temperature to make it non-condensing, and whether that would be compatible with the electronics.
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The traditional approach is an enclosure of some kind that would be open on the bottom in this case so the sensor can see the water level but closed on all other sides. Then a low-wattage heating element or incandescent light bulb is used to heat the smaller enclosure and the sensor. Obviously you would have to keep the whole thing at a reasonable operating temperature (<85C I would think).
The keywords to google are “anti condensation heater”.
I watched the hydrophobic spray video and that looks cool!
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I wonder what wattage it would take to do that, and whether I could use constant power, or I would have to have a temperature sensor and some kind of control circuitry to adjust the power depending on the outside temperature. Right now I only have cat-5 wire running to the top of the tank (through a buried conduit), so I’d probably have to find a way to get power to the enclosure.
I wonder if wouldn’t be easier to go with a wet pressure transducer instead, or use multiple SS or gold pins to measure conductivity as you mentioned above; that was the way I was originally considering.
Why not have a look at this thread?
That only works if you’re getting mostly good readings. After a couple of days, I’m getting nothing but bad ones.
O… That’s bad news @Ric !
I did not read the whole thread and didn’t realize it was you who had this problem. 
Mine is still waiting to be mounted in place…
This works great but is kind-of expensive since you want everything that can get wet to be either stainless, PVC or teflon (PTFE). PTFE coated stainless stranded wire and stainless lug terminals are not off-the-shelf items at the local big box home improvement places, but can be found.
If your tank holds drinking water, you should think about what components would be drinking water safe. Also rain water and normal drinking water can be measured using resistance quite easily, but de-ionized or distilled water cannot.
My tank is for drinking (as well as irrigation) water, and it’s quite high in minerals, especially iron, so it’s plenty conductive enough; I’ve tested it with a simple voltage divider on one of my Photons. I was originally thinking of using gold wire wrap pins, since I thought I had a bunch of those around from the “old days”; but alas, I can’t find them. Another approach I was considering was to use a donut shaped float containing a magnet that would ride up and down on a pvc pipe containing a bunch of hall effect devices; this would only expose the water to the plastic parts, and the electronic components would all be in the dry space. Either of these approaches will require the use of multiplexing since I’ll probably want to have 1" spacing (1" = ~30 gallons) for the top 2 or 3 feet, with wider spacing below that (I do have several CD4067 16 channel multiplexers lying around).
I’m still considering the heating to control condensation on my HC-SR04’s. Wondering if I could paint the sensors black, and use several IR LED’s to provide the energy for heating? I looked at a some more expensive ultrasonic sensors that are supposed to be self-cleaning; self-cleaning it turns out is a heated sensor – those used ~70 ma continuous to heat the sensors. One other problem I’m worried about is whether I will get mineral deposits on the sensor which could compromise its long time reliability. My tank has an ozone bubbler in it (for sanitation and oxidizing iron) which does cause some aerosolization in the head space of the tank. A lot of variables to think about for what would seem to be a fairly simple project.
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