The contamination of flow cell windows is a frequently encountered problem wherever process photometry is used for continuous measurement. With time, the steady flow of sample liquid through the flow cell will often cause buildup on the windows that falsifies the readings. Sigrist has developed a number of systems for eliminating the effects of window contamination under specific measurement conditions (differential measurement method; dichromatic method; color compensation).
If the path length used for measurement is 10 mm or less, the free-fall flow cell is employed to eliminate all contact between the liquid and windows. The measurement light is passed through the sample as it drops by gravity through the detection zone.
A precondition for the use of this flow cell is that the sample be fed to it at constant pressure. Excessively low pressure reduces the diameter of the liquid stream too much, and excessively high pressure causes contamination as a result of splashing at the supply pipe outlet. Sigrist solves this problem by supplying a supply receptacle mounted just above the photometer; its steady overflow ensures constant hydrostatic pressure. An added bonus is the elimination of any air bubbles in the liquid.