Electronic solution to compensate the effects of the temperature and the humidity on the measurements of a capacitive sensor dedicated to an injection insulin pen

This article presents the climatic effects on a capacitive sensor device which measures drug volume inside an injection pen. The variations of the temperature and the humidity have a direct impact on the value of the capacitance. Therefore, experiments have been performed by modifying the temperature from 32 deg C to 23 deg C at a constant relative humidity of 30%, inducing a variation of absolute humidity. The errors due to temperature and humidity induce a capacitance variation corresponding to a volume error of ~130 muL That represents a dose error of 13 International Units (IU) of insulin. Two main approaches are presented in this paper. Firstly, the effect of water absorption is decreased by encapsulating the analog Printed Circuit Board (PCB). Secondly, reference electrodes are added to the system to correct the climatic variation by differential measurement. It will be shown that these two combined techniques have a beneficial effect on the measurements with a reduction of the error by 70%. The remaining error corresponds to a volume error of 40 muL, equivalent to 4IU of insulin.