Thermometers emergent stem correction

The Beckmann thermometer used with the bomb calorimeter should be calibrated for the normal depth of immersion with which it is used. To cover the normal range of laboratory temperatures, this calibration should be obtained for three settings of the zero on the scale, convenient values being 10, 15, and 20°C. Such a series of calibrations allows automatically for emergent stem corrections and variations in the value of the degree on the thermometer scale with different quantities of mercury in the bulb, in addition to those arising from inherent variations in the diameter of the capillary bore. 

Total immersion means standardization with the thermometer immersed to the top of the mercury column, with the remainder of the stem and the upper expansion chamber exposed to the ambient temperature. Partial immersion means standardization with the thermometer immersed to the indicated immersion line etched on the front of the thermometer, with the remainder of the stem exposed to the ambient temperature. If used under any other condition of immersion, an emergent-stem correction is necessary to obtain correct temperature readings. 

The methods of determining boiling-points at atmospheric pressure are described in all the practical manuals 7 a simple apparatus consists of a distilling flask in which the liquid is boiled, connected with a condenser (if necessary, a reflux condenser), and with a thermometer in the neck, the bulb being in the vapour suitable corrections for the variation of atmospheric pressure from 760 mm. Hg ( 14.VIIIJ) and the emergent stem corrections ( 3.VI A) must... 

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... 

Emergent Stem Correction for Mercuiy-in-glass Thermometers.—k large proportion of the thermometers now sold are graduated and standardized for the condition of total immersion, that is, for the condition that the bulb and all the portions of the stem which contain mercury, are heated to a uniform temperature. 

Emergent Stem Corrections for Liquid-in-Glass Thermometers 75 Ed. 

EMERGENT STEM CORRECTION FOR LIQUID-IN-GLASS THERMOMETERS... 

With aU these corrections discussed, it may be useful to practice an actual loss calculation, using the data in Table 4.2 (answer AT = 1.3710 K). If the temperature is measured with a calorimetric mercury-in-glass thermometer, an emergent stem correction becomes necessary if the thermometer extended out of the bath liquid. This emergent stem correction can be made as the last correction for the calculated AT, using the equation derived in Fig. 4.4. 

Emergent Stem Corrections for liquid-in-dass Thermometers Emissivity of Total Radiation for Various Materials Emissivity of Timgsten Fats and Oils... 

The most frequently encountered source of error when using liquid-in-glass thermometers is the misuse or complete neglect of the emergent-stem correction. This correction derives from the use of the thermometer with a portion of the stem exposed to a different temperature from that of calibration. A common example is the use of partial immersion of a thermometer calibrated for total immersion. For detailed information on this correction, see the American Society of Mechanical Engineers Power Test Codes Temperature Measurement. 

When a total-immersion thermometer is used, correct for the temperature of the emergent stem by the formula... 

Record as the softening point the temperature of the thermometer at the instant the sample touches the lower horizontal plate (see Fig. 4CW) or the bottom of the container (see Fig. 41e). Make no correction for the emergent stem of the thermometer. 

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. 

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. 

It should be noted that all the thermometers listed above except the ground-joint type (Fig. 363) are calibrated for total immersion in other words, they read correctly only when the whole of the mercury thread is at the measured temperature. Short thermometer wells can thus give rise to appreciable errors. According to Piatti and Marti [20] the error is small only if the mercury bulb is just immersed in the oil in the well. In practice, total immersion thermometers should be mounted so as to be completely immersed in the vapour or liquid, although the ease of reading is thereby adver.sely affected. Otherwise a correction for emergent stem has to be applied (r/. 

In some thermometers, the so-called partial immersion thermometers, these corrections have been made at the factory. A partial immersion thermometer can be recognized by an engraved mark 76 mm from the bottom of the bulb. Such thermometers must always be immersed to this mark, and the stem must be kept at room temperature in order to have a precise reading. A partial immersion thermometer which is not immersed to its immersion mark, or which does not have the emerging stem at room temperature, still needs to be corrected by an expression analogous to that shown for total immersion thermometers. A final note of caution no temperature measurement is possible if the bulb is not safely immersed to the beginning of the constant-diameter capillary. 

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