water level measurement

Im looking for an economical way to measure the water level of a tank that fills and drains. I need an accurate constant level measurement instead of just using float switches. This will be going through a PLC. I've been looking at a pulse output flow meter and thinking about hooking it up to a counter, and I've also looked at a paddle wheel flow meter to measure the water input instead of the tank level, although those seem to be a little spendy. I see pressure transducers, (not the submersible ones) and I'm assuming they're for air only? Or would it work to mount the sensor on the bottom of the tank to get overall pressure with a 4-20ma output and set the span accordingly? It's only a 25 gallon tank.

Any help pointing me in the right direction would be greatly appreciated.

Thanks

If it is an open tank, I think a pressure transducer mounted on the bottom will work. If it is a closed tank, I would suggest using a differential pressure transducer for better accuracy. You can also use a a diff. pressure transducer for open tanks.

Pressure transducer will work. How tall is your tank? I prefer submersible hydrostatics but if the task isn't that tall you may struggle to find a fixed span sensor that won't be massively overscaled.

The only thing to watch with the pressure sensors is if you Mount it on the outlet pipe, at high flowrates it can read low due to headloss in the pipe.

If the tank is "fatter" than it is "tall", you might consider an Ultrasonic sensor pointing down from the top.
There are many good ones, and all can interface with a PLC. My personal fav is SENIX, but you'll need to do your own research.

Keep it as simple as possible. I use ultrasonic transmitters for obnoxious fluids or when conditions don't permit access. When possible I go to submersible pressure transmitters. I've had success with this unit:

http://www.kelleramerica.com/submersible-level-transmitters/

or for more obnoxious fluids (slurries, wastewater, etc.) this type:

http://www.dwyer-inst.com/Product/Level/LevelTransmitters/Submersible/SeriesPBLT2-PBLTX

In your situation, if this is just water in a small tank, get a pressure transducer and install it on the bottom of the tank. Scale your 4-20mA to the inches of water column range of the transducer (get one with a range as close to the level of your tank as possible for the best accuracy, but don't scale it to your tank level), and you're done.

Thanks for all the reply's guys, this is a big help. Im looking at a pressure transmitter that outputs a 4-20ma based on a 0-100 inches of water scale. I understand an inch of water to be the specific pressure at 4 deg Celsius which works out to about 39 degrees Fahrenheit. Will this still measure accurately at say 60 deg Fahrenheit? or 150? or is there an equation I can write in the PLC to convert so it will read right at different temperature? Im assuming I have to go with an inches of water as most psi transmitters are scaled to high since this is such a small tank.

The ultrasonic sensors are sounding pretty appealing, although would be a little more tricky to implement and all the fittings are there for a pressure transmitter

Thanks again guys

Handcraftedsince87 said:

Will this still measure accurately at say 60 deg Fahrenheit? or 150? or is there an equation I can write in the PLC to convert so it will read right at different temperature?

Click to expand...

Probably and yes.

Water does expand with temperature increase, but unless you are doing custody transfer or need really, really precise control with large fluctuations in temperature it probably won't matter. From 4 C to 15 C the volume change is about 1%. If you use the specific weight of water at your nominal temperature you may not need to consider temperature. That is how it is usually done.

And here is the formula:
http://www.engineeringtoolbox.com/volumetric-temperature-expansion-d_315.html

You can also use two contacts as an encoder (cheap) pull the float up, reset and let it go again, this will measure the real level, in digital form. We have many floats here in the river, they are pulled up by a small motor resolution is 1 mm.

Another method to consider would be load cells under the tank. This can be highly accurate for about any material and more reliable if done right, than any ultrasonic, optical, or pressure sensor.