Problems Thermometers

There are problems to be considered and avoided when using Hquid-in-glass thermometers. One type of these is pressure errors. The change in height of the mercury column is a function of the volume of the bulb compared to the volume of the capillary. An external pressure (positive or negative) which tends to alter the bulb volume causes an error of indication, which may be small for normal barometric pressure variations but large when, for example, using the thermometer in an autoclave or pressure vessel. 

A particularly difficult problem in microwave processing is the correct measurement of the reaction temperature during the irradiation phase. Classical temperature sensors (thermometers, thermocouples) will fail since they will couple with the electromagnetic field. Temperature measurement can be achieved either by means of an immersed temperature probe (fiber-optic or gas-balloon thermometer) or on the outer surface of the reaction vessels by means of a remote IR sensor. Due to the volumetric character of microwave heating, the surface temperature of the reaction vessel will not always reflect the actual temperature inside the vessel [7]. 

The other limit is the problem of temperature measurements. Classical temperature sensors could be avoided in relation to power level. Hence, temperature measurements will be distorted by strong electric currents induced inside the metallic wires insuring connection of temperature sensor. The technological solution is the optical fiber thermometers [35-39]. However, measurements are limited below 250 °C. For higher values, surface temperature can be estimated by infrared camera or pyrometer [38, 40], However, due to volumic character of microwave heating, surface temperatures are often inferior to core temperatures. 

The reactor has facilitated a diverse range of synthetic reactions at temperatures up to 200 °C and 1.4 Pa. The temperature measurements taken at the microwave zone exit indicate that the maximum temperature is attained, but they give insufficient information about thermal gradients within the coil. Accurate kinetic data for studied reactions are thus difficult to obtain. This problem has recently been avoided by using fiber optic thermometer. The advantage of continuous-flow reactor is the possibility to process large amounts of starting material in a small volume reactor (50 mL, flow rate 1 L hr1). A similar reactor, but of smaller volume (10 mL), has been described by Chen et al. [117]. 

The problems encountered in performing accurate measurements of T with a primary CVGT can be overcome by the use of the dielectric constant gas thermometer [27-31] or by the acoustic gas thermometer [3,32,33],... 

The main problem encountered in the realization of a vapour pressure thermometer is the correct measurement of the pressure p. In this case, however, the dead space is not a problem and larger tubes may be used (instead of capillaries). Since the pressure gage is usually at room temperature, also in this case, a gradient in temperature between the cold and the warm end of the sampling tube produces a thermomolecular pressure difference [44,45, p. 48] ... 

A precious quality of Ge thermometers is their stability (better than 0.1% after repeated cooling cycles). Problems at very low temperatures are ... 

Theoretically, the noise thermometer is a primary thermometer and as such has been used [85,86], In practice, besides the low level of the signals to be detected, there are other problems [87,88], such as ... 

Unfortunately, most capacitance thermometers are not stable and must be recalibrated at every cool down. They may also present problems of heat release [95] moreover, their thermalization times at low temperatures may be long since the materials used present low thermal conductivity and high specific heat. 

Low-temperature thermometers are obtained by cutting a metallized wafer of NTD Ge into chips of small size (typically few mm3) and bonding the electrical contacts onto the metallized sides of the chip. These chips are seldom used as calibrated thermometers for temperatures below 30 mK, but find precious application as sensors for low-temperature bolometers [42,56], When the chips are used as thermometers, i.e. in quasi-steady applications, their heat capacity does not represent a problem. In dynamic applications and at very low temperatures T < 30 mK, the chip heat capacity, together with the carrier-to-phonon thermal conductance gc d, (Section 15.2.1.3), determines the rise time of the pulses of the bolometer. 

Figure 7b. Thermometer reading as a function of time, Problem 7.2...

At relatively high temperatures thermocouple thermometers are most commonly used to measure temperature. The thermoelectric power of three frequently used thermocouples is compared in Figure 10.2. The choice of thermocouple depends on the temperature range, the chemistry of the problem in question, sensitivity requirements and resistance towards thermal cycling. The temperature range and typical uncertainty of some of the most commonly used thermocouple thermometers are given in Table 10.2. 

Here you use a loaded melting point capillary tube (see Loading the Melting Point Tube ) and attach it directly to the thermometer. The thermometer, unfortunately, has bulges there are some problems, and you may snap the tube while attaching it to the thermometer. 

In addition, Lavoisier and his colleagues introduced programmatically into the chemical laboratory apparatus other than the furnace, the crucible, and the retort, describing and illustrating the new instruments construction and their use in texts like Lavoisier s Traite elementaire de chimie. Lavoisier employed not only the balance and the thermometer but pneumatic apparatus, the electrical machine, the burning lens, and the calorimeter. 80 As the instruments of the chemical laboratory proliferated, so, too, did the problems chemists dreamed of posing and resolving. 

Difficulties develop if the thermometer is exposed to certain types of radiation. However, calculations indicate that under normal circumstances, these radiation fields raise the temperature by only about 10 °C, which is a quantity that is not detectable even with the most sensitive current-day instruments [4]. Similarly, we shall neglect relativistic corrections that develop at high velocities, for we do not encounter such situations in ordinary thermodynamic problems. 

The same problems encountered in mineral-water isotopic thermometry could be present in T estimates based on isotopic equilibrium between brine and dissolved species, such as SO ". Isotope geo thermometers based on mineral couples or gaseous couples (e.g., CH4-CO2), however, are not affected by isotopic salt effect in brines (Horita et al., 1993b). 

You must be very careful to recognize and understand the units of a gauge that appear in a test question. For instance, a temperature gauge (commonly called a thermometer) could use either degrees Fahrenheit or degrees Celsius. Mistakes on units can cause major problems, so be careful The table on page 200 shows some common types of gauges, what they measure, and the kind of units they use. 

Some researchers, aware of the temperature problem, elect to use electrically heated wires for heat sources. The same wires can be used as resistance thermometers with satisfactory accuracy. A drawback, however, is that tests cannot be made in the transition region of boiling, because of instability. In addition, unless the wires are quite small the currents needed become very great. For example, if a -in.-diam. copper tube with a 0.03-in. wall is intended for use with water near the critical AT, a current of about 8,000 amp. is required. 

One would not think that a shiny little sliver of mercury sealed in a thermometer could cause problems, but broken thermometers have resulted in a number of hospitalizations. Children are especially susceptible to the effects of mercury vapor, because of their small size. The symptoms they experience may... 

One strategy for avoiding these problems is to use the services of a mercury recycler such as Quicksilver Products, Inc. of Brisbane, CA This company is an EPA licensed handler of mercury wastes that is able to extract the mercury from fluorescent tubes and mercury vapor lamps, as well as from batteries, switches, thermometers, contaminated soil, and other sources. 

Faulty thermometers and thermostats can lead to problems in trouble shooting Plugged up sight glasses can be misleading... 

An alternative method of calorimetry that gives less accurate results, but is simpler in concept, uses only a single insulated container and a thermometer. Temperature changes in the calorimeter are brought about by adding hot (or cold) objects of known weight and temperature. Calculations are based on the principle that the heat lost by the added hot object is equal to that gained by the water in the calorimeter and the calorimeter walls. This simple approach is illustrated in the next two problems. 

The calorimeter of the preceding problem is used to measure the specific heat of a metal sample, A 100 g sample of water is put into the calorimeter at a temperature of 24.1°C. A 45.32 g sample of metal filings is put into a dry test tube that is immersed in a bath of boiling water until the metal is at the temperature of the latter, 99.1°C. The hot metal is then quickly poured into the calorimeter and the water stirred by a thermometer that is read at frequent intervals until the temperature reaches a maximum of 27.6°C. Compute the specific heat of the metal. 

The spatial uniformity of temperature in the cell is difficult to determine, and we are not aware of a careful study of this problem. In most experiments, it is the temperature of the electrode-solution interface or that of the diffusion layer that is relevant. A possible internal thermometer could be created by measuring a temperature-sensitive voltammetric function, for example, the peak separation in the cyclic voltammogram of a reversible reaction, which is 2.22RT/ F. The resolution is not likely to be outstanding, but such a technique would probably allow detection of serious differences between the thermocouple reading and the actual temperature of the electrode-solution interface. 

KEY CONCEPT PROBLEM 1.15 What is the temperature reading on the following Celsius thermometer How many significant figures do you have in your answer ... 

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