When the CPM cannot detect the swing accurately, the user is provided incorrect information.
The solution to this dilemma is to use a wider bandwidth CPM in conjunction with a recording device such as an oscilloscope or data recorder so that the actual product /device voltage swing can be monitored. This combination also allows the user to separate the effects of displacement current from ion current based upon wave shape or by blocking the ion current and measuring and subtracting the displacement current.
Figure 9 shows a CPM with a Digital Sampling Oscilloscope (DSO) attached to its analog output. The analog output of the CPM has a small signal (± 100 V referred to the plate) bandwidth of ~60 Hz making the composite bandwidth of the DSO and CPM system acceptable for most ionizer measurements.
If the ionizer is measured with this higher bandwidth system, it will produce a result which is representative of the actual wave shape and the features can be observed in figure 10.
Figure 9: CPM with and digital oscilloscope. Oscilloscope display shown in Figure 10.
A test to compare the swing voltage reported by a conventional CPM as compared to a high frequency system was performed and is shown in figure 9. The ionizing bar was operated at a frequency of 16 Hz while measurements were taken with the CPM plate 60 cm from the ionizer. The conventional CPM indicated voltage readings over the range 0 to 14 V, representing a swing voltage of 14 V while the CPM recorder indicated 62 Volts as seen in figure 10. Also, inspection of the wave shape shows an inflection point in each edge of the waveform, indicating that the displacement current represents >50% of the swing. With a lower bandwidth CPM the oscilloscope would not be able to discern the inflection in the waveform allowing the user to recognize the presence of appreciable displacement current. See Figure 4.
Figure 10: Offset voltages. The swing is 62 V p-p referred to the plate.
Based upon this experimental work, we believe that the use of commercially available products incorporating a higher bandwidth CPM with an appropriate indicator to show peak-to-peak swing voltage may be necessary to properly adjust the performance of AC ionizers. It would also make sense, to block the ion path and re-measure the voltage swing. MORE