Chapter 5 LIQUID - IN - GLASS THERMOMETERS

“… the degree of their scales agree with one another, and their variations are within limits …” Daniel Gabriel Fahrenheit (1706 )

5. 1 Principles and Definitions A liquid -in -glass thermometer [ 1 ], [ 2 ] is a temperature- measuring instrument consisting of a thin-walled glass bulb ( the reservoir for the thermometer liquid ) attached to a glass stem ( the capillary tube through which the meniscus of the liquid moves with a change in temperature ), with the bulb and stem system sealed against its environment. The portion of the bulb-stem space that is not occupied with the thermometer liquid usually is filled with a dry inert gas under sufficient pressure to prevent separation of the thermometer liquid.

A scale is provided to indicate the height to which the liquid column rises in the stem, and this reading is made to indicate closely the temperature of the bulb. A contraction chamber (an enlargement of the capillary) often is provided below the main capillary to avoid the need for a long length of capillary or to prevent contraction of the liquid column into the bulb. An expansion chamber ( an enlargement of the capillary ) often is provided above the main capillary to protect the thermometer in case of overheating. A reference point (usually marking the height of the thermometer fluid in the stem when the bulb is at the ice point) should be provided as a check against changes in the bulb volume.

The operation of a liquid-in-glass thermometer thus depends on the coefficient of expansion of the liquid being greater than that of the containing glass bulb. Any increase in the bulb temperature causes the liquid to expand and rise in the stem, with the difference in volume between the bulb and the stem serving to magnify the change in volume of the liquid. In establishing a thermometer scale, certain conditions prevailed during the calibration. Specifically, there was some definite amount of the thermometer liquid exposed to the calibration environment, and some definite ambient temperature prevailed during the test.

Variations in these important conditions are distinguished by the following three definitions (see also Figure 5.1). Partial Immersion A liquid-in-glass thermometer designed to indicate temperatures correctly when the bulb and a specified portion of the stem are exposed to the temperature being measured.

Figures 5.2 Partial, total, and complete immersion thermometer. (After ASME PTC 19.3, 1974, p.44.)

Total Immersion A liquid-in-glass thermometer designed to indicate temperatures correctly when just that portion of the thermometer containing the liquid is exposed to the temperature being measured. Complete Immersion A liquid-in-glass thermometer designed to indicate temperatures correctly when the entire thermometer is exposed to the temperature being measured. Whenever the calibration conditions are not duplicated in the application of a thermometer, an emergent-stem correction must be applied. These corrections are discussed next.

5.2 Stem Corrections Most liquid-in-glass thermometers in use are of the total immersion type. This is because ambient temperature variations can introduce large uncertainties in the indications of partial immersion thermometers. Countering this advantage of the total immersion type, however, is the difficulty necessarily experienced in reading temperatures when the thermometer liquid column is totally immersed in the environment of the bulb; for example, in measuring a bath temperature, a short length of the thermometer liquid in the stem is usually left emergent from the bath so the meniscus will be visible.

But this practice may expose the stem to an appreciable temperature gradient near the bath surface. This will, of course, cause the reading of the thermometer to be either too high or too low, depending on the ambient temperature level with respect to that of the bath. The equation usually used to account for this effect is [ 3 ] C S =KN(t 1 -t 2 ) (5.1) where C S =stem correction in degrees, to be added algebraically to the indicated temperature, K = differential expansion coefficient of thermometer liquid with respect to thermometer glass (for mercury thermometers the values usually recommended are for Celsius scales and for Fahrenheit scales),

t 1 = temperature of bulb (for first try, use indicated temperature; then adjust by C s1 and try again for a better approximation to C S ), t 2 = average temperature of emergent liquid column.

Figure 5.3 Thermometer calibrated for total immersion and used for partial immersion. (After ASME PTC 19. 3, 1974, p. 50.)

Example 1. A total immersion thermometer indicated 780 ℉ when the mercury column was immersed to the 200 degree mark on the scale. The mean temperature of the emergent column was 1700 ℉. What was the corrected temperature of the bulb?

Example 2. A total immersion thermometer indicated ℃ when the mercury immersed to the 80 degree mark on the scale. The mean temperature of the emergent column was 60 ℃. What was the corrected temperature of the bulb? A partial immersion thermometer also can be used at conditions other than specified. The emergent stem correction in such cases is given by C S =KN(t sp -t 1 ) (5.2)

where C s, K, and t 2 are as defined previously, N = length of emergent column from immersion mark to top of thermometer liquid, expressed as number of degrees on the thermometer scale (note that even ungraduated scale lengths must be considered ), t sp = specified mean temperature of emergent stem.

Example 3. A partial immersion thermometer indicated ℃ when used improperly as a total immersion thermometer. The equivalent length of the submerged column was 110 ℃ (taken from the correct immersion mark to the top of the mercury column). The specified mean temperature of the emergent stem was 75 ℃, whereas the approximate mean temperature of the emergent stem was 250 ℃. What was the corrected temperature of the bulb? C s = × 110 × ( ) ＝ -1.7 ℉ t corrected = = ℉.

A general stem correction curve that can be used for either total or partial immersion thermometers is given in Figure A final question concerns the immersion effect for shielded thermometers. Sometimes a metal case is used to protect the more-or-less fragile thermometer, such situations the stem corrections differ significantly from those of (5. 1) and (5. 2). This point to the fact that every temperature sensing element must be treated as an individual. Some wear metal jackets, others are clothed in ceramics, and still others face the environments bare. Whenever deviations from specified conditions are encountered, individual tests can provide the only answer.

5. 3 Note on Glass Characteristics Glass flows when subjected to temperature stresses, and the progressive change in bulb volume causes a corresponding change in the scale calibration. According to J. Busse [4] of the NBS, the bulb of a Fahrenheit mercury thermometer has a volume equivalent to about 10, 000 degrees of the stem scale. Thus a change in bulb volume of only 1 part in 100, 000 will introduce a 0.1 ℉ change in the stem scale, and such changes are to be expected. To note the effect of a change in bulb volume on the temperature measurement, the ice point indication should be observed with respect to the original etched mark.

Since the change in bore volume is relatively small compared with the change in bulb volume, it is assumed that all points on the thermometer scale change by the same amount at that noted as the ice point.