A Linear Absorbance Converter for Continuous-Flow Colorimeters

The photocell transmittance output from continuous-flow colorimeters can be converted into linear absorbance by means of a simple solid-state circuit. The circuit includes a field effect transistor operational amplifier and a logarithmic operator, plus several stabilizing circuit networks. The working range is 1.2 absorbance units, which for most applications is an improvement of 50% over the unmodified colorimeter, at wavelengths of 420 to 700 nm. The output can drive the conventional AutoAnalyzer recorder, or a 2-input, low-cost recorder such as the Heath Kit IR-18M.