Thermometer, accuracy partial immersion

The accuracy attainable with a liquid-in-glass thermometer is limited by the characteristics of the thermometer itself. Instability of the thermometric liquid, nonuniformity of capillary bore, and inaccuracies in scale graduation are the important factors. Uncertainties in corrections for the emergent stem may greatly limit the accuracy of partial-immersion thermometers. Generally, partial-immersion thermometers are assigned an uncertainty of 0.3°C in their calibration, whereas total immersion thermometers may have an uncertainty as small as 0.03°C. Observer errors add to the uncertainty but with care these can usually be made relatively small. 

Partial-immersion thermometers have a greater tolerance (and therefore less precision) than total immersion thermometers. Interestingly enough, when a total immersion thermometer is only partially immersed and no stem correction is made, the accuracy is likely to be less than a partial-immersion thermometer. 

Because partial-immersion thermometers are designed for a specific test, uniformity of procedure is more important than overall accuracy. 

The boiling point is one of the most important physical constants of a liquid. It is also easily determined with sufficient accuracy for most purposes if a reasonable quantity of a pure substance is available, for then it is only necessar to carefully distill the sample, noting both the vapor temperature and the barometric pressure. A particularly convenient apparatus for distillation of small quantities is shown in Fig. 1-10. The shape of the flask here is a desirable one because it confines the liquid to a smaller area than does a round-bottom flask. Thermometers with standard taper joints (1-in. immersion) are very convenient and reduce the possibility of contamination, and in vacuum distillation, of leakage. They are also calibrated for partial immersion, thus making emergent stem corrections (page 83) unnecessary. They... 

Partial Immersion Thermometers.—As a means of avoiding the necessity for applying the correction for the emergent stem, it has become customary to use partial immersion thermometers which are graduated to read correctly when immersed to a definite depth, say three inches, in a bath. Such thermometers should be, and usually are, marked to show the proper depth of immersion, and many of them have a line engraved around the stem to show this depth of immersion. While the accuracy attainable with a partial immersion thermometer is not so high as can he attained with a total immersion thermometer under favorable conditions, it is true that these favorable conditions are not often realized, and therefore in the great majority of cases, the partial immersion thermometer is to be preferred. It is probable that the partial immersion thermometer will be extensively used in this country. 

Partial-immersion thermometers are sometimes graduated in smaller intervals than shown in these tables, but this in no way improves the performance of the thermometers, and the listed tolerances and accuracies still apply. 

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