Teach me about seed cleaners........

I understand grain cleaners---- we have and use two  2-stage cleaners, and run corn and soybeans thru them when unloading into bins.

However.......I know nothing about how seed is cleaned thru an official "seed cleaner".

What happens inside an official seed cleaner that cannot be accomplished with different screens in our grain cleaners?

We are combining our own oats and rye for cover crop purposes, and I want the seed to be as WEED-FREE as possible......that is the primary target.........weed seed.

So.......

1.  Can I use our existing cleaners with reasonable expectations of success?

2.  If not, what are some small, low-priced cleaners that could be used for the 2000-3000 bushels that we would be handling annually?  (I see "seed cleaners" coming up on auctions, but don't know enough about them to bid half-intelligently.)

Local seed dealer wants 1.00--2.00 per bushel for cleaning------- I am too cheap to pay that before exploring buying our own cleaner.

Thanks.