I need to sense the level of water in a tank. I want to sense between 2 inches (5 cm) and 6 foot (183 cm).
Can you suggest a sensor that I can use (affordable).
Accuracy of about 10% would be ok.
Dip tube and gauge sensor: http://david.neonquill.com/projects/rain_barrel/
For more complex/expensive ideas: http://www.sensorsmag.com/sensors/leak-level/a-dozen-ways-measure-fluid-level-and-how-they-work-1067
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Ultrasonic sensors are very popular too... bounce a chirp off of the surface of the water, measure the time it takes to receive the echo.
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2" = 5cm
Sorry. I do a lot of 3D modeling in millimeters for 3D printing, but I was born and raised in Tennessee, so my brain does these calculations a lot (at least on dimensions 200mm ~8" or less). 
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@wgbartley, don’t be sorry for correcting me. I see now that I was wrong, thank you for pointing it out. I will edit the entry.
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You can miss a planet that way 
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Great information guys. I appreciate your input, and have learned a lot from it.
I liked the idea of the preasure sensor, http://david.neonquill.com/projects/rain_barrel/
but the last paragraph (status) said it did not work, losing pressure at the sensor.
I liked the Ultrasonic sensors, but, the ones I tested was not very accurate (less than 10%) after about 5 foot.
Can anyone address my doubts about either or both of these solutions ? Or other options, please.
Any chance you could use a scale? Subtract the weight of your empty container, then the weight of the liquid will be easy to convert to volume. And a little more math would get you the height, based on the radius of your container.
Though, with that much water, I’m not sure how much it will cost to get a sensor that can handle that much weight and still give decent accuracy for your low-level condition.
I’m wondering if you could use some sort of plumb-weight and floater, with a string on a roller, and a rotational encoder of some sort? Does that make sense? I’m thinking a self-rewinding (with a spring) roller, a string with a weight heavy enough to force it to unroll, then countered by a float sufficient enough to keep the weight from pulling it under the water.
Then if you can somehow track the up/down rotations as the water level changes, you can know how far the string has been let out.
I have no idea if that exists in a pre-packaged way, or how hard it would be to do from scratch, though.
@dougal, That’s a couple interesting ideas.
I don’t see how I can put it on a scale for weight. This tank is only 300 gallons (how many pounds is that), but I cant see an easy way to weigh it.
the idea of a plumb-weight and float is intriguing . I have seen lots of radio volume controls that you just turn, and seem to just produce pulses to change the volume or channel. I can almost see that. Do you know of any hardware that would send a pulse as it is turned? I will look for it to after supper. Wait, maybe it does not have to produce pulses… How about a 10 turn pot?
So this guy couldn’t make his sensor leak-free - I don’t think that invalidates the solution.
Just make yours not leak (having fewer transitions might help.)
@AndyW, My thoughts to, Just adjust for the problems, and keep trucking.
What do you think if we keep everything below water level? I think his tube, and maybe the sensor was above water level.
The sensor usually is above the overflow level, so it always stays dry.
I looked at the specs, of the MPX4250GP, and could not determine if it was wet, dry, or either. What do you think?
Usually, for these sensors, you run an open ended tube to the bottom of the tank, with the top end where the sensor is above the overflow level. The sensor should never have water up against the diaphragm, but the humidity in the tube between the sensor and the water will be high, so a sensor that can take humidity is a must.
The sensors are cheap enough to just try it and see how well it works. My guess is that if this is outdoors, you will likely need temperature compensation for the readings as well.
That’s about as far as my experience/knowledge of this goes, so I’m going to take a step back and see what you end up with for a design, good luck.
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Thanks @AndyW, you provided me lots of info. Based on your info, I have ordered a MPX4250GP . When I get it and test it, I will post the results.
As far as weighing goes; you could use these load cells under an X-construction. The load gets split over the 4 support points which should each measure 1/4 of the weight: http://www.seeedstudio.com/depot/Weight-Sensor-Load-Cell-0400kg-p-2379.html
Why not think of using the ultrasonic sensor, it could work
Hi,
There are also capacitive sensors you could try. These can be non-invasive so you don’t have to worry about leaking.
Best regards,
Clive
Yes, I now see how that would not be to hard to setup.
the MPX4250GP sensor seems to have that restriction.I would like to find one that did allow water.
I think these sensors are used a lot in washing machines as you describe. That seems to work well since the air tube is re-primed with air ever 20 minutes. But I think that if it is not re-primed for 20 days, the air will go out, causing wrong readings. Like a balloon. It will hold air for 20 minutes fine, but probably not for 20 days.
Any easy way to re-prime the air hose before a reading? Readings normally ever hour.