Standard errors and detection limits are reduced by use of multiple stages.
An induction charge detector with multiple sensing stages has been conceived for use in characterizing sprayed droplets, dust particles, large ionized molecules, and the like. Like related prior single-stage devices, each stage yields a measurement of the electric charge and the time of flight of the particle. In effect, an n-stage sensor yields n independent sets of such measurements from the same particle. The benefit of doing this is to increase the effective signal- to-noise ratio and thereby lower the charge-detection limit and the standard error of the charge measurement.
The principle of operation of the induction charge detector with multiple sensing stages is best described by reference to the figure, which shows a cutaway drawing of a three-stage prototype. In this case, there are eight electrically conductive tubes. The first and eighth tubes are electrically grounded. The second through seventh tubes are the sensing tubes; they are held in place by electrically insulating supports and are connected to operational amplifiers for measurement of the potentials as described in the next paragraph. Following the eighth tube is a final stopping electrode, which collects the charged particle and is connected to another operational amplifier for measurement of the potential associated with the charge. An electrically grounded housing surrounds all of the aforementioned parts.
The second, fourth, and sixth sensing tubes constitute a three-stage sensing electrode. They are electrically connected to each other, the potential on them is denoted V1, and they are connected to the input terminal of an operational amplifier. Similarly, the third, fifth, and seventh sensing tubes constitute another three-stage sensing electrode; they are electrically connected to each other, the potential on them is denoted V2, and they are connected to the input terminal of another operational amplifier. The potential on the stopping electrode is denoted V3, and this electrode is connected to the input terminal of a third operational amplifier. In operation, V1–V2 is measured as a function of time. As a particle travels along the sequence of six tubes, it induces a three-cycle V1–V2 waveform, each cycle representing the reading from one of the three sensor stages. The charge measurement for each stage can be calculated as the product of (1) the magnitude of the corresponding V1–V2 peak reading and (2) a calibration factor obtained from the V3 reading.