That could be a good option. Couple slices of copper / aluminium tape, resistor and readings with arduino CapacitiveSensor.h / CapSense.h:
Shielded wires should be used for better performance...
Just need to map the values according to the shape and properties of the tank..
Yes the capacitive sensing option is a good idea. Difficult to say if it would fit the shape which is not flat by any means if you look at an Aqua-Roll container.
I think you can put tape on almost every kind of shape, but the values that you'll get might not follow that predictable pattern (linear or exponential). This is why mapping tuning is perhaps needed if you want it to be as accurate as possible...
Thanks. It's a very convoluted shape and needs to be rolled around to fill the container, In addition the container will be outside subject to the elements, rain, snow etc. So this method is not very practical in the circumstances, but thanks for the suggestions.
Investigate these - those with a longer shaft have multiple switches contained so you can resolve the depth .
Maxbotix has a number of ultrasonic sensors with narrow beam angle, some specifically intended for tank service. They're about US$110. The tank ones have a minimum range of 50 mm (maybe mounting could get you your 30 mm). Others have a 20 mm min. All with appropriate IP ratings.
Thanks for the image TomGeorge. DaveEvans - The sensors look good, but a tad expensive to try without knowing they would work in my application. The issue is the water pickup pipe which is dangling into the container through the same opening as the sensor. The container only has a 60mm opening and the pickup tube is about 20mm. Leaving only 40mm space for the sensor.
The way I found round that issue using the A02YYUW sensor was to use a 50mm oval shaped pipe going down into the container keeping the pickup tube away from the beam. While that worked it also caused errors. Water droplets on the side of the tube would give false readings rather than the water surface level. As you can imagine it is in a very high humidity environment so the droplets don't evaporate but form randomly on the tube surface.
While those problems could be sorted by using a coating of some sort, a narrow tube is hard to keep clean on a longer term use basis. All in all a non contact method will be best. The pickup tube is easy to wipe clean as the inside is constantly full of water and also easy to periodically clean with cleaning agents. The beam tube will always be partially submerged most of the time leaving surfaces exposed to bacteria formation. How much of a concern that is in reality I don't know, but my feeling is it should be best avoided if at all possible.
Also thought about hanging the container from tree branch or putting it hanging in the middle of an aluminium stick tripod then measure the weight constantly with tension meter? Should work in "camping" environment too...
And the scale could be used to other measurements too if needed...
Another way of measuring level by pressure is to have a slow air pump bubble air down and out the bottom of a tube, and you measure the pressure of air up at the top of the tube. When the air bubbles out of the bottom, the air pressure inside the tube is equal to the water pressure at the bottom of the tube, and you can maintain your gauge in dry, accessible conditions.
While weight would be a good way of measuring the volume, hanging 40kg of water would require a very sturdy branch. The same goes for the tripod idea.
Pressure is a good way of doing it but a bubbler is out of the question as too complex to set up and maintain. Thanks
In your shoes, I'd phone Maxbotix and talk with an application engineer about the situation.
Thanks DaveEvans. Maybe I'll fire off an email. Time zone issues prevent calling I think. I'm in UK.
I'm pretty sure that tripod made of 20mm diameter 1,5mm wall thickness aluminium tubes (1m-1,5m) would take care of the weight easily...
This can take 40kg 
Thanks for the idea ,but way too complicated in practice. Initial view is that the container won't be level when suspended, the handle is not centered. Also lifting 40kg awkwardly onto a hook won't be easy. Lastly where the container is situated there wouldn't be space to set up a tripod. But thanks for the ideas.
I contacted Maxbotix and it is as I thought. The tube idea won't work due to water droplets in the tube. In their opinion the tube would need to be at least 6 inch diameter which is way too big to fit in the opening. So despite their narrow beam the idea of ultrasound seems to be impractical for my application. Thanks for the link though as it was worth having a conversation with people who know a lot lot more than me about ultrasound.
So the only method left that seems to be easy to use and accurate enough will be pressure and a drop-in pressure sensor. So back to the drawing board on this one I think.
Thanks for ALL your comments, they were very welcome.
Looks robust. What model is that? Tell us how it works after you have installed it.
In the last month I have finished a fuel level measuring project . It uses a QDX50A transmitter from aliexpress. I bought the lowest pressure one 0-0.1bar ( the tank is only 50cm high) and 4-20mA. Works great. Only time will tell if it is reliable. Says it can be used for water . Possibly worth a look
