The Influence of Radiant Heat and Air Movement on the Cooling of the Kata-Thermometer

Abstract

The factor of the kata-thermometer as deduced from the relationF = 0·27θ × cooling time in secondsis not constant, but increases as θ decreases. The relation between F and θ is shown, and it is recommended that the factor should always be corrected to the value corresponding to a temperature of 60° F.The rate of heat-loss calculated from the factor determined by the still air method is not the true value, but under-estimates it. If the true value is required it can be obtained with reasonable accuracy by increasing the still air value of the factor by 20 per cent, in the case of the body-temperature kata, and 31 per cent, in the case of the high-temperature instrument. For ordinary use however, it is recommended that the still air factor should continue to be used.Observations were made in still air with standard kata-thermometers and also with silvered instruments. From these observations, and from determinations of the emissivity of the bulbs, the heat loss by radiation was estimated. The remainder after deducting the radiation loss from the total heat loss gave the loss by convection. Estimations of the convection loss from the silvered and plain katas corresponded closely. An equation is given for the convection loss in still air.The whirling arm method was used for investigating the effects of air velocities up to 1 m. per sec., corrections being made for swirl. The equation deduced from these observations on the standard body-temperature katas was practically identical with the equation of Hill, Vernon and Hargood-Ash. An equation of similar form was found for the silvered body-temperature kata-thermometers, and equations are also given for the high-temperature instruments. When the true rate of heat-loss is used it is found that one equation can be made to fit the observations with both high-temperature and body-temperature instruments, whether the bulb surfaces are of glass or silver.It is shown that in places where the temperature of the surrounding surfaces differs from the air temperature, a much more reliable estimate of the air velocity can be obtained if a silvered kata is used instead of the plain glass instrument.A method is given for measuring the emission of radiation from the surroundings by means of ordinary and silvered kata-thermometers, and it is shown that estimates so made correspond, to within 1 per cent., with the values obtained by direct thermopile measurements.