Infusion device for administering a medical fluid to a patient, comprising: - a pumping mechanism for exerting a force onto a delivery set (2, 3) for delivering a medical fluid from the delivery set (2, 3) towards a patient, - a sensor device for measuring the force exerted on the delivery set (2, 3) by the pumping mechanism, the sensor device being constituted to output a sensor signal indicative of the force exerted onto the delivery set (2, 3), and - a processor device for controlling operation of the infusion device, wherein the processor device is constituted to perform a first and second diagnosis routine during which a measured sensor signal of the sensor device is obtained and compared to an expected sensor signal, to allow for detecting a drift in the sensor signal of the sensor device, wherein the different diagnosis routines allow to detect a zero drift on the one hand and a span drift on the other hand, the processor device is constituted to perform the first diagnosis routine while the pumping mechanism is not in operative connection with the delivery set (2, 3), the delivery set comprises a syringe having a tube containing a medical fluid and a piston movable with respect to the tube, the infusion device comprising a receptacle for receiving the syringe and a pusher device for acting onto the piston for pumping the medical fluid from the tube towards a patient, wherein the sensor device is arranged on the pusher device, which by means of an anti-siphon arm is configured to fix the piston with respect to the pusher device, wherein the anti-siphon arm is constituted to press the piston against the pusher device and hence into abutment with the sensor device or with a pressure transmitting element constituted to act onto the sensor device, wherein the processor device is constituted to perform the second diagnosis routine while the anti-siphon arm fixes the piston with respect to the pusher device, wherein during the second diagnosis routine the expected sensor signal corresponds to a predefined load by which the anti-siphon arm presses the piston against the pusher device and hence towards the sensor device. - a pumping mechanism for exerting a force onto a delivery set (2, 3) for delivering a medical fluid from the delivery set (2, 3) towards a patient, - a sensor device for measuring the force exerted on the delivery set (2, 3) by the pumping mechanism, the sensor device being constituted to output a sensor signal indicative of the force exerted onto the delivery set (2, 3), and - a processor device for controlling operation of the infusion device, wherein the processor device is constituted to perform a first and second diagnosis routine during which a measured sensor signal of the sensor device is obtained and compared to an expected sensor signal, to allow for detecting a drift in the sensor signal of the sensor device, the different diagnosis routines allow to detect a zero drift on the one hand and a span drift on the other hand, the processor device is constituted to perform the first diagnosis routine while the pumping mechanism is not in operative connection with the delivery set (2, 3), the delivery set comprises a syringe having a tube containing a medical fluid and a piston movable with respect to the tube, the infusion device comprising a receptacle for receiving the syringe and a pusher device for acting onto the piston for pumping the medical fluid from the tube towards a patient, wherein the sensor device is arranged on the pusher device, which by means of an anti-siphon arm is configured to fix the piston with respect to the pusher device, wherein the anti-siphon arm is constituted to press the piston against the pusher device and hence into abutment with the sensor device or with a pressure transmitting element constituted to act onto the sensor device, wherein the processor device is constituted to perform the second diagnosis routine while the anti-siphon arm fixes the piston with respect to the pusher device, wherein during the second diagnosis routine the expected sensor signal corresponds to a predefined load by which the anti-siphon arm presses the piston against the pusher device and hence towards the sensor device.