Water pressure (depth) sensors?

A quick google for "pressure transducer waterproof" found

which has sensors as low as $75

Caution: I don't know _anything_ about this stuff - I just took it as a Google challenge :-) I couldn't begin to guess whether this is what you're looking for!

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derek

Thanks, every once in a while I need someone to whack me over the head with a "Google Is Your Friend" reminder.

Turns out "submersible pressure sensor" isn't a bad search term. I'd like to hear from anyone with experience with a particular product, if there's anyone out there...

Thanks!

Not a sensor but... I'm going to use 4 Crydom RSF85Y100R switches to indicate level and automate filling of a 6 foot tall tank. They'll be mounted on Tees in a length of PVC pipe. The system uses a timer alone now (with occasional manual intervention) so that pumping takes place in the middle of the day (or at multiple specific times when we we're watering cattle but don't want the well over-pumped). I'll keep the timer but add automatic shutoff when the tank is full.

Wayne

For cheap, put a tube into the tank and a pressure gauge (or U tube) outside the tank. Blow into the tube, excess pressure will bubble out the bottom, read the gauge.

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Put a electrode on the bottom of the tank...float another electrode on a float....measure the resistance between them....can give you approximate depth....resistance will increase with depth....

thank you for listening to my thoughts......sno

Nick Hull wrote:

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Hey Bill, Have you looked at the Motorola solid state pressure sensers? I have used these in air water and even some chemicals. They are fairly cheap and seem to be above average for ruggedness.

Me

Yeah, my main problem is that the sensors have to be submersible (there are no holes in the bottom of the cistern, for obvious reasons), and have to be compensated for atmospheric pressure, as an inch of mercury is about a foot of water depth...

Looks like it's not as trivial as it seems.

True. I searched very hard for an affordable solution. A submersible pressure transducer, as you say, must be vented all the way up the cable. Lowest price I found was around $500, but I needed 150' of the vented cable. For your probably shorter cable the price might be half that. Ultrasonic sensors are cool, but again the price bottomed at close to $500, and the range didn't include my 150'.

I ended up with a tube to the bottom of the well (1/4" drip tubing), a tiny air compressor, pressure tank and differential switch, and a carefully adjusted needle valve to keep the tubing just barely bubbling.

With the pressure transducer aboveground and measuring air pressure, it can be very low cost. The first type in the chart at

cost me about $20. Output is 0-5 VDC, which was perfect for my control system, but they have a chip to convert it to 4-20 mA if you have a long cable run or a current input.

A lot of hassle compared to dropping one cable down the well, but an order of magnitude cheaper...

Loren

a) what accuracy do you require ? b) what medium do you have (potable water / rain water or poluted / salt water) ? c) is this for permanent use or only temporary ? d) do you need continuous readout ?

You would require a "vented gauge" sensor sealed to IP68. This is not cheap. $500 sounds reasonable.

Possible alternative is the good old bubble principle. Mount a pipe almost to the bottom in the cistern, and slowly pump air through the top of tube, (like limonade with a straw). You can measure the air pressure (which is the same pressure as the hydrostatic pressure at the bottom of the tank) with a cheap (maybe

Very trivial. The SENSOR does not need to be submerged. all you need is a fine bore tube to the bottom of the tank with the sensor at the top. Small bore will not allow the sealed tube to fill with water. You want a differential pressure sensor. If you can get one with a 0-5 volt output you can put an adjustable shunt across it and connect it to a 0-1 volt meter, with the shunt adjusted to make the meter read full scale when full. It will then show 1/2 when half full, and be accurate. In a cistern, likely a good idea to put a pickup filter on the bottom of the tube to keep it from being blocked by a bit of dirt. Use the pickup filter from a chainsaw fuel line. About $2 each.

I solved a similar problem measuring fuel in a dual-wall diesel tank that does not have outlets in the bottom, only the top.

I used a tube a little longer than 3 times the height of the tank, forming it into a 'U' over the top, placing a 'T' on the outside end, placing a differential pressure sensor from one side of the 'T' (air pressure on the other side) and using the remaining tubing to run from the 'T' to above the top of the tank. Then a little suction on the top can form a siphon in the 'U' and up to the level of the fuel. The sensor sees the same pressure it would see if inside the tank at the same level. Minor bubbles forming in the tubing do not affect the reading.

You can use clear plastic tubing for water, but will need a sensor designed for water. With the right sensor you can resolve 1/2" depth.

If you only need a switch action, there are wire probes that work well. Most well supply companies have these. Bill Kaszeta Photovoltaic Resources Int'l Tempe Arizona USA snipped-for-privacy@pvri-removethis.biz

Unless you specifically need remote indication... two simple methods..

1] siphon tube , drop one end into tank with weight, over the top, down to the ground, and back up to top of tank again, vacumn on end [ie..suck] and tube will fill with water, and visible level indication on outside, and pretty accurate too... 2-] brass [or similar] contacts on insulated rods, spaced vertically for indication needed [ie each foot] and driver circuitry to light leds or whatever.. 3] if you want to drill an hole when it's emplty. a simple sight glass works as well..cheap too

Try

This is a low cost wireless water level system specifically designed for water tanks. Minimal maintenance and 1 mm resolution.

Regards

Pete

If you can't submerge the sensor, the easiest way is to use a 'dip tube' type of setup. A small plastic tube with a weight on one end to hold it down in the bottom of your tank. On the other end, a 'tee' with a gage pressure sensor on one leg, and a source of very low pressure air on the other. Trouble with this is you must adjust the air flow to *very* low flow, so just a bubble every second or two percolates up from the bottom end of the dip tube.

If you want continuous measurements, you need continuous air supply. If you just want to check the level once/twice a day, a tiny hand pump can be used. Pump the air until you're sure the line is cleared, then stop pumping. After you stop, the pressure in the line will stabilize and you can take a reading. For intermittent use, you can get just a direct-reading pressure gage of the right range. To help stabilize the readings, you can attach a sort of 'diving bell' to the lower end (think of an inverted drinking cup). This will provide a larger volume of air that has to leak off before affecting the reading very much. Of course, it takes more weight to hold such an air volume under water. Maybe permanently attach it on the inside wall while the cistern is empty for maintenance.

Using a 'gage' pressure sensor means it will sense the difference between the sensing tap and the atmosphere around it, thus compensating for changes in the weather. If you can only find 'absolute' instruments, then you must get two and use one to measure the atmospheric pressure, and one to measure the pressure in the sensing 'dip tube'. Then the pressure difference is caused by the depth and density of the liquid being measured.

Along with the various differential pressure schemes, there is also the use of a float pulling up/down on a string/wire and looping the wire around a form of revolution counter (these can be direct reading, or electronic). Also the float with a magnet riding up/down a tube with a series of reed-switches. Or, an insulated wire inside a free-flooding tube to measure the different capacitance constant between air and water (the insulated wire forms one plate of a capacitor, the tube the other plate, with air/water as the dielectric). Although a bit high-tech, I've also seen the ultra-sonic range finders such as some cameras used to have to measure the distance from the top to the surface of the water.

Using two uninsulated wires and measuring the relative conductivity can be pretty impractical if you don't know the variability of the water's purity (although a separate 'reference cell' in the same tank could be used in some form of bridge). Best to use a very low voltage AC in such a setup to minimize glavonic-like corrosion of one side.

The capacitance and resistivity probes can have calibration issues if there is any growth of 'organics'. And obviously, floats sometime sink, and tubing sometimes can clog. They all have various advantages/disadvantages.

As you can see, there are *lots* of ways to measure the depth of the water besides differential pressure.

daestrom

You can convert any 4-20 mA sensor to 1-5V indication with a 250 ohm resistor, or set up a resistor circuit with a regulated power supply of your choice to give you a suitable range (example, 4.0V = 4000 liters). There are shareware programs out there where you can create a replacement meter scale and thus you have calibrated a 5V full scale meter movement to any range you want.

One other overlooked and nearly free solution is in your washing machine. A pressure switch in the washing machine control panel turns off your water fill solenoid in the laundry tub. When you change the amount of water you want in the tub you are actually changing the spring pressure on the diaphram pressure switch. Find scrapped washing machines and you have a free source of diaphram pressure switches. Of course, this is only on/off, not a range.

Good luck.

I have seen quite cheap designs for measuring fluid levels, using ultrasonic transducers to measure distance between the top of a tank and the fluid level. Sorry I don't know of any cheap submersible pressure sensors.

..and I saw a circuit that used a simple pair of wires as a capacitor with the water acting as a dialectric - that 'cap' then controlled the frequency of an oscillator, and another chip converted frequ to voltage - very neat, and very cheap to make

David

J> >

NivoTube detectors come to mind.