Looking for a sensor that can "see" through water

[ECSI]

I am hoping to find an analog sensor that can detect and transmit the level of a mound of salt in a tank. The tank is 13 feet high and 8 feet diameter. The level of salt will be as high as 10 feet, and gradually decrease to about 3 feet as it mixes with water and is taken out of the tank. The level of water in the tank is kept up to 10 feet at all times, so as the salt level decreases it will become more and more submersed in water. I need to detect and transmit the salt level, while ignoring the water level. Any suggestions on what I can use for this application? Thanks!

[JRoss]

Maybe some kind of capacitive sensor would be able to sense the difference between saline and salt. There are capacitive based level probes, but typically they only have to distinguish between liquid and air.

[DanW]

The Siemens 'low-end' point level sensor, the CLS-100, could easily distinguish between water and the resin beads (immersed in water) used for water softening. I've never used one or seen one, but the description of the technology for the LC-300 series continuous capacitive sensor could do the same, but I'd strongly recommend trying it (or any capacitance unit) before buying. https://www.automation.siemens.com/mcms/infocenter/dokumentencenter/sc/pi/InfocenterLanguagePacks/Catalog%20sheet%20SITRANS%20LC300/sitransl_lc300_fi01en.pdf

[ECSI]

Thanks for the suggestions. I'll contact Siemens and get more info from them. I was hoping to go to a non-contact type such as sonar or radar. I figure if a fish-finder can do it on a boat then there must be an industrial type available. Then I come across a company called Hawk. Seems they may have the solution for me. Hawk Measurement

[waterboy]

I have used submersed probe ultrasonics on clarifiers to detect the "blanket" level. They work great as long as the sensor stays submerged and there is a big difference in density at the material interface. If you need a model number I can get that tomorrow.

[ECSI]

A model number would be great if you can get it. Thank you!

[waterboy]

The one I have used is a Binminder 9300 (now obsolete) from Entech. We tried thier new model "Echosmart" in an application where solids were fluidized and that didnt work at all compared to when the material was settled. So if the stuff is settled, this should do the job. Bear in mind, the probe is looking for the first change in density so it must be submersed in water at all times. http://www.entechdesign.com/products/echosmart/ I wonder if guided radar would work as well. Haven't tried that.

[andybr]

Endress and Hauser have recently been pushing a hybrid Guided Radar/Capacitive solution for interface measurement but I think this is aimed primarily at oil/water interfaces. Still it may be worth talking to them. One thing to be wary of, however, is that you are not really looking at a solid in water. In your case you are trying to measure the level of salt in saturated brine which may not work well with many instruments. The actual interface may also be difficult to pin down as the salinity will probably cause stratification in the liquid above the salt.

[waterboy]

The Binminder we used worked VERY well... when the settled material is noticeably denser. In the application I used it, the fluid was water and the material level I was detecting was water with dirt and coagulants in it. If there was less coagulant, the material would become "fluffy" with a less defined boundary and it would not work well then. If the salt stayed granular I'll bet it would see it. if it becomes a supersaturated fluid Im not sure it would. I asked Rosemount and was told that guided radar can't tell much difference in materials once submerged. If the material is under water, it wont notice the density change. Food for thought.

[DanW]

another 2 cents: I had some experience with a submersible ultrasonic transducer in a clarifier. It was a miserable experience. Apparently temperature gradient at different water levels can affect the bounce of the ultrasonic echo, in addition to the density and clarity of the water.

[ECSI]

Thanks for the info guys! I'll contact the suppliers that you mentioned and go from there. I'll let you know the results if we do make a purchase.

[Bob O]

I think the device andybr is referring to is the E&H FMP55. This device is looking for the difference in the dielectric constants(D.C.) of oil [1.8 - 1.9] and water [80]. Here is a link.....http://www.us.endress.com/#product/FMP55

[TConnolly]

I suggest you use a load cell under the tank. If the water level in the tank is always constant then a load cell will tell you how much of that mass is salt. Load cells are a common solution (pun intended) to tricky tank contents control problems and can be less expensive than some other measurement technologies.

[JRoss]

Neat idea! Could you combine the load cell with a level sensor to handle variable water level?