Infrared photometric measurement of the saturation temperature and supersaturation of syrups

This paper describes experiments with the infrared photometric measurement of supersaturation of a sugar syrup, which is based on the fact that a scan against temperature of light transmission of seeded syrup has a minimum at its saturation temperature. The supersaturation of sugar syrup was obtained by determining its saturation temperature along with its actual temperature, and then using sucrose solubility data. The photometric measurement, while it is direct, operates in a batchwise (scanning) manner. The apparatus includes a heating stage assembly and a dedicated TP-801 computer with A/D converter for data logging and processing. It is stated that, with good equipment and a responsive thermal probe, the measurement time could be reduced to about 5 min. An examination of a variety of methods for seed preparation was carried out, and seed crystals of 38 to 75 micro m, dispersed with a little surfactant and crystallization inhibitor, is found suitable. The systematic errors of the measurement are analyzed in some depth. The magnitude of these is shown to depend on the size and quantity of seed, pre-crystallization delays, thermal probe delays, etc. It is concluded that the theoretical estimate of the error in the saturation temperature could be as low as -0.5 K, implying an error in the supersaturation value of some 0.0015 units. Some theoretical aspects of the measurement are discussed. Diffusion layering is examined and considered to be helpful in amplifying the photometric discontinuity at the saturation point. The rate of change of the light transmission with temperature above the saturation temperature, i.e. the slope of the X/Y trace) is considered to be proportional to the seed number density divided by its mean size. Finally, suggestions have been made for the use of the equipment off-line, as well as for modifications of the equipment to enable on-line measurements to be made from sugar pans. If on-line measurement could be accomplished it is said to represent a major advance towards optimal control of sugar pan boiling operations.