Proper Non Contact Sensor for Water Based Solution in Glass Tank

deanfran

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Apr 2011
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We've had an intermittent issue with this for years. The application is sensing a level, or more accurately the absence of level in smallish ~5-10 liter Pyrex buffer tanks. The OEM solution i.e. a capacitive prox switch for the tank fill signal works very well, except for when the product is high conductivity, such as a 7% NaCl or 24% sugar solution. We don't know if the switch picks up whatever liquid is still adhering to the interior surface of the pyrex, or what. We've tried different sensing distances, but that doesn't seem to matter. The operators end up constantly fiddling with the sensitivity adjustment on the prox switch, and that just doesn't cut it. Any ideas, experience or, suggestions are appreciated.
 
The capacitive proximity sensor is the sensor for the application.
What about using something with a push button teach function? Similar to the following:
https://cdn.automationdirect.com/static/specs/prox30mmcapct.pdf

The other option that comes to mind is the ultra sonic sensor. This must be mounted from the top looking down into the liquid to sense the level.
https://www.automationdirect.com/adc/overview/catalog/sensors_-z-_encoders/ultrasonic_proximity_sensors

Hope this helps you out.
Regards,
 
Load cell, take into account the specific gravity of the fluid. So you need to know what's in the tank and when as a parameter, which includes the temperature
 
The Siemens CLS100/200 capacitive point level sensors claim some discriminating sensing technology that can handle difficult level sensing apps.

I've used them for solids sensing - corn level through a glass sight window in a bin, and the level of resin media in industrial water softeners both by contact and through a sight glass. I was amazed that it could adjusted to tell the difference between water-and-resin and water only.

It's designed as a contact device with a tapered teflon coated sensor body so mounting externally is always a challenge and I'm amazed at what people come up with.

A Siemens distributor can probably come up one or the other to let you try one and see if it can avoid a false level reading due to residual liquid film on the pyrex (I think you're right on that point). My gut feeling is that it probably can.

Dan
 
Load cells aren't really an option here, and punching through the 1 inch thick stainless steel top on these vessels isn't something I want to get into if I can help it. We've actually tried a couple of the teachable cap prox's from AD. They exhibit the same problem. I've contacted Banner Engineering. They claim to have some sensor solutions. I'm going to reach out to whomever our closest Siemens rep is too. One idea we've come up with is a plastic float if you will, that just stays inside the Pyrex container and would sit on the surface of the liquid. We could sense that easy enough. This is an aseptic application, so our float will have to meet a few requirements, clean, smooth, be able to survive steam in place operations. The tanks fill and empty from the center bottom, so a doughnut shape probably fits the bill.
 
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in a glss tank you can use a simple light reflector, the light will reflect different when there is water in the container.
in very old days the level of the steamboiler worked with a light thru the glass tube.
 

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