Searching for metal detector

The trashcan operates by using a scale and metal detector. The assumption is if the correct relation between metal and weight is detected we can assume that it is a 12 oz can.

I've been hunting around for a good strategy for metal detection. There are a few do it yourself strategies, the best of which is this one at instructables. There are a whole slew of other metal detector schematics at discovercircuits. My understanding is that the bfo metal detector operates by having two oscillators that each have the same frequency. When metal is present between the two oscillators it will vary the frequency. I think it is the same principle in this video below.

The concern I have with this approach is that it appears to provide feedback on whether metal is proximate. It definitely does not provide precise measurements for the amount of metal detected. Therefore I think i need to get a profesional metal detector that can provide more information on what amount and perhaps type of metal is nearby. This brings us into the realm of those treasure seekers. There is a large group of treasure hunting enthusiasts who use these devices to find lost jewelry and ancient coins. Their high end devices can pin point distance from the surface, distinguish metal from minerals in rocks, and provide on board diagnostics. Not exactly what I need. There are other metal detector systems as well, for instance metal detectors are used poultry farms to make sure there aren't any metal bits in the birds. Also, metal detectors are used by various security sensors at night clubs and airports.

It appears that at this point the best solution is to buy one of the low end professional units (around $50) and get a better idea of exactly how well it can work. I can test it around the pressure sensor and then try to dissassemble it and reassemble into the shape needed for the Trash can. Time to start hunting through craigslist and ebay!