Liquid thermometers

Liquid-in-glass thermometers measure the thermal expansion of a liquid, which is placed in a solid container, on a length scale. The mercury thermometer is one example of liquid thermometers. Alcohol is also used with this type of instrument. The temperature range is -80 to a-330 °C depending on the liquid. The quality, stability, and accuracy vary considerably. The advantages are a simple construction and low price. A disadvantage is that they are not compatible for connection to monitoring systems. 

Any substance that somehow changes with alterations in its temperature can be used as the basic component in a thermometer. Gas thermometers work best at very low temperatures. Liquid thermometers are the most common type in use. They are simple, inexpensive, long-lasting, and able to measure a wide temperature span. The liquid is almost always mercury, sealed in a glass tube with nitrogen gas making up the rest of the volume of the tube. 

Once equilibrium has been reached, the height difference between the two liquid surfaces is all that remains to be measured. The primary factor to note here is that capillaries are used to minimize the dilution effects. This means that corrections for capillary rise must be taken into account unless the apparatus allows the difference between two carefully matched capillaries to be measured. We discuss capillary rise in Chapter 6, Sections 6.2 and 6.4. Finally, there is an extremely important practical reason, in addition to the theoretical requirement of isothermal conditions, for good thermostating in osmometry experiments. The apparatus consists of a large liquid volume attached to a capillary and therefore has the characteristics of a liquid thermometer The location of the meniscus is quite sensitive to temperature fluctuations. 

Thermal stresses are more difficult to imagine than physical stresses, but we can observe what causes them anytime we look at a liquid thermometer. When materi-... 

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 ... 

In this section, the design and operation of familiar liquid thermometers, thermocouples, platinum resistance thermometers, thermistors, and optical pyrometers are discussed in detail. Briefer descriptions are also given of a variety of special thermometric devices such as quartz thermometers, germanium resistance thermometers, and sihcon-diode thermometers. 

Special Liquid Thermometers. For low temperatures, one can use several kinds of liquid-in-glass thermometers. Toluene thermometers may be used down to — 95°C, and pentane thermometers will operate as low as — 130°C. However, it is usually more convenient, as well as more accurate, to use thermometric devices of other types, especially thermocouples or resistance thermometers. 

The gas thermometer was discovered by Galileo. Liquid thermometers were introduced later. Fig. 1 is a diagram of Gahleo s air thermometer, Fig. 2 of a liquid thermometer as used at the present day. 

Had we chosen another liquid instead of mercury for the definition of our scale of temperature, the correspondence between the degrees of the gas and liquid thermometers would probably have been much less exact. Even in the case of the mercury thermometer, there is no absolute correspondence, but the discrepancy between the various gas thermometers and the mercury thermometer is so small at ordinary temperatures, that it may be neglected for most practical purposes. The scientific standard of temperature is not the mercury, but the hydrogen thermometer. The deviation of the mercury thermometer from the air or the hydrogen thermometer is only a few hundredths of a degree at temperatures between 0° and 200°. At 300°, however, the difference is nearly 2°. ... 

Recording Pyrometry.—The pyrometers which can be made to record automatically fall under the following classifications (1) Gas, saturated vapor, and liquid thermometers (2) resistance thermometers (3) thermoelectric pyrometers (4) radiation pyrometers. 

Reliable temperature measurement in a microwave reaction presents a considerable challenge to the experimentalist, as the microwave field directly affects conventional instruments such as thermometers and thermocouples. Although thermocouples may be used if they are suitably shielded and earthed, there is an inevitable perturbation to the microwave field pattern. A number of other methods are available that are appropriate to use at moderate temperatures. A gas pressure thermometer, or a microwave-transparent liquid thermometer, may be used as inexpensive options, whilst thermal imaging and fluoro-optic thermometry, although expensive, provide more reliable, higher precision information. 

PYROMETER - A device for measuring temperatures above the range of liquid thermometers. 

Liquid thermometers are the most common instrument employed to measure atmospheric temperature. They have an excellent range and can measure temperatures from as low as -200 °C to above 600 C. Similarly to gas thermometers, they are based on the dilation properties of liquids when subject to changes in temperature. Many liquids have been used to measure temperature including water, alcohols, mercury, paraffins, and aromatics. 

Newton used a liquid in glass-thermometer to study heat radiation. Rumford and Leslie used a difierential gas thermometer. Herschel reverted to the liquid thermometer, but this was soon replaced by the thermopile (Melloni [3.4]). Some time later (Langley [3.5]) the first bolometers were used. More recently the use of the gas thermometer, in the shape of the Golay [3.6] and Luft cells has been reintroduced and is now widely used in spectrometers. Another type of thermal detector now widely used is that utilizing the pyroelectric effect. In addition to these, several other detection processes have been suggested, including thermal expansion and changed dielectric properties with temperature. 

Pyrometer pl- ra-m9-t9r [ISV] (1796) n. (1) Instrument for measuring temperatures beyond the upper limit of the usual liquid thermometer. They may operate on the differential expansion of two metallic strips joined together, the measurement of changes of resistance, and the measurement of current flowing through two... 

Because of these error sources, liquid thermometers are rarely used nowadays for precision measurements (Hall and Leaver, 1959). 

Temperature is measured by thermometers. The first temperaturemeasuring device was a special type of liquid thermometer, the thermoscope (discovered by Galilei in the sixteenth century). These days there are gas thermometers, liquid thermometers, infrared thermometers, liquid crystal (LC) thermometers (cholesteric), thermocouples, and resistance thermometers (these latter ones are the most important in thermal analysis). 

The temperature sensors depend upon range and sensitivity requirements. They could include gas-liquid thermometers, platinum resistance thermometers, thermocouples or thermistors. It is advised to have more than one device in the sample and on the copper adiabatic shield. Very good computer adiabatic controllers are easy to construct. One must take into account in programming, the power required for various temperatures to match the heat capacity of the shield, that is, one needs to adjust the power and damping that the power supply puts out according to calorimeter and sample. This may take some preliminary runs to adjust it correctly. 

The starting point for the problem definition is develop a model for a liquid thermometer as shown in Fig. 1.3. This looks like a simple problem, however, it is easy to get lost in details. Important questions that could surface are how does the thermometer function exactly and what can be ignored . First the problem definition and the goal should be clearly stated. What should be modeled and how ... 

The first liquid thermometer was built in 1631 by Jean Rey a French physician and was used to take the temperature of patients. The stem was, however, open and the water evaporation effected the accuracy of reading. The first thermometer with a scaled stem was built in 1641 by the Grand Duke Ferdinard II of Tuscany, using alcohol instead of water. 

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