if what you are looking for has the same conductivity as the alum trash. If you are mainly looking for coins then you can ignore trash items that have a diff. reading than coins you expect to find. If you think a particular site has the possibility of yielding valuables then the common practice is to dig as many signals as possible. One thing for sure, the more you dig, the more you will find, both good stuff and trash. You will have to determine how much time you are willing to invest at a particular site. If the site has only the promise on modern coinage and other modern items, you could use more disc. By careful listening to signals, you may be able to ignore some alum. Alum. generally has to be larger than good targets of the same conductivity so by listening to length of the signal can give you a good idea of the size of the target but it's not an exact science. Also larger alum targets can be detected at higher distances so if you lift the coil up off the ground at a distance greater than what a good target could be detected then you know that the target is large and by watching the depth meter while scanning say if you lift the coil 8" off the ground and the depth meter is close to the top of the scale then you know also the target is large. Of course, most of this doesn't apply to pulltabs. Hope this helps and others here may be able to give you some good ideas. HH
Steve(MS)