A portable hemodialysis system is provided including a dialyzer, a closed loop blood flow path which transports blood from a patient to the dialyzer and back to the patient, and a closed loop dialysate flow path which transports dialysate through the dialyzer. In addition, the hemodialysis system includes two reservoirs which can be alternately placed in the dialysis flow path using various controllable fluid valves. The weight, and therefore the level of dialysate, of each reservoir is measured by a preferred level sensor having a lever arm, a load cell, and a tilt sensor. The load cell and tilt sensor are electrically connected to a processor for sending force and tilt measurements to the processor. The processor may analyze the tilt measurements to correct for any inaccurate measurements of the load cell caused by the tilt.