SUPER soft gel sealant / silicone for pressure sensors

[DTJ]

I've starting to look around for some super soft gels for protecting the actual sensing element face on some pressure sensors.

I looking for sources of the gel like used on the Motorola MPX type pressure sensors its a fluorosilicone that is applied directly to the silicon sensor element.

1) Has anyone any recommendations on a suitable product to try?

2) What units are used to measure the hardness of these gels? They are much softer than the usual Shore A ratings. I imagine the gels in the pressure sensors are even softer than the low end extra soft OO Shore ratings.


Has anyone played with this stuff before?

[coppercone2]

do they actually harden into a solid or is it paste like?

One method you might be able to measure its relative hardness if its solid and does not flow is to make a cylinder and wedge it between two plates and weigh it down and measure its bowing/deflection.

If it flows then you might just want to look at its viscosity (so rectangular tube), unless its really viscous, then you end up with a tar-drop experiment. Maybe you can do flow at increased pressures if you make a special apparatus.

[e100]

The re-entrant silicone gels commonly used for potting circuits have a high hysteresis. If you disturb the surface it takes several seconds for it to return to its original shape. Presumably this would have a negative impact on the frequency response.

[floobydust]

I've learned to just call application engineering for assistance. There's too many products and things to consider.
DOW Selection guide shows over 24 gels: https://consumer.dow.com/en-us/industry/ind-pcb-systems-assembly/pg-pcb-gels-encapsulants.html

[coppercone2]

The re-entrant silicone gels commonly used for potting circuits have a high hysteresis. If you disturb the surface it takes several seconds for it to return to its original shape. Presumably this would have a negative impact on the frequency response.

I meant like to cast a small cylinder and then just weigh it down and look at it from the side with a camera or boroscope or something with a scale in the background to see how much it mushrooms, it should be a indicator of which one is softer at least.

[DTJ]

do they actually harden into a solid or is it paste like?

One method you might be able to measure its relative hardness if its solid and does not flow is to make a cylinder and wedge it between two plates and weigh it down and measure its bowing/deflection.

They are gel - the best description I can give is that they are will bounce back after you touch them, but they are so soft you can hardly feel it against your finger.

They need to be so soft to transfer pressure to the sensing element without losses.

[DTJ]

The re-entrant silicone gels commonly used for potting circuits have a high hysteresis. If you disturb the surface it takes several seconds for it to return to its original shape. Presumably this would have a negative impact on the frequency response.

I thought about the reentrant gels. I saw some inside some automotive electronics once and it was super sort. My concern is that this pressure sensor will be exposed to potentially dirty water and cycles of wet and dry and I'm worried crud might build and damage the surface of the gel.

[DTJ]

The re-entrant silicone gels commonly used for potting circuits have a high hysteresis. If you disturb the surface it takes several seconds for it to return to its original shape. Presumably this would have a negative impact on the frequency response.

Thanks - I didn't know about the hysteresis. We're measuring (almost) static water levels so this may not be a problem but it's good to know about.

[DTJ]

I've learned to just call application engineering for assistance. There's too many products and things to consider.
DOW Selection guide shows over 24 gels: https://consumer.dow.com/en-us/industry/ind-pcb-systems-assembly/pg-pcb-gels-encapsulants.html

I think you are correct! I will contact them for advice.

[babysitter]

Ten years ago I would have said Visilox type 191. Dont know if still available.

[DTJ]

Ten years ago I would have said Visilox type 191. Dont know if still available.

Visilox - thanks for the lead. I'll see if I can track it down.

I ordered some RVT6136 today to try out. The Visilox (from q quick search) seems like its aimed at my application.