Liquid stem correction

The comparatively inexpensive long-scale thermometer, widely used by students, is usually calibrated for complete immersion of the mercury column in the vapour or liquid. As generally employed for boiling point or melting point determinations, the entire column is neither surrounded by the vapour nor completely immersed in the liquid. The part of the mercury column exposed to the cooler air of the laboratory is obviously not expanded as much as the bulk of the mercury and hence the reading will be lower than the true temperature. The error thus introduced is not appreciable up to about 100°, but it may amount to 3-5° at 200° and 6-10° at 250°. The error due to the column of mercury exposed above the heating bath can be corrected by adding a stem correction, calculated by the formula ... 

The stem correction may be avoided by employing a thermometer with a short scale (sometimes termed an Anschutz thermometer) and of such size that the entire column of mercury is immersed in the vapour or in the liquid. In practice, it is generally more convenient to use thermometers which have been graduated for partial immersion for a short and con-... 

When a thermometer which has been standardized for total immersion is used with a part of the liquid column at a temperature below that of the bulb, the reading is low and a correction must be applied. The stem correction, in degrees Celsius, is given by... 

Tn addition to the error due to the exposed stem, ordinary chemical thermometers of low cost are subject to errors due to irregularities in the bore and sometimes the scale graduations may not be very accurate. " It is therefore essential to check the thermometer at several temperatures against the melting points of pure solids or the boiling points of pure liquids as described below. The application of an exposed stem correction will of course be unnecessary if the thermometer is calibrated in this way. A calibration curve may then be drawn upon graph " paper from the data thus obtained. Temperatures at intervals of about 20° are marked as abscissae and the corrections to be added or subtracted as ordinates the points thus obtained are then connected by a smooth curve. The thermometer correction at any temperature may be read directly from the curve. 

Stellite-type aboys carbides in, 4 647 Stellite-type aboys, 7 220-226 Stelometer, 11 614 Stem corrections, in liquid-in-glass thermometers, 24 464-465 Stem cell technology, 11 13-14 STEM-drilling, 9 599-600 Stem bbers, 11 285... 

K = 0.00016 for centigrade mercurial thermometers and K = 0.001 for centigrade organic liquid thermometers To better understand stem corrections, consider the following example ... 

Liquid in Glass Thermometers. Mercury-in-glass thermometers (or better yet, mercury-in quartz) function well between -25°C and 360°C their typical precision is 0.1 K. They must be corrected for (1) relatively small pressure effects and (2) a relatively large "exposed-stem correction," due to the different coefficients of thermal expansion of mercury and glass in the part of the thermometer not immersed in the system being measured. 

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... 

Comments. Methods for obtaining melting-points are not usually given in the specifications, but it is absolutely necessary that a definite procedure and definite apparatus be agreed upon. The following points should be taken into consideration calibration and stem correction of thermometers, size of tubes, amoimt of substance taken, rate of melting and vessel of liquid to be used as bath. 

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... 

Liquid-in-glass thermometers are almost exclusively used to determine the temperature of fluids that are relatively uniform—that is, they contain no large temperature gradients. Conduction along the glass stem can affect the temperature of the glass as well as that of the liquid. Therefore, thermometers are usually calibrated for a specified depth of immersion. They should then show the correct temperature when inserted to that level in the fluid whose temperature is to be measured. When a thermometer is used in situations where the immersion is other than that for which it was designed and calibrated, a stem correction [8] should be applied. 

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

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

Emergent Stem Corrections for liquid-in-Glass Hiermometers Emissivity of Total Radiation for Various Materials Emissivity of Tungsten Fats and Oils... 

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. 

When measuring temperatures above 150°C, thermometer errors can become significant. For an accurate boiling point with a high-boiling liquid, you may wish to apply a stem correction to the thermometer, as described in Section 13.3, or to calibrate the thermometer, as described in Technique 9, Section 9.9. 

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. 

The vapor pressure against temperature data obtained with a Knudsen cell set-up are handled as already described for a low boiling temperature liquid. The main difference stems from the very low pressure of the vapor in equilibrium with the solid, which justifies the adoption of the ideal gas model in this case. ASub ° at the mean temperature can then be derived from equation 2.40 (with Z = 1) and the correction to 298.15 K can be made with an equation similar to 2.41. 

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