Actually I found out the truth. It is that there is a 10% margin of error on the use of the dialysate by the cycler. That means up to 3 liters . So foor example if I use 30 liters of a 60 liter sak the first treatment, it in reality could be from 27 to 33 liters actually used. Then the second use of the Sak is effected either up or down accordingly. This is due to the cycler not being 100% accurate. It kind of surprised me to find that much of a margin of error, but that is what I was told by Nxstage.