A few questions before pulling the trigger on getting a rock tumbler. If I were to get a single barrel unit and intend on doing my pennies in one run, and dimes and all the rest of the coins in a second run, do you change the gravel between runs or do you just rinse it after the pennies? Is it a mistake to not just get the 2 barrel unit in the first place? A run or two will basically get me all caught up, so a single can easily do the short term job. Thanks. Martin