To assure safe arrival of the thermometers, they should be packed firmly in their individual cases. A rigid material, such as wooden slats or corrugated paper, can be wrapped around the thermometer case as an additional aid against breakage. The package should contain enough packing material to absorb any shock that it may receive. Included with each shipment should be a packing list stating the number of thermometers shipped, the name of the company sub

purchase order number.

Unless otherwise specified, the thermometers will be returned via the best method in the judgment of NBS. Shipping charges, both to and from the NBS, will be paid by the applicant. All possible care will be taken in handling thermometers at NBS, but the risk of damage either in shipment or testing must be assumed by the agency, firm or individual submitting them.

Bulb: The reservoir for the thermometer liquid. The bulb of a thermometer will contain a volume equivalent to a specific number of degrees of the scale depending upon the coefficients of expansion of the thermometric liquid and bulb glass. For mercury in a bulb made of "normal" glass, the bulb volume is equivalent to approximately 6,222 times the volume of a 1 degree length of the capillary on the scale for Celsius thermometers, or 11,200 times the volume of a 1 degree length of the capillary on the scale for Fahrenheit thermometers. For organic thermometric liquids with higher coefficients of expansion than mercury, the bulb volumes are correspondingly less. Stem: The glass capillary tube through which the thermometric liquid advances or retreats with changes in temperature.

Main Scale: The scale graduated in degrees or multiples or submultiples of degrees; in many instances the main scale constitutes the only scale.

Auxiliary Scale: A short scale including a reference temperature such as the ice point, to provide a means for checking the thermometer for a change in calibration with time (see Sec. 7.4). This scale is added when a suitable reference temperature is not included in the range of the main scale.

Expansion Chamber: An enlargement at the top end of the capillary bore having a volume equivalent to not less than the volume of a 20 mm length of unchanged capillary. Smaller chambers are not regarded as expansion chambers. The expansion chamber is

provided to prevent the buildup of excessive pressures in gas-filled thermometers as the liquid filling advances toward the top of the scale (see Sec. 7.1).

Contraction Chamber: An enlargement of the capillary bore which serves to reduce a long length of capillary or to prevent contraction of the entire liquid column into the bulb. This chamber is introduced below the main scale or between the main scale and an auxiliary scale.

Reference Point: A reference temperature, such as the ice point or steam point, used periodically to check the thermometer for changes in bulb volume. (See Sec. 5.2). The reference point can be included in the main scale or on an auxiliary scale.

Total-Immersion Thermometer: A thermometer designed to indicate temperatures correctly when the bulb and the entire liquid column is exposed to the temperature being measured. (See the definition for complete-immersion thermometer.)

Partial-Immersion Thermometer: A thermometer designed to indicate temperatures correctly when the bulb and a specified portion of the stem is exposed to the temperature being measured. The remaining portion of the stem, referred to as the emergent stem, will be at the ambient temperature, usually different from the temperature being measured. Such thermometers are generally marked with an immersion line to indicate the proper depth of immersion.

Complete-Immersion Thermometer: A thermometer designed to indicate temperatures correctly when the whole thermometer, including the expansion chamber, is subjected to the temperature being measured. In gasfilled thermometers the reading will be different for complete, as compared to total immersion, as a result of the effect of temperature on the gas pressure in the thermometer (see Sec. 8.2). Although the difference in readings under the two conditions is particularly significant at high temperatures, it is also significant at moderate temperatures if the bulb and expansion chamber are both relatively large.

Calibration Points: The temperatures on the thermometer scale (i.e., 0°C) where calibrations are performed and corrections reported.

to the difference between the measured value and the true value of the quantity being measured. In this Monograph the accuracy of a given thermometer refers to its ability to indicate temperatures correctly on the International Practical Temperature Scale of 1968 [1] within the uncertainty stated (See tables 5-12) provided all corrections are applied and the thermometer is used in the same manner as when it was calibrated. The accuracy attainable is principally limited by the characteristics of the thermometer itself (See Sec. 6).

at the surface of the thermometric liquid, nonuniformity of capillary bore, and inaccuracies in scale graduation are among the important factors. With partial-immersion thermometers, uncertainties in corrections for the emergent stem may greatly limit the accuracy. Observer errors are also involved, but with care these can usually be made relatively small.

Precision: The precision of measurement refers to the degree of agreement amongst repeated measurements at a given time of the same quantity.

The scale to which measurements of temperature should ultimately be referred is the Thermodynamic Kelvin Temperature Scale (TKTS). Values of temperature expressed on the TKTS are designated by the symbol T. The unit of temperature is the Kelvin, symbol K, which is a base unit of the Systeme Internationale (SI), and is defined as "the fraction 1/273.16 of the thermodynamic temperature of the triple point of water" [3].

Because of the difficulties that are encountered in the practical realization of the TKTS, it has been necessary to define and utilize practical temperature scales. The International Temperature Scale and defining text was first adopted in 1927 and later revised in 1948. The Eleventh General Conference of Weights and Measures in 1960 changed the name of the scale to the International Practical Temperature Scale (IPTS) of 1948 and adopted a revised text of the scale [4], although numerically the scale was changed. In 1968, in accordance with the power given to it by Resolution 8 of the Thirteenth General Conference of Weights and Measures, the International Committee of Weights and Measures adopted the International Practical Temperature Scale of 1968 [1], henceforth referred to as IPTS-68. The IPTS-68 replaced the International Practical Temperature Scale of 1948 and may be expressed either as a Kelvin scale or as a Celsius scale.

The present Celsius scale has its zero 0.01 K below the triple point of water (essentially the ice point). The value of a temperature expressed on the Thermodynamic Celsius Temperature Scale is designated by the symbol t and is related to the value on the TKTS by

68

The units of Tes and tes, like the thermodynamic scales, are the Kelvin, symbol K, and degree Celsius, symbol °C.

Due to the use of more sophisticated equipment and the ability to measure with more accuracy the defining fixed points, the new temperature scale more closely agrees with thermodynamic temperatures. This scale, IPTS-68, is intended to provide scientific and industrial laboratories throughout the world with a common basis for stating temperatures. Calibrations of thermometers at NBS, therefore, are made with reference to values of temperature on the IPTS-68.

Shown in Table 1 is the approximate difference in degrees Celsius between the values of temperature given by the IPTS-68 and the IPTS-48 [1]. The differences are listed in two separate sections for convenience and clarity. It can be seen, by studying these values, that the correction is important if quality control in the laboratory is to be maintained to the nearest 0.01 °C. If, on the other hand, accuracies no better than the nearest 0.1 °C are desired, the correction becomes less significant in the liquid-in-glass thermometer region (-200 to +600 °C) of temperature

measurements.

Although values of temperatures on the IPTS-68 are expressed in degrees Celsius, thermometers that are graduated on the Fahrenheit Scale can be calibrated with reference to the IPTS-68 by using the conversion formula:

temperature value in °F = 9/5
(temperature value in °C) + 32

The National Bureau of Standards has been performing calibrations with reference to the IPTS-68 since July, 1969. On all Reports of Calibration and Reports of Test issued after this date, it has been clearly indicated that the new scale was used.

In the range of temperatures normally covered by liquid-in-glass thermometry, the IPTS-68 is defined. by four fixed points: the equilibrium between the liquid and vapor phases of oxygen (normal boiling point of oxygen) at -182.962 °C; the equilibrium between solid, liquid, and vapor phases of water (triple point of water) at +0.01 °C; the equilibrium between the liquid and vapor phases of water (nor