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If you want "something cheap, reasonably accurate (say 10%, 20%, etc) and low powered that I can hook up to my emonTH clone.", then I haven't come up with a solution because I don't have that as an application, but I'd suggest a float and a counterweight linked by a cord running around a drum on the top edge of the tank. Put the drum on the end of a multi-turn potentiometer and read off the voltage. You only need to energise the pot when you make a reading. The other sensors are powered in that way too, so most of the time it is no-powered, not low-powered. You might find the cord creeps on the drum over time, but a scale on the outside to read the position of the counterweight should allow you to quickly and easily re-establish the calibration.

Coincidentally I've just been playing with the load cell from a redundant kitchen scale.   How about one from a cheap bathroom scale to weigh the water?  Mine has a discrete op-amp to amplify the tiny voltage from the Wheatstone bridge, which is easy for an Arduino to digitise.  The rest of the scale electronics are encapsulated.  I don't know if continuous loading would affect the output - scales normally zero themselves at power-up.

I haven't tackled the harvested rain water measurements yet, I had pondered on the "repurposed scales" idea for a while but as I have 5x210ltr tanks I quickly ruled it out, not only do digital scales usually measure a difference in weight so a permanent load may not be possible, but 210ltrs = 210kg, not many bathroom scales would take over 33 stone, and needing 5 makes it a bit more expensive,

My current preferred method would be air pressure sensors tubed to the bottom each tank, the recommended pressure differential sensors do get a bit expensive but I wonder whether that level of accuracy is required and a basic pressure sensor would do the job.

Another half baked idea I had was to use a number of cheap leds and one light sensor, using a simple float arrangement to move the led array or sensor whilst the other was fixed so that when the leds are blinked sequentially the led blink that coincides with a detected pulse indicates the level. 

Paul

I went with the precision hydrostatic level sensor, but I agree it's difficult to justify the cost and the need for that much accuracy.  Although, as with many measurements, once you have the data you can find unexpected uses for it.  I now know how much water various activities use.  For example, in the fist pic I can see that Karcher'ing the pool cartridge filters costs me about 75L of water.  In the second pic I get a feel for how clogged the anti-mosquito mesh is on the overflow drain, by how quickly it decays back down to 3500L.

Hi Paul

Which sensor did you use?

Regards 

Ian

Still there Paul? :)

Who? me? Yep I'm still here!

Sorry Ian I missed your post,

I haven't implemented anything yet. It's quiet low down on my to do list, but it is under consideration and although I see a lot of benefit in using differential pressure sensors, they are quiet an expensive option so I did buy a very cheap air pressure sensor off ebay just to try, but I didn't get around to it as I started wondering about blockages due to it being (unfiltered and untreated) harvested rain water and with no secondary (manual/visual) method to double check the pressure sensor I started thinking of other options.

Paul

I've used one of these before on a boat...

http://mcsboatproducts.co.uk/portfolio/waste-water-gauge/

It fits in a push fitting tee'd into the outlet and measures the pressure of the head of water.

You can see inside here.

http://mcsboatproducts.co.uk/wp-content/uploads/2012/01/mcs_sender_gauge...

They retail for about £80, so I suspect the sensor is about £10 to £20 of that cost.

Think the sensor on the above looks like one of these

http://cpc.farnell.com/honeywell-s-c/24pcefa6g/pressure-sensor-0-0-5psig...

Which are about £14 [£17 inc VAT]

From what I recall that price seems in line with the pressure differential sensors I found at the time, although I would of been looking for lower operating voltages. I (currently) have 5 tanks so the costs start to ramp up, the sensor I bought was only a pound or 2 so I would still be tempted to try a "non-atmospheric pressure compensating" option first as I'm only needing an approximate indication.

Paul

Er --- I think that might not work. Normal atmospheric pressure goes from about 970 mb (a deep winter low) to 1040 mb (a summer high) at sea level, a swing of 70 mb. That equates to a water column head changing by 710 mm.

At the time I had thought I could "adjust" the values "enough" using weather station data, this is only an indicator the submersible pumps have float switches and the the tanks have overflows. Granted I hadn't expected the swing to be 710mm but I could live with anything under 300mm which doesn't sound totally un-achievable.

Paul

Are all the tanks outputs plumbed together?  If so, you'd only need 1 sensor?

They are interlinked but not all connected all the time, I can open/close valves as required so individual levels are required to optimise their use, possibly even prompting or automating down the line.

Paul

HC-SR04?   £2 on ebay

I never used one of these, but I have been meaning to get around to it at some point.  I do however know someone who uses this device to measure fuel oil levels.

I would be tempted to place it at the top of a section of vertically mounted drain pipe placed in the tank (with holes drilled at intervals horizontally or open at the end) so it's just sensing the level in the pipe rather than getting echos from the side of the tank but no idea if that is useful or over the top)

In fact... I think I'll order one and give it a go!

(The only potential issue is the need for 15mA @ 5V)

That ultrasonic will will but only for a week. It's a very wet, humid, hostile atmosphere in a water tank. It'll kill that sensor as it's not waterproof... Shame really.  :(

Maybe you could use the ultrasonic device outside the tank on the counterweight for a float inside the tank?

That could work. Bit of drainpipe mounted down the side of the tank. Ultrasonic in the *bottom*, two pulleys, two weights linked by some thin nylon rope. Careful playing with the weights and a small float inside the tank and as the water is used the column of air in the pipe gets longer! Nice...

Still an issue with the 5V in my case though.

The same could apply to your idea of the multi-turn potentiometer (never thought of that!) just mount the pot into the rope with another pulley.

Sounds a bit too mechanical to me though, too many moving parts may hinder long term reliability? Though it is a sold Plan B and very cheap and very accurate!

My latest thought is have a play with capacitance based measurement, I'm waiting for some big resistors to arrive, I'll hopefully have a play this week if I get some spare time! 

In my experience, capacitance is hairy. What distances are you thinking about, and where is the capacitor - what forms the 'plates'? If one plate is the (earthed) water and the other a conducting surface somewhat above the highest level, remember that you've got another (stray) capacitor to the top and around the edges of the upper plate that's at best fixed and much larger, and at worst completely unknown and variable, depending on the material that the tank is made of. And of course capacitance is proportional to 1/d, so highly non-linear, and in the order of picofarads - depending on the area and depth of the tank of course, and in parallel with those stray capacitors that could easily be an order of magnitude larger.

Let's say your tank is a 1 m cube, and the sides and top are perfect insulators, C = ε × A / d where ε = 8.854 × 10 ^-12 for air. You've got 8.8 pF empty and 79.2 pF at 900 mm depth of water for the 'wanted' capacitor. Hmm.

I see what you're saying Robert, I'm just going by a few examples on the net, using pipes within pipes and also aluminium angle with a wire strung down it. My tank is 1500L ex-orange juice container like this http://www.dvcontainers.co.uk/product.asp?pid=19 6ft tall 3.5ft wide, half buried outside my kitchen window! :) It has a small 'shed' built around the half above ground in an attempt to keep the cold out to prevent it freezing during the winter (hence the current emon temperature sensor in there).

_{http://www.instructables.com/id/Capacitive-Fluid-Level-Sensor/}

_{http://www.instructables.com/id/Building-a-Capacitive-Liquid-Sensor/}

_{http://www.medpants.com/liquid-level-sensor}

_{http://njhurst.com/electronics/watersensor/new/}

There is also this slightly more complicated(!) version using a 555 timer.

_{http://dl2sba.com/index.php?option=com_content&view=article&id=177:capacitive-water-level-sensor&catid=30:arduino&Itemid=61}

Your thoughts please gentlemen.

Edit - duplicate posts deleted - Moderator, BT
Please do not make duplicate posts!

@BT Sorry about the duplicates, I posted it once and it didn't appear. So I wrote it again and posted it again, the second version appeared on it's own, I've just edited that first post and from my viewpoint it's disappeared. Anything up with the site?

What's "up" with the site is the link probably alerted the spam filter and the post went into the moderation queue. Duplicate posts waste everyone's time and we delete them. It's explained in "Read this before posting", which you'll see top right when you scroll up to the top of this page.

Cross-posting dilutes the answers, so we delete those too.

I think your problem with all those ideas will be preventing them from turning into conductivity sensors after a while.

Robert, with tap water I'd agree. But rain water has nothing in it. On my admittedly basic multimeter there is zero conductivity. I started down this route with a conductivity test rig linked to an Arduino but it didn't work with rain water, hence trying other avenues.
 

I can't say that about the rain here. I've measured about 600 kΩ between a metal can with about 10 mm of rain water in it and my meter probe dipped in to a depth of about 5 mm.

re: cheap ulttrasonic sensor 

I see references to covering each of the two halves individually in cling film to make a more waterproof version.

Might be worth a try?

An interesting idea, I assumed that would just stop/attenuate the ultrasonic sound from exiting the emitter or entering the receiver, but some googling reveals that it barely affects the readings. 

What I can't tell though is for how long it worked afterwards or what sort of conditions it worked it.

Inside my tank, it's very wet and humid with water dripping off the lid of the tank due to condensing up there. I suspect even with clingfilm covering the sensors, if a drop of water forms on the clingfilm it will completely mess up the readings. If your tank is open to the elements, or inside like Felix's (from low power labs) sump is, you won't have any problems. Horses for courses...