Based on an observed decrease of effective thermal conductivity of milk during coagulation, a heated thermistor sensor was developed for monitoring curd formation. The thermistor was heated with a constant direct current, and its voltage was measured during milk coagulation, indicating a drop after the enzymatic reaction. The sensor was studied in the laboratory for different milk acidities, rennet concentrations, added calcium chloride, and coagulation temperatures. It was also tested in a commercial cheese plant for more than a month. The cutting time established by the cheesemaker in 16 experiments was compared with a prediction equation, and the standard error of prediction was 1.9 min.
RESUMOEsta pesquisa teve como objetivo desenvolver um anemômetro de fio quente para operar à temperatura constante usando-se, como elemento sensível, filamento de tungstênio de lâmpada incandescente, como braço da ponte de Wheatstone. Um circuito eletrônico foi construído e testado, permitindo controlar a tensão no sensor e restaurar o equilíbrio da ponte de Wheatstone em processos de troca de calor pelo sensor. Os resultados mostraram que a corrente elétrica e a temperatura de operação do sensor, mediante compromisso entre vida útil e sensibilidade, foram de 160mA e 140°C, respectivamente; a calibração com o sensor no circuito ponte de Wheatstone mostrou sensibilidade para medir a velocidade do ar no intervalo de 0,00 e 5,00m.s -1 e que o circuito eletrônico apresentou saturação para velocidades do ar inferiores a 0,50m.s
ABSTRACTThis research was carried out with the purpose of developing a hot-wire anemometer operating at constant temperature, using as a sensitive element a tungsten filament fan incandescent lamp as a Wheatstone bridge arm. An eletronic circuit was constructed and tested to control the voltage in the sensor and to restore the equilibrium of the Wheatstone bridge in the transfer of heat by the sensor. The results showed that the sensor's eletrical current and temperature of operation, considering useful life and sensibility, was 160mA and 140 o C, respectively. The calibration with the sensor in the Wheatstone bridge circuit showed sensibility to measure the air velocity in the interval between 0.00 and 5.00m.s -1. It also showed that this eletronic circuit presented saturation in measuring the air velocity lower than 0.50m.s
INTRODUÇÃOAnemômetros são instrumentos usados para medidas de velocidade de ar e de outros fluidos; existem vários com características de construção e operação diferentes. Para baixas velocidades, os anemômetros que operam no princípio das trocas de calor são os mais indicados e seu funcionamento se baseia nas trocas de calor entre sensor aquecido e o fluido, provocando alteração da resistência elétrica do sensor, que pode ser correlacionada à velocidade do fluido. Estes anemômetros
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