Disclosed herein is an instrument suitable for processing cells for example culturing, concentrating or washing said cells, the instrument comprising: a housing for accommodating mechanical elements including at least one fluid pump; and a disposable processing kit complementary to the mechanical elements within the housing and comprising a fluid circuit including a fluid reservoir and plural fluid paths capable of carrying fluid flow caused by said pump(s), the instrument further including a mechanism for determining the quantity, or change in quantity of the fluid in the reservoir resulting from said fluid flow, the instrument yet further comprising a controller operable to control at least the pump and operable to perform a fault determination process, which includes the steps of determining the expected flow rate of said pump(s) calculated from the speed of the pump(s) and comparing that expected flow with the change in quantity of the fluid in the reservoir as determined by said mechanism.