A Practical Method of Determining Response Time of Thermometers in Liquid Baths

Abstract

Liquid baths are a convenient and practical tool for determining the response time of thermometers; but, unless the heat-transfer coefficient of the liquid is known, the results obtained by dip testing cannot be reduced to a common denominator or compared with the results obtained from different liquids or by other methods. Since it is impractical to compute the heat-transfer coefficient for the three-dimensional flow pattern around a thermomenter when it is suddenly immersed in an agitated liquid, a different approach is required. This paper describes an experimental method for determining the heat-transfer coefficient in any liquid. A relationship between the heat-transfer coefficients and the physical properties of liquids agitated by stirring is developed from dimensionless parameters. Coefficients determined experimentally are correlated by these relationships for molten salt, water, and oil. Correlation in a salt bath at various rates of agitation is within ±10 per cent as compared to the ±20 per cent usually obtained with the same parameters for flow of fluids around a cylinder, and is even better in a water and an oil bath at a fixed rate of agitation.