I have an acid injector on my pool and I want to sense the level of the acid in the bucket. Can't use a float, this is HCl and will rot anything that isn't an invulnerable plastic. Can't use ultrasonic, the fumes rise and will eat the tranducers. Can't use optical, the liquid is clear.
How the heck can I do this?
capacitance?
optical IR?
How the heck can I do this?
Want my opinion, don't mess with anything inside the ACID tank. Use a load cell/strain gauge under the tank and measure the weight of the tank and it's contents. Not so hard to set-up once you have the initial end point weights determined (tank empty = ?counts, tank full = ?counts, map() that count range to 0-100% variable.
At least that's my story. At the refinery I worked on we used differential pressure transmitters to measure tank head pressure to calculate level. For the Acid tanks we had to buy the pressure transmitters with optional and expensive "Hastelloy" metal for all 'whetted' parts. Of course plants were designed and built to run for 40+ years so you did what was required.
Lefty
Actually, that's not a bad idea at all. It never occurred to me to measure the weight of the bucket! I kept thinking in terms of level, not weight. Adafruit has a little one that maxes out at 20 pounds and the bucket weighs far less than that so this is an idea that should work if I can work out the details of how to build something under the bucket that can measure the weight.
Thanks
Capacitive sensor on the outside. $16 on Ebay or build one using 2 bits of metal stuck to the outside. You may need to electrically ground the liquid to avoid picking up interference.
Retrolefty clearly has the experience, but there is one other option; search Mouser for "liquid level" and you'll find a bunch of sensors that use refractance to sense level so only a lens is exposed; they can tolerate acids I think. None of them are cheap, though. 
Thanks for the help folks, but I've yet to succeed on this silly little thing. The inexpensive weight sensors jam up if they are left too long with weight on them and don't like intense sunlight at all. The acid proof level sensors are too darn expensive. The capacitive sensors are too prone to noise; they are a huge antenna and just slam the noise into the arduino. This bucket sets next to the pool motor and it's one of those permanent magnet devices that really pumps out the RF.
The next thing I'm going to try is a simple microswitch; one of those with a lever on it. I'll tie a teflon string to it and suspend a PVC float in the bucket. This way the string will be loose until the liquid drops and pulls down on the lever. I'll work out some water protection for rain and such and the teflon should withstand the heat here so maybe this silly little attempt will work. Talk about Rube Goldberg.
Wish me luck.