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Forum Index : Other Stuff : Water tank level meter
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Been thinking about adding a PicAxe based water tank level meter/alarm project to the site. I've seen several tank level projects out there, that use a variety of sensors including resistive, floats, capacitive, etc. Problem is, they need to measure the water level at the tank. This can be a problem when the tank is some distance away, like up on a hill at the other end of the farm, sort of thing. If the tank feeds water to a house or shed, I was wondering if measuring the water pressure at the tap would give a accurate indication of tank level. See the diagram below. The figures I found on the net indicate 32 feet or 9.7 meters for 15 PSI ( 1 atmosphere ). Keeping it metric, thats 650mm ( 0.65M ) per PSI. So with a pressure guage at the bottom of a tank that holds 2 meters of water, we get a reading of 3psi when full, and 0psi when empty. If we lift the tank 10 meters up, we get 18.4psi full and 15.4 psi empty. How far away the tank is on the horizontal, or the diameter of the pipe, dont come into play, only vertical height will affect static pressure. 
A pressure guage screwed into the line, or even fitted to the tap, will tell us how full the tank is. All good in theory as far as my understanding goes, but I have a few questions that maybe someone could help with. The plan is build a PicAxe based controller with a pressure sensor. Calibrate it by pressing a button when the tank is empty, and another when the tank is full. With the two readings, the code can work out how full the tank is by measuring the current pressure, and use this to display a reading, switch on a pump, etc, the software is easy. My questions are, does the pressure change with temperature, and what would work as a pressure sensor? Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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powerednut Senior Member  Joined: 09/12/2009 Location: AustraliaPosts: 221 |
I think the pressure will change slightly with atmospheric pressure, which will change over the course of a day (low or high pressure cells moving over etc). There may be some correlation to temperature, but I don't think it will be a direct correlation. I'm not sure how much the difference in pressure is over the course of any given day, but I suspect it would be noticiable with a reasonably accurate pressure sensor. If it turns out to be a problem you could compensate for it easily enough with a second sensor measuring the atmospheric pressure. |
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mid north Matt Regular Member  Joined: 06/01/2011 Location: AustraliaPosts: 58 |
what about a map sensor out of a turbo efi car,i think they use a diaphram so water wouldnt effect it and they have sensors that go to 3 bar or 45 psi,ever non turbo map sensors will read up to 10 psi Pt Wakefield Matt |
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Tinker Guru  Joined: 07/11/2007 Location: AustraliaPosts: 1904 |
Glenn, if you want to convert water pressure to air pressure fit a T- joint near the outlet. With the outlet closed pressurize the other line with air until the pressure no longer raises. The air will bubble out at the tank end when that happens. The air pressure at that moment bears a direct relationship to the amount of liquid in the tank. The nice thing about doing it this way is that it matters not what kind of liquid is in the tank. You could use it for a fuel tank or even for black water tanks by just having a manifold to select the tank to measure. Once calibrated it was very reliable a builder of a yacht told me who used this system with just a single pressure gauge and marked scales on it for each tank on his boat. Klaus |
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| RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495 |
Glenn, if you use a differential pressure sensor (most are), the change in air pressure shouldn't be a factor as the air will be operating both on the water in the tank, and the reference side of the pressure sensor. What I'd be most concerned about is the water-hammer when you turn a tap off, you'll get a fairly decent pressure spike - quite possibly enough to damage your sensor. Calibrating with a full/empty tank sounds like hard work. The pressure relationship should be linear (unless you have water flowing in the pipes, then you've got a slew of other things to take into consideration). A calibration at a single point should be enough. Measure water in the tank (poke a broom handle in to the bottom, take it out and measure how long the wet patch is). Press the button, give it the measurement and you should be right from there. |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
A car map sensor might be one way to go. It would be reasonably accurate within the range of tap water pressure, and would be designed to handle a backfire in the inlet manifold, so should cope with the water hammer Ross mentions. I'll have a look into the output signal, it might be digital instead of analogue. I was also thinking of using a oil pressure sender, typically measuring 40 to 80 psi, or a water level sender from a washing machine, though these would only normally read up to a couple of PSI, so I might need to do some testing. Might scan Farnel too, there might be a cheap pressure sensor that would suit. The thinking behind calibrating the meter with a tank empty and tank full is the sender might be an unknown, if we take a reading with tank full, we dont know what reading to expect for a tank empty ( -3 odd PSI ). On my water tanks I fit a tap at the output, so to take the tank empty reading, I would turn off the tap and disconnect the pipe. That would give the same pressure as a empty tank. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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norcold Guru Joined: 06/02/2011 Location: AustraliaPosts: 670 |
Glen, Jaycar have a kit KC5460 in their Silicon Chip kit section, uses a PIC and has a pressure sensor included. Have been looking for such myself recently, couldn`t justify the cost so have stuck with the lead weight-rope-float ie. the treasurer put her foot down. Vic We come from the land downunder. Vic |
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fillm Guru Joined: 10/02/2007 Location: AustraliaPosts: 730 |
Hi Glenn , I will also throw my situation in to the ring, both my tanks are below the house level and rely on a pump to deliver the pressure to the house . A system that measures the actual water level in the tank with some type of sensor might also be a good option . PhillM ...Oz Wind Engineering..Wind Turbine Kits 500W - 5000W ~ F&P Dual Kits ~ GOE222Blades- Voltage Control Parts ------- Tower kits |
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MacGyver Guru Joined: 12/05/2009 Location: United StatesPosts: 1329 |
Hi Glen Ya know . . . I don't want to be a stick in the mud, but water tank levels have been measured for hundreds of years using a mechanical float. Seems to work well and is quite reliable. If your tank is a bit of a distance from your house, you can run a pipe, tube or hose from its base to your metering location. Remember: water seeks its own level, so no matter how far away the tank is, the water level in the tube will be exactly the same as it is inside the tank. If your tank is elevated 10 meters high, all you'd have to do is erect a column using something clear like glass or plastic and toss in a ping pong ball, then calibrate the outside of the column and you'd have it done. While you're at it, you can calculate the static pressure and label the column. For each foot of height, your liquid gains approximately half a pound of static pressure. Just a thought from an old fart that believes "simple is best". 
. . . . . Mac Nothing difficult is ever easy! Perhaps better stated in the words of Morgan Freeman, "Where there is no struggle, there is no progress!" Copeville, Texas |
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GreenD88 Senior Member Joined: 19/05/2009 Location: United StatesPosts: 104 |
Like Mac said simple is better , If you need to control a pump the simplest way would be a Dual Float Switch, kicks the pump on when it gets low and shuts it off when it's full. Cost about $37 over here, could easily build one for cheap out of some micro switches or a couple magnetic switches. And could still be integrated with a PicAxe if you wanted too. And to answer your OP question. Other things to note are with temperature changes, your static water pressure is gonna change, so unless you have sensors for those your gonna have to calibrate that thing a lot especially if it's sensitive. Static Pressure is dependent on three things; the weight of water (density) and with a temperature change the density changes; Height of Fluid and the Acceleration of Gravity.
---------- Edit: http://www.lmnoeng.com/Statics/pressure.htm Below shows pressure difference with Density/Temperature Change 10 Meters High Water Pressure 0*C = 0.999841 = 14.221psi 15.5*C = 0.999023 = 14.209psi 30.9*C = 0.995371 = 14.157psi 12 Meters High Water Pressure 0*C = 0.999841 = 17.065psi 15.5*C = 0.999023 = 17.051psi 30.98*C = 0.995371 = 16.989psi Licensed Master Plumber / EPA 608 Universal License / 410a Safety Certified / Medical Gas Brazer/Installer |
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| Barry T Coles Senior Member Joined: 30/07/2009 Location: AustraliaPosts: 109 |
Hi Glen The simplest & cheapest is the pressure guage & as said before you would need to know the vert height between bottom of tank & where the guage is located; you can buy reasonably cheap inline guage over pressure protectors that wont let the pressure over a given to exceed the presure rating of the guage. I am in the water industry & we use these protectors on guages that are at the top of the bore & where we have airlines down the pump column; by pumping air with a hand pump through a schrader valve set in a T piece between the guage & the airline we know how much water we have above the bottom of the pump, we introduced the protectors when the lazy ones started using high pressure 12 volt air compressors instead of the hand pump & where blowing the pressure chamber in the guage, these protectors would eleviate the water hammer when turning off a tap quickly. I would go for a metre head guage, that way you know the exact height of the water in the tank/s minus the elevation difference. Cheers Barry I need to learn from the mistakes of others. I dont have the time to make them all myself. |
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| Air Bender Senior Member Joined: 25/01/2011 Location: AustraliaPosts: 206 |
Hi Glen We use pressure sensors in a couple of 70 kl tanks at work that hold 2% dilute caustic, becouse of foaming issues other sensors are not suitable. Over the years these pressure sensors have been acurate and have not given any trouble not like half the sensors we have in our other tanks. Thest tanks are heated and cooled with steam and hot water jackets and temperature change has never afected the calabration enough to be an issue. All the best Dean. |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Yeam I'm starting to think a pressure guage would be the easiest way to go. A couple of pen marks to show full and empty. Finding a cheap electrical pressure sensor thats accurate and reliable is asking a bit much I think. I did consider a water level sensor from a washing machine, but they are only good for low pressure, and wont work for tap pressure over a couple PSI. And now that I know Jaycar have a kit based on a pressure sensor, it takes all the fun out of designing my own 
I've used a float switch to control a pump before http://www.thebackshed.com/windmill/PumpControler.asp, but there are situations where the tank is too far away to see or run a cable or 2nd pipe too. So thats where the idea of measuring pressure came from. GreenD88's post about temperature and pressure is handy, not much change in pressure at all really, good to know. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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