Expansion-Based Thermometers

Instruments based on the contact principle can further be divided into two classes mechanical thermometers and electrical thermometers. Mechanical thermometers are based on the thermal expansion of a gas, a liquid, or a solid material. They are simple, robust, and do not normally require power to operate. Electrical resistance thermometers utilize the connection between the electrical resistance and the sensor temperature. Thermocouples are based on the phenomenon, where a temperature-dependent voltage is created in a circuit of two different metals. Semiconductor thermometers have a diode or transistor probe, or a more advanced integrated circuit, where the voltage of the semiconductor junctions is temperature dependent. All electrical meters are easy to incorporate with modern data acquisition systems. A summary of contact thermometer properties is shown in Table 12.3. 

A mercury thermometer is based on the differential volume thermal expansion of liquid mercury (about 1.8 X 10 and glass (about 0.2 X 10 " K ). In the manu-... 

Given this situation, it is not surprising that, over the last few years, products based on platinum thin film technology have been finding their way into the home. With the growing use of electronic control systems in the new generation of domestic appliances, platinum temperature sensors have been more widely used in ovens where they have replaced electromechanical regulators such as capillary tubes, solid expansion thermometers and NTC thermistors. Typical sensor applications in the food preparation sector are shown in Fig. 5.3. 

All thermometers, regardless of fluid, read the same at zero and 100 if they are calibrated by the method described, but at other points the readings do not usually correspond, because fluids vary in their expansion characteristics. An arbitrary choice could be made, and for many purposes this would be entirely satisfactory. However, as will be shown, the temperature scale of the SI system, with its kelvin unit, symbol K, is based on the ideal gas as thermometric fluid. Since the definition of this scale depends on the properties of gases, detailed discussion of it is delayed until Chap. 3. We note, however, that this is an absolute scale, and depends on the concept of a lower limit of temperature. 

In Birmingham and its environs Hutton met other members of the Lunar Society, including Matthew Boulton and Erasmus Darwin. On a visit to the latter in Lichfield, Hutton used Darwin s house as a base for expeditions into Derbyshire and also participated in an experiment with an airgun and thermometer to demonstrate how the expansion of air cools it.31 Darwin was later to make use of this fact in explaining processes of devaporation in clouds and in steam engines, as we will see in a moment. 

Fahrenheit decided to substitute mercury for the alcohol because its rate of expansion was more constant and could be used over a wider range of temperatures. Fahrenheit, like Isaac Newton before him, realized that it was more accurate to base the thermometer on a substance that changed consistently based on temperature, not simply the hottest or coldest day of the year like the Florentine models. Mercury also had a much wider temperature range than alcohol. This was contrary to the common thought at the time, promoted by Halley as late as 1693, that mercury could not be used for thermometers due to its low coefficient of expansion. 

Since temperature measurements are required over such a wide range and diversity of situations, a large number of different types of thermometers with varying levels of accuracy and convenience have been developed over the years. Those most frequently used are based on the expansion of a gas, liquid or solid on changes in electrical resistance on the thermoelectric effect on changes in the thermal radiation of a system on changes in the thermal (Johnson) noise of electrical resistors on changes... 

Thermostats can be controlled to 300°C by contactthermometers expansion regtilators are used up to 500°C. The latter are based on the difference in linear expansion between a quartz rod and an aluminum or iron tube. Alternatively, a contact thermometer may be located in a cooler part of the furnace, provided a fairly constant temperature difference can be maintained between the hot reaction area and the cooler measurement section. The frequently recommended arrangement consisting of an auxiliary furnace in series with the main furnace, whereby the contact thermometer is placed inside the smaller furnace, can only work if both furnaces are very well insulated and have equal heat losses. This is not easily achieved in practice. 

In terms of the primary definition of temperature which we have for the time being adopted, Charles s law, which states that gas pressure is proportional to the absolute temperature, would be a tautologous statement. It is not necessarily so, and was not so in its historical setting, since other scales of temperature, notably that based upon the expansion of a mercury column by heat, are possible. The mercury thermometer and the gas thermometer provide scales... 

Pyrometer pl- ra-mo-tor n [ISV] (1796) (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 joined pieces of metal. In addition, radiation pyrometers are based on the measurement of heat radiated from a hot body, and optical pyrometers on the measurement of the intensity of light emitted from a hot body. (2) An Infrared Pyrometer. 

Connecticut) vapour pressure thermometer (up to 6(X) F), which could be supplied with recorder 4) a device based on differential expansion of metals (up to 1500 F), typically based on copper and iron, and available from H. W. Bulkley, Schaffer Budenberg, Edward Brown Co. (Philadelphia), and (Jueen Co. 5) Le Chatelier thermo-electric pyrometer, available from (Jueen Co. at 140 6) Siemens electrical resistance thermometer, as modified by H. L. Callendar (no supplier of this instrument is given) 7) and, although it did not give continuous measurements, the optical pyrometer of Nouel and Mesure, supplied by ()ueen Co. Although these were being sold by suppliers in the USA they were nearly all instruments of European origin. 

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