Gold and platinum have a much lower conductivity than copper and silver, that is why I did not include them earlier when I talked about high range valuable metals. These two metals will be discriminated out if you have your discrimination set to ignore foil, pull tabs and/or screw caps The trick here is to know your machine as well as the area you are detecting in.

What you need to do is decide if the area you are hunting in has a lot of aluminum trash, and if you want to dig it, then adjust your discrimination accordingly. Pull tabs came out in the early 1960's, (with screw caps coming a little later). If the site you are detecting, has seen little or no human activity since then (some examples are areas scraped of a few inches of top soil, where pavement or concrete has been removed, old schools, empty houses, old parks), then I would suggest using little or no discrimination. However if you are searching a 'modern' site where a lot of people are currently using the area, and there are a lot of tabs and screw caps, then a higher setting may be desirable. Just keep in mind that you may be missing gold or platinum. I try to set a either a time limit, or a set number of targets I'll dig in the pull tab and screw cap ranges, depending on how much ambition I have on a particular day as well as the location. This way I feel I am giving myself at least a chance to find jewlery and gold coins.

Some machines like the Whites DFX and XLT have what they call Tone ID, which assigns a differant tone depending on the conductivity of each target. This makes it easy to 'open up' the discrimination, but still allows you to decide on what you want to dig.

The bottom line is simply that you have to decide for yourself what you want to 'see' and dig, and adjust your discrimination accordingly.