Sensor to measure tank volume

Hello I want to measure the level of water in a tank. At first I wanted to use a hc-sr04, but we are talking about a septic tank so I don't want to make any holes in it or put any electronics into the tank itself. Is there any sensors that can measure the waterlevel from outside the tank? Either measure distance from the top until it hits the water (like you woild with the hc-sr04) or placing the device on the side of the tank displaying if there is water above this horisontal line or not. It is a metal tank Thank you for answers

The septic tanks I'm familiar with stay full all the time, when new "stuff" comes in, old water runs out into the drain field.

This one doesn't though. When it's emptied it's compleatly empty Yu can get devices at boatshops that works the way I have described, I'm just not sure how they measure

What material are the tank walls made from, and how close is the outside of the tank to anything?

MarkT: What material are the tank walls made from, and how close is the outside of the tank to anything?

It's made of steel or aluminium. It's connected to the wall on two sides, the other two are facing a storage room, the top is about 15cm from the celing

How is an empty tank a septic tank? This thing is indoors? Why are you wanting to measure what's in there? Is there some sort of action that needs to happen manually when it reaches certain level? What triggers it to empty? Where does it empty to? What device at the boat shop are you talking about?

Okei, I'll try to explain in more detail. I want to measure the volume of the septic tank in a sailboat. When you start the trip the tank is compleatly empty. Every time you use the toilet, the tank gets a little bit fuller. The tank is small, and when it is full you cannot use the toilet anymore. Therefore it is emptied, in some contries you have to do this at the docks ,other places you can do it in the ocean. Anyways, no matter what rules apply you cannot empty it anywhere you please. Therefore it is useful to know if the content is above a certain level, say 3/4. I want to know either "yes it is above this level" or "no it is not". The people onboard then decides what to do about it, but this has nothing to do with the arduino. The monitoring does not have to be constant (I was thinking more like turn it on, check what it says, turn it of). This is the device from the boatshop. The description simply says that it lets you know when the tank is 3/4 full and that it goes on the outside of the tank, no need to take it apart or drill any holes http://www.seatronic.no/toaletter-septiktankmalere-c-104_389/gobius-1-septikmaler-p-2991

Looks like an ultrasonic sensor but more along the lines of a stud finder. Takes a reading on an empty tank as a baseline, figures some kind of threshold for difference when 'fluid' is on the other side instead of air.

Not sure what purchasable sensor can accomplish that for a DIY. Is there a way to pressure gauge the outlet or does the stuff get sucked out like a porta potty? If there's a rubber section of hose or something at a low point, you can figure volume from pressure.

First, is the tank metal or non-metallic? If the later then my first thought was a capacitive device. If the dielectric is air then the value will be low, and if liquid, it would be higher. I have no idea what the sensitivity would be but you can perform a simple experiment to find out. Take a container, like an empty one gallon milk jug. Stick on the outside at the corner on adjacent sides two patches of copper foil with wires attached. Measure the capacitance when empty and then filled with water. If this works then place patches at the height you need. You can use an astable 555 timer at the tank and measure the pulse width with the pulseIn() function. You also might be able to put two long vertical strips of copper from bottom to top and get it to read like a fuel gauge. Read the values at empty and full and use the map function to convert pulse width, Ce/Cf to tank volume, 0/100.

If the tank is metallic then you would have to put two insulated rods into the tank and measure the capacitance between them.

PS The other posts appeared while I was typing mine so did not know the tank material.

Post #4 was made a day ago. If that post took you a day to write, you get an award for dedication :)

I would drill a small hole and stick two flat plate leads in there at the level I want to sense. When submerged, circuit gets completed to be sensed in any number of simple ways. That'd be my dead simple no extra cost solution. I'd spend more time making sure that hole was well sealed than I would building the sensor.