Poly beads level detection

what are you looking for? I assume you're not really too concerned with the weight of the content of the silo, just the level of the beads, when it's full and then again when it's at the time to refill.
I'm thinking that your silo is basically a pipe stood on end with a funnel welded to the inside of it and I assume there is a hole for access to the inside of the base below the funnel, if so, then the strain gauges could be attached to the inside of the pipe around the base where they would be protected from weather, damage, static electricity, filling hoses, Etc.

A strain gauge is glued to the surface of the metal and works similar do a load cell.

I would suggest that you call or spend a few minutes online chatting with https://www.omega.com/

Ah i may have misunderstood, what you said about the grounding and the trucks. My take was you were going to use two ropes with electrodes on (welding rods), one for each plate if you see what i mean. but then if you ground it out while filling up and blowing them through surely your going to get a wrong reading?? Or are you saying its always grounded and the electrodes are acting as a single plate and the silo as the other??

I think i have missed something you said. At the moment it now sounds like your saying its always grounded and the electrodes are one plate? if so then yes mark me up for another stupid of the week nominee, if not then i dont get how your going to use two strings in the silo and get a reading when grounding it out??? Sorry if i have confused you, i have also now confused me.

If you cannot load cells under the silo, that is a proven process. The wind loading will esseially cancel as the cells the wndward side will show decreased weght and leeward side will show increased weight.

If you cannot install load cells, and you only need 5% accuracy, we would occasionally measure current draw on the impeller used to mix the contents. Assuming the blades on the impeller were stopped and contents still, integrating the current draw for the first five seconds after starting the motor showed a pretty good estimate of tank level - assuming there was no cavitation near the impeller blades.