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I have a 18" tank that will be filled with water (not pressurized). I am looking for a cheap solution to continuously measure the water level. It must have a 4-20mA output.

There's fairly cheap 4-20mA pressure transducers out there that will do the job, assuming you can get near the bottom of the tank. Find one in the inches of water column range. Try Dwyer or Omega for the cheaper stuff.

In order measure the level of water (with no pressure and normal water temperature) it is very easy to install level transmitter magnetic float type. Magnetic float will change the positions of read switches causing the increase the circuit resistance as level increases. It has transducer installed on the top to convert this in to 4 ~ 20 mA with zero & span adjustment so that you can adjust it according to you site requirement. (Make Kubler)

Cheapest way to do it with a 4-20 mADC output I can think of is a (cigar shaped) submersible pressure transmitter. There are a dozen or so manufacturers out there, at least. It can be suspended on its waterproof cable, and it can be purchased with a direct 4-20 output, or you can get one with a millivolt or voltage output (1-5 VDC is common) and use a converter.

You didn't say whether the tank is 18" in diameter, or 18" high.

The real issue is if the tank is 18" high. If it is only 18" high, and you want some real granularity in the reading, you will have to spend more money, because very low range pressure transducers are expensive. Think of it this way. If your entire measuring span is less than 18", you will need a transducer with a range of 0-24" maximum. You can use a transducer with a higher range, say 0-100 psi, but your readings will be innaccurate and not very granular.

If this is a "science project" and the tank is maybe made of plastic, write back, because there are some very inexpensive "kludges" you can come up with.

Good luck,

Walt Boyes Editor in Chief Control and Controlglobal.com www.controlglobal.com Mailto:wboyes@putman.net Read my blog SoundOFF!! At www.controlglobal.com/soundoff

Do you really mean 18 inches?, this is very short for a DP cell.

A few more Options
Ultrasonic
Capacitance
Radar
Magnetorestrictive

It's quite easy to make your own capacitance level transmitter with a length of teflon covered wire running to the bottom of the tank.

If your tank is 18 Feet strain gauge pressure transmitters are very cheap, you just need to be careful that they are compatible with the tank contents, some are good for dry gas only.

You can use a bubble tube and pressure transmitter if you don't have a bottom connection.

Roy

Roy, all of the options you mention will work just fine, but they range from spendy to spendier to spendiest. (grin)

I like your capacitance tape idea. The bubbler could be built with a very inexpensive Honeywell automotive dp transducer, and an aquarium airpump, too...but there's the requirement for a 4-20 output.

At that point, you start either having to be a circuit board maker, or a hobbyist with an account at your local distributor for Burr-Brown. You need a switching power supply, and a A/A ASIC the output of which is 4-20.

You are right about the range being short. I covered that too in another post.

Hopefully, Martin will give us more ideas and we can help him figure it out.

Walt Boyes
Editor in Chief
Control and Controlglobal.com
www.controlglobal.com
Mailto:wboyes@putman.net
Read my blog SoundOFF!! At www.controlglobal.com/soundoff

I'm not sure what your price range is but you might try http://www.flowline.com
They have moderately prices level solutions. For example, a model LV50-S001-24-3.5 which is a 24" sensor goes for around $500.