High speed precision weighing of pharmaceutical capsules

In this paper we present a method for fast and accurate in-line weighing of hard gelatin capsules based on real-time processing of the capsule capacitance profile resulting from 5000 capacitance measurements per second. First, the effect of the shape and size of the capacitive sensor on the sensitivity and stability of the measurements was investigated in order to optimize the performance of the system. The method was tested on hard gelatin capsules with the capsule weight in the range from 50 mg to 650 mg. The results showed that the capacitance profile was exceptionally well correlated with the capsule weight with correlation coefficient exceeding 0.999. The mean precision of the measurements was in the range from 1 mg to 3 mg, depending on the size of the capsule and was significantly lower than the 5% weight tolerances usually used by the pharmaceutical industry. Therefore, the method was found feasible for weighing pharmaceutical hard gelatin capsules. Moreover, the proposed measurement system can be calibrated by using only two or three sets of capsules with known weight depending on the size of capsules and measurement range. However, for most applications it is sufficient to use only empty and nominally filled capsules for the calibration. Finally, a practical application of the proposed method shows, that precise and accurate weighing of up to 100,000 capsules per hour is possible using just a single measurement system.