Wrong! The air only has to bubble if is lost somewhere between sensor and water tank buttom to take it to the level corresponding to water pressure. It might (or part of it, nitrogen) also diffund into the water.

The water pressure compresses the air according to its height. Therefor it is important to dunk the hose into the water without pressure losses since the compressed air inside the hose is equivalent to the water pressure.

Even with a little bit of water in the tank (just covering the end of the pressure hose) pumping air with excessive pressure into the hose will indicate an almost full tank.

Also a differential pressure sensor compensates automatically for local air pressure variations by leaving the negative side of the sensor unconnected (access to ambient air).

Boncuk

Attached Thumbnails

 

__________________
Proper Planning Prevents Piss Poor Performance

 

Some very interesting ideas here as I am also looking at this as a project.

First the problem with a float system that measures bouyancy is any change in water temperature will alter the calibration but it is a neat solution.

The 2 rods detecting the change in capacitance is currently marketted by VDO and they are very accurate, the sensor outputs 0-180ohms for use with a standard meter. I have used one for years but want to change to a pressure based system with a tapping at the tank bottom and differential pressure sensor. This however will not fit your case as you dont want to drill holes.

The air blow down a tube system used to be popular and was called tank tendor but after a while they tend to lose accuracy and are quite sensitive to the air flow pushed through the tube. If you (hand pump) give it a little too much energy the readings change quite dramatically.

Out of all the working systems we have tried the best and most reliable is the rotary method. A simple flat bar (say 1mm by 5mm) is evenly twisted from top to bottom and has a float attached with a slot to fit the bar, a second round rod keeps the float from spinning and so when it raises and falls the flat bar twists and output can be read from a simple pot. Very efficient and works well and is not subject to temperature changes. If you want super accuracy you could even use a 10 turn pot if you can get the twisted bar right?

 

No, not wrong.

Dunking the hose into the water will not result in the true hydrostatic pressure at the bottom of the tube. Extra air must be added to the tube.
If you start with an empty tube and dunk it, the air will compress as the bottom of the tube is submerged and some amount of water will rise up into the bottom of the tube, The pressure reading will be low by a value equal to the height of the water column in the bottom of the tube.

You could calibrate so this pressure represents a full tank, but you will be susceptible to error from temperature changes and, to some extent, gas dissolving into the water column.

 

Electronic pressure sensors are calibrated on the basis of on U-type manometer.

I guess temperature changes are negligable.

Let's assume two degC between water tank and air column (normally having the same temperature as the water in the tank) which don't lead to significant inaccuracy - say 1%.

Are you satisfied with your fuel meter in the car?

What worries me is gases dissolving into the water. I wouldn't drink that stuff - even boiled prior drinking.

Best method yet is capacitive liquid level determination. No temperature, air pressure and gases in the water will have effect on it.

Another cheap method is using a newspaper. Dunk it into the tank and check if it gets soaked. Then measure the arm length to termine the water height above buttom.

Boncuk

__________________
Proper Planning Prevents Piss Poor Performance

 

I was talking about change in temperature over time. I don't know where the OP's tank is, but if its outdoors where the temp could go from 40-80 degrees F the change in pressure in the tube could be significant (6%-ish if I'm doing the math right) depending on the ratio of the air in the system (length of tube volume of air chamber in the sensor) vs. the I.D of the tube, this could be negligible or very significant. Particularly if the tank level was low when the air temp went up and air escaped out the bottom of the tube, at which point the calibration of the entire system would be off.

If this tank is holding water or some other non fouling liquid, I think the twisted rod/ float/potentiometer idea would be the cheapest to implement, but I think binding might be a problem in taller tanks.

 

I don't like the twisted rod idea much. I can see binding problems etc and the thing gluing itself together with algae. All moving parts should be out of the liquid if possible.

What about a scissor mechanism (like a multi-stage scissor lift) with the float at the bottom of the scissor and the top scissor hinge as a pot to measure how far the scissor is extended?

It wouldn't be linear ratio of scissor extension to pot angle but that could be compensated in software. It should be real easy to build and gives a lot of vertical movement for a smallish pot rotation.

 

Quantised,
I like the twisted bar idea, but heres what im going with...

Ultra sonic sensors that measure the distance from the water and compare them with a preset distance (the empty distance)
I will simply drill a hole at the top, and mount my device on.
No holes in the side, no contact with the water, no corrosion, rust, engineering issues..
and it will finally teach me how to use these sensors!

I will encorperate a temperature sensor in the tank to tell the PIC to change a variable.
(the variable being time taken for sound to travel through air)
So when the µC is making the calculations, it takes air temp into consideration.
In summer the tanks internal air will rise dramatically on hot days with sun on it, so using the temp sensor will keep the readings accruate.

__________________

 

The twisted rod system is pretty reliable as 2 of mine have been working for over 20 years with no servicing, but it is invasive and the ultrasound is not so I like this idea very much.

Also the temperature correction is a good idea.

What is the beam angle of your sensor as it may get reflections from the tank wall when the level is low?

Looking up on the web there are lots of ultrasonic distance meters for as little as £20 and some even can do preset calculations - this may be a way to go for my project but they all seem to have a minimum distance of around 2 feet which is a problem. The infrared versions work to 1 inch minimum and so may be better.

If you get it to work please let us know!