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I'll try to explain it as lucidly as possible, but if it unclear, I'll answer questions.
What I'm trying to do is make cheap plastic bags contain a built in scale, some kind of flexible color system similar to the film thermometers used on foreheads and aquariums.
This is based on the assumption
that either A) stress on a strip with a thin chemical film within could squeeze a colored liquid up a number scale or B) the plastic itself, upon stretching, can have alternating color bands made of variable density plastic which would stretch for given weights.
("A" would allow for items to be removed as well. "B" would probably be cheaper, but would need some kind of plastic net skeleton, or once the 1/2 pound band stretched, that plastic band would rip if you added more).
This is, admittedly an overly complex solution to a non existing problem, but I think plastics manufacture has reached a point where it could do this. Just a little added convenience; those blasted scales always seem to be on the other side of the produce section.
Probably would have other applications as well.
Digital hanging scales
Hang your bag off this. [wagster, Dec 13 2004]
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||hmmn, I'm not sure I want to encourage landfill in the way you describe so:
for the 5 items or fewer type shopper:
what about having scale-in-a-shopping-bag, so as you go round you can see how much you're carrying (or adding)
scales integrated into the trolley for the more thorough shopper, you could then self-price.
||The most elegant solution to this problem is one already employed by many supermarkets: Hanging scales nearby.
||[neilp] - It strikes me that digital hanging scales [link] could be made cheap enough to install them on trolleys or simply take shopping with you. To go back to the original idea: if you could make plastic bags out of a material that changes colour as it comes under more strain, it probably wouldn't be accurate enough to weigh with but it might give you advance warning that the bag's about to tear and drop three bottles of good chardonnay onto the tarmac.
||Points taken, the scale would be best inside of the cart (trolley) and it would promote waste. I will not delete however, and will give myself a fishbone. Still, maybe an early tear warning system would be a good idea. I leave that to Wagster.
||No need to give your self a fishbone--just retitle and edit the idea, nothing that you've done so and why. Plastic which could give an indication of strain approaching the breaking point should not be difficult to produce, and could be quite useful. One approach I would expect should work would be to have an thin opaque outer layer which is not stretchable, bonded to an inner layer which is of a contrasting color (or is light if the outer layer is dark). Any significant plastic deformation of the inner layer would then cause visible cracks in the outer layer.
||You could have thinner and thinner layers of plastic that tear up to a certain point. The length of the tear would indicate the weight. (A scale could be printed horizontally above the tear.)
||I like the concept. Once developed and validated, it would be inexpensive and convenient. I could use a precision version of this and keep it in my my dive bag for weighing fish I catch.
||I'm never able to answer the question " How much did it weigh ", since keeping a precision piece of weighing equipment with me rates low on my list.