Jag said Why the hell would you weigh it??
To know how much product their is in the truck , most bulk product ( rock, steel scrap , etc,,,,) is bought and sold by the ton .
How??
Here is one way
http://www.mt.com/mt/product_detail/...y=I3Mjg4NjM1MD
I installed the first one of these ever and walked it thru the government testing on site . Have you ever seen 400,000 lbs of known and un known weight ?? I have pics if you would like .
Some kind of pneumatic pressue system? (random guess)
No with strain guage load cells . Pnuematics wouldn't work because air expands and contracts to much , no to mention the water that would accumulate in the system .
How ever there are scales that use hydrostatic systems , great for explosive areas and for large weird applications out side were lighting resistance is a consideration .