Balance Indication and Density-Range Extension in a Split-Beam Densitometer using Low-Frequency Beam Chopping

Abstract

A split-beam densitometer is briefly described in which the sample is placed in one beam and the comparison wedge in the other. The two beams are chopped alternately at a low frequency (7 c/s) before being directed to a common photo-cell. An electrometer valve converts the photocell output to a low-frequency error signal proportional to the density error. This signal is sampled by a photoelectric switch, applied to a simple memory circuit and used to operate a centre-zero balance indicator.

An optional refinement is a range-extension circuit which automatically increases the amplification at higher densities. The density range over which sensitivity is uniform is thus extended by approximately 1.0 units. Alternatively, a smaller sample spot may be used over the same density range as before. The densitometer is characterized by the simplicity and stability of its amplifier and power supply.