Fluorosilicone gel chemical reactions. Alternatives? 3

[bill318]

We are using 0-2 PSI pressure sensors to measure the fluid level in recreational vehicle holding tanks. The sensor assembly is made by another company using pressure sensor dies from a well known sensor manufacturer. The dies are housed in an ABS plastic fitting and the wet side of the sensor die is protected by a layer of what I believe is Fluorosilicone gel. This gel forms the protective diaphragm of the sensor.

The problem we are having is that the sensors are failing due to water and/or gas migration past the edges of the gel diaphragm. This migration is causing corrosion of the sensor die underneath the gel and its subsequent failure. The failures are only occurring on sensors installed in the gray water and black water tanks. The gray water tanks hold waste from sinks, showers, dishwasher and cloths washer. The black tank holds waste from the toilets only. The fresh water tanks have very few failures.

Upon removing the defective sensors, it appears that the gel is shrinking and curling up on the edges, loosing adhesion to the plastic housing. In some cases we have also noticed an off white concretion that forms a layer over the sensor diaphragm and plastic sensor housing. This layer is very hard and prevents the diaphragm from transmitting the small pressure changes to the sensor die. So there are two failure modes at work.

Can Fluorosilicone gel oxidize or even shrink in the presence of household chemicals and human waste products found in these tanks?

The sensor is very cost effective and we would like to continue to do business with the company that makes them for this purpose. However this problem needs to be solved and I think the answer lies in finding an interface material that can take the place of the silicone gel in this harsh environment. The layer of gel is no more than 1/4” thick and 3/8” in diameter. The only material I could think of to replace the gel would be synthetic oxygen safe grease. The grease should stick to the plastic parts and provide a good seal and not react with oxidizing agents/chemicals in the water. Then too, I was hoping that the grease would not be as prone to letting concretions form on its surface. This replacement interface material must not react with the very thin layer of Fluorosilicone gel that covers the surface of the stock sensor die. It must also be plastic safe and its own surface tension and adhesion properties must hold it in place. It would be nice if the new interface material were non-toxic and food safe so that we could use the same sensor for all three tanks.

Sorry for being so long winded, but I wanted to provide enough information to work with. Any help on what is happening to the current gel or possible gel alternatives would be appreciated.

-Bill-

 

[kenvlach]

Suggest a vapor-deposited coating of Parylene on the silicone.

"Parylene: A Highly Effective and Efficient method for coating Rubber and Silicone Products
Parylene provides a barrier against leaching of elastomer bi-products and protects elastomer surfaces from organic solvents and inorganic reagents, acids and saline solutions.
Parylene substantially improves surface lubricity or slippery-ness..."

http://www.scscoatings.com/parylene_applications/rubber-silicone.cfm

 

[Compositepro]

Place the sensor at the end of a vertical tube connected to the bottom of the tank there will be an air pocket in this tube that prevents liquid contact with the diaphragm.

 

[bill318]

Thank-you for the postings!

I had read a paper on using Parylele and Fluorosilicone together to enhance the life of a pressure sensor, but to be honest, most of the chemical jargon used was a bit over my head. I'll see if I can find the paper and post a link to it here. Maybe it will make more sense to readers here than it did to me at the time. I'm assuming vapor-deposition takes specialized equipment?

About 6 months ago, we had begun placing the sensors on a right angle fitting to trap an air-bubble between the fluid and the sensor gel. This certainly helps, but we still are seeing some failures. The failures went from about 2 months to more like 6 months. So I'm not sure if it is capillary action (fluid creeping up the sides of the sensor bore, past the air pocket) or gassing that is still causing the problem. Our biggest concern is that we have installations in the field that cannot be easily retrofitted with an angle fitting or extension tube without having the tube or fitting getting clogged with debris.

The sensor manufacturer is now trying some sort of primer on the plastic sensor parts and a "stiffer" fluorosilicone gel to see if this will mitigate the problem. They seem to think that without any water pressure on the sensor that the gel is migrating away from the sensor housing and when fluid is re-introduced, it slips by the gel and attacks the sensor element.

-Bill-

 

[moltenmetal]

On a liquid-filled application, a gas pocket in a piece of tubing is only a temporary solution at best. Over time, the gas pocket will dissolve in the water, leaving the sensor in contact with the solution and subject to corrosion by it. Unless there's a way for that pocket to drain itself of liquid and recharge the gas pocket each time the tank is emptied, it's only delaying the inevitable.

Instead, you could try mounting the unit below the pressure tap and fill the sensor and tube leg with a fluorocarbon oil such 3M Fluorinert. Although this product is not inexpensive, it is remarkably insoluble in water, much denser than water, and non-reactive. Short of going with a more expensive sensor construction or an intermediate wetted diaphragm made of another material, this may offer you some additional life from your cheap sensors.

 

[btrueblood]

And at some point, the cost of assembly of the gizmos to prevent water from reaching the chip, and cost of failures (including lost sales), you'd have been money ahead to have used a stainless OEM sensor in the first place.

 

[bill318]

moltenmetal The tanks get drained about every week so there is not a problem of refreshing the air pocket. Interesting idea about the Fluorinert.

"you'd have been money ahead to have used a stainless OEM sensor in the first place." We used to use a stainless diaphram industrial sensor, the problem with these is their over-pressure range. We use a 0-1 psig sensor and the over-pressure on these sensors is 2psig. We had numerous problems with customers overfilling the tanks, over-pressurizing the system and blowing the sensors. These sensors were over 10X the cost. We have had no over-pressure problems with the new sensors, just media compatability issues.

The paper I was referring to earlier is "Reliability test methods for media-compatible pressure sensors." and a google search brings up the following but I'm not sure if you can download it anymore, however the cache copy is still there.

http://www.google.com/search?hl=en&lr=&q="Reliability+Test+Methods+for+Media-Compatible"&btnG=Search

Bill

 

[moltenmetal]

If you don't make your impulse line large enough in diameter, it won't drain and recharge the gas pocket even if you do dump the tanks once a week. It'll contain a stagnant layer of liquid which won't exchange with the bulk. If your intent is to protect the sensor with a gas pad, make sure the tube is large enough in diameter to gravity drain.

 

[itsmoked]

Have you tried just a simple membrane. It can be pretty cheap. Something that you could screw in in place of your sensor and then screw your sensor into. That would allow easy refit. Personally I don't think greases or chemicals or anything else will LAST very long in that sloshing chemical mess without being constrained behind a membrane anyway.