Product description Please insert the tubes into the machine symmetrically, or will not work properly in an asymmetric state. Please add a tube of clean water to achieve symmetrical operation if you have a single sample. Please check the rotor for corrosion damage berfore operation. Please balance the weight of the tubes down to about +/- 0.1 grams. If your opposing pairs of tubes are off by more than that, it starts making a lot of noise. It has 3 adjustable speed ranges and a adequate timer (+/- 6 seconds). Although it vibrates slightly at speeds <500 rpm, it smooths out and runs very nicely > 1, 000 rpm.