yea i totally agree with that, the sensors I have been using I put right at the top of the septic tank so they are never actually even under water, and if they are you got bigger problems to worry about lol, but thank you for the advice I appreciate it
There's some new information.
Your best bet is pressure device, the industry I work in supplies submersible pressure sensors for the sewage industry.
Tom.. 
Would you be able to recommend any?
I though there might be a problem using a drop in sensor in a holding tank having all the solids sit at the bottom with the liquids on top, are those just normal pressure sensors or are they made to combat that problem. or is that not even a problem
sorry for so many replies, but I live in Canada so freezing might be a problem, I have asked some people and they say their tanks have frozen before but idk if that's just the top layer or if the bottom was frozen too.
If it's frozen, who gives a fug, it's obviously not going anywhere.
Don't know what is wrong with using a float level, worked for yonks before all these other sensors arrived on the scene.
I don't know my only experience with drop in pressure sensors is in a temperature controlled environment, and the ones I have used don't work with solids, I was thinking that if it is frozen around the sensor it might sense more pressure because water expands as it freezes and then it would give the wrong level reading, and if the top layer freezes, and is also frozen to the side of the tank then any new liquid on top of the frozen layer wouldn't be sensed.
I have to agree with Jremington. You use a differential pressure gauge. The gauge has two ports and one is vented to atmosphere while the other is measuring pressure at the bottom of the tank. Forgive me but I will use US measurement.
How many inches of water in 1 psi?The answer is 27.679904842545.We assume you are converting between inch of water [4 °C] and pound/square inch. While the water temperature can have an effect the reality is unless you really want to split hairs on resolution the results likely will not come in light of your intended application. Note also if you want to measure tank level in a pressurized tank things get a whole lot more expensive.
So we know that 27.7 inches of water column will be 1.0 PSI. How tall is your tank? This will tell you how many inches of water to determine your gauge (sensor) range. Once you know how many inches of water are in the tank you can get really cool knowing the tank dimensions and calculate how much water is in the tank by just adding some simple math.
You mention the tank could freeze? You get some silicone rubber heat blanket material, set it and forget it. Use heater wire as used in gutters for your piping or heat tape. You really don't need much material of each. This stuff is easy to use and inexpensive but watch where you shop and buy. Your tank will not freeze.
Outfits like Honeywell make good differential pressure gauges in a wide range of features including temperature compensated. The more and accurate a gauge is, the more expensive it is.
Another method is the bubbler method. Fill a glass 1/2 way with water. Extend a drinking straw to the bottom of the glass. Now blow just enough to have air escape the bottom of the straw. Now add water till the glass is full. You should notice you need to blow harder (more pressure) with the full glass, actually twice as hard, A Google of "bubbler method water level images"
Ron
Hi, @tfunk5
The sensirs we use are $200 plus, due to the pressure and material they are in.
What is the maximum depth?
Is this a domestic or industral tank?
Tom... 
Thank you Ron
If this was just a water tank I would definitely do this, but I feel like there would be problems with a septic tank with having all the solid sitting on the bottom. If it was just some solids that would be fine but it gets to the point where there’s just a thick layer of solids and all of the liquid ontop and I feel like that would not be able to give an accurate reading then? Please correct me if I’m wrong as I said I don’t have a lot of experience with using pressure to determine level
It would be an domestic tank, max depth I’d need to read is between 10-15ft
Is it a traditional septic tank or a sewage holding tank? If it is the former then you need not worry about level. If it is the latter we need to know what your end objective is.
Use the level measurement to control a pump that pumps the sewage to an actuall septic system?
Use the measurement to know when to call the pumper service to come empty your tank?
Something else?
The answer will help determine how concerned you need to be about accumulated solids.
For freeze protection your best bet is foam insulation. The regular sewage from a house has considerable heat content. Insulation on top of the tank and maybe the sides will hold that heat and allow heat from the depths of the earth to prevent freezing.
One possibility for a noncontacting measurement would be to devise a system that keeps a float underneath your sensor. The float can be made of a material that is detectable by your lidar sensor. Of course you still have to deal with all the adverse environmental conditions.
Yea that’s a good idea, I was thinking I could find a sensor that could fit my needs to be able to get around doing this but I think I might just have to do this, I think using a PVC pipe as a rail to guide the float would be the best bet.
Sorry yes it would be a holding tank, and I would be using it to know when to call someone to come empty the tank
Could you use a ping sensor at the top of the tank to measure the distance to the top surface of the water?
Or hang a float on a spring arm so when the level reaches and lifts the float the arm rises?
Maybe a float switch?
Where I worked we used these to replace some sophisticated capacitance level sensors that couldn’t tolerate the environment which was significantly less harsh than the inside of a sewage holding tank.
A float switch would work if I only wanted to know when the tank was full, I should’ve elaborated more before but I also want to see the level of the tank the whole way.
I think I would have the same issue with the viewing angle being to wide
The time of the quickest echo of a directional sound impulse is going to give a false reading how?
You could drip water drops from above and listen for the sound to come back up. Then there would be fall time and speed of sound that mainly varies with temperature returning .... and what happens when the water dropper freezes?
Ultrasonic peep is very directional and so are the echoes. Did you set up a ping sensor and collect data to see what you get or just think about it? What have you actually tried?
Set up a float with a foil top and see what a metal detector does at different depths.