1. Field of the Invention
This invention relates to a current normalizer, and in particular to a current normalizer having a digitally controlled variable impedance for automatically effecting current normalization in measuring the resistance of an electrolyte type particle sensing zone.
2. Description of the Prior Art
Sensing zones for particle analysis apparatus are constructed in various configurations, one of the most widely used being of the type disclosed in U.S. Pat. No. 3,345,502 of Robert H. Berg. This type of sensing zone comprises a beaker having an electrolyte containing suspended particles therein and an orifice tube containing electrolyte emersed in the electrolyte of the beaker. One terminal from a current source is positioned within the orifice tube and the other terminal of the current source is positioned within the beaker whereby modulations of the developed voltage across the orifice in the form of particle pulses may be sensed as a sample of suspended particles to be analyzed is caused to flow through the orifice. As noted by Geoffrey T. Haigh in his U.S. Pat. No. 3,745,455, issued July 10, 1973, and as the present invention, assigned to Particle Data, Inc., when a particle traverses a given orifice, there will be a change in the resistance of the orifice proportional to the product of the volume of the particle and the resistivity of the electrolyte. For an orifice which measures 10,000 ohms, for example, a 10-ohm change for a particle entering a given orifice might be obtained. If the resistivity of the electrolyte is changed such that the orifice resistance is 20,000 ohms, a 20-ohm change will occur for the same particle. However, when a current is forced through the electrolyte, some degree of back voltage is generated due to polarization at the electrodes.
Haigh observed that if the current through the electrode is held constant, the momentary current change caused by passage of a particle is independent of electrolyte resistivity, and that if the voltage drop across the orifice alone is held constant, the voltage change caused by passage of the particle is independent of electrolyte conductivity. He further observed that holding the voltage constant is not practicable; whereas, holding current constant may be practically realized. This results because, when the amplifier input impedance is nearly matched to that of the orifice, as is generally desirable for the best signal to noise ratio, the amplifier is sensing partly voltage change and partly current change at the orifice. Therefore, the sensed signal is proportional to the volume of the particle and some fractional power of the resistivity of the electrolyte. Correspondingly, the provision of a voltage source to program the orifice is insufficient because the counter-emf generated by electrode polarization is a variable.
In view of the above, Haigh provided current programming of the orifice so that a constant voltage drop is provided across the resistance of the orifice. As an implementation, the resistance of the orifice for a given electrolyte is measured and the system is adjusted to compensate for that resistance. The details of the implementation may be had by reference to the aforementioned Haigh patent which is fully incorporated herein by this reference. It should be pointed out, however, that the current programming is effected in that disclosure by means of a potentiometer or the like while utilizing the instrument operator for visually observing a scope and/or pulse count rate indicator from which to manually adjust the supply current.