Measurement of Temperature Differences

Thermocouples are usually not accurate enough for the precise measurement of temperature differences and are not fast enough to detect highspeed variations in temperature. Resistance temperature detectors (RTDs) can detect temperature differences of 5°C (10°F) at a measurement error of 0.02°C ( 0.04°F). If higher-speed response is desired, thermistors or infrared detectors should be considered. 

Experiments on dropwise condensation are difficult as they entail the measurement of temperature differences of 1 K or less for the determination of the heat transfer coefficient. At these small temperature differences the wall temperature fluctuates with time and also locally. The cost of the measuring techniques for the achievement of accurate results is, therefore, considerable. 

To the bottom of each crucible the hot joint of one thermocouple is attached. Both the thermocouples are linked outside the ceramic block by equal wires. Such a linkage enables the measurement of temperature difference of both the crucibles during heating and the difference versus time is recorded. Such a connection, however, yields zero voltage when... 

Every exchange of heat needs a temperature difference to enable a heat flow. Therefore, the measurement of heats and heat flow rates can be reduced to the measurement of temperature differences (i) as a function of time, that is, AT = T(t2) — T(ti), inside a calorimeter, or (ii) as a function of position, AT = T x2) — T x ), along a heat conducting path. 

One of the major applications of calorimeters with measurement of temperature differences as a function of time is the determination of specific heat capacities by measuring the rise of the sample temperature following the supply of a known amount of electric energy. A number of versions of these devices are described in Chapter 7. 

Figure 2.2 Principles of measurement of temperature by means of thermocouples. A, B, and C are different metals, (a) Simple arrangement for measurement of temperature difference T2 —Ti. (b) Measurement of...

As can be seen from their principle of operation (Figure 2.2), thermocouples are particularly suitable for the measurement of temperature differences, there being no need for a thermostat for the reference junction. Series connection into thermopiles, comprising up to 1000 thermocouples, permits the rapid measurement of temperature differences of 10 K, which is not achieved by other measuring techniques. Thus, thermocouples constitute ideal measuring instm-ments for the determination and continuous monitoring or control of minor differences of temperature. They are consequently used in many calorimeters. At temperatures above 1300 K, thermocouples surpass resistance thermometers for absolute temperature measurements because the precision of the latter instruments deteriorates rapidly in this range. The measurement uncertainty is on the order of 1% of the absolute temperature. 

Figure 7.15 shows schematically such a heat flow calorimeter. The measuring system is connected with the surroundings through the thermal resistance J th-With the current commercial heat flow calorimeters, the thermal resistances serving for the measurement of temperature differences are often difficult to identify and define. They may occur in the form of thermopUes or disks made of metal or ceramic and bearing the sample containers. 

While in principle this scheme appears to be beneficial, it may not perform well. One issue is timing. The reflux dmm introduces a large process lag and so its exit temperature will change later than its inlet. Using the condenser outlet temperature would result in the reflux being corrected too early - although it would be possible to lag the measurement of temperature difference. Alternatively the drum exit temperature could be retained and the inlet temperature lagged. 

Thermal electroanalysis — is an analytical method based on the measurement of temperature differences between the electrodes of an electrolysis cell. The temperature differences between the anode and the cathode as well as the electrodes and the solution are measured. A relationship between the temperature differences and the electrolyte concentration can be estab-Hshed. 

Figure 4.37. Thermocouples for measurement of temperature difference between the active and inactive sample in the DTA analysis.

The luminometer index (ASTM D 1740) is a characteristic that is becoming less frequently used. It is determined using the standard lamp mentioned above, except that the lamp is equipped with thermocouples allowing measurement of temperatures corresponding to different flame heights, and a photo-electric cell to evaluate the luminosity. The jet fuel under test is compared to two pure hydrocarbons tetraline and iso-octane to which are attributed the indices 0 and 100, respectively. The values often observed in commercial products usually vary between 40 and 70 the official specification is around 45 for TRO. 

Optical metiiods, in both bulb and beam expermrents, have been employed to detemiine tlie relative populations of individual internal quantum states of products of chemical reactions. Most connnonly, such methods employ a transition to an excited electronic, rather than vibrational, level of tlie molecule. Molecular electronic transitions occur in the visible and ultraviolet, and detection of emission in these spectral regions can be accomplished much more sensitively than in the infrared, where vibrational transitions occur. In addition to their use in the study of collisional reaction dynamics, laser spectroscopic methods have been widely applied for the measurement of temperature and species concentrations in many different kinds of reaction media, including combustion media [31] and atmospheric chemistry [32]. 

Glass-creep errors are also encountered. The Hquid-in-glass thermometer should always be used to measure temperatures in ascending order. If the thermometer is stored at room temperature, a temporary ice point depression results, which may be as much as 0.01 K for 10 K of temperature difference, when the thermometer is heated above room temperature. If the thermometer is used to measure a temperature and must then be used to measure a lower temperature, the thermometer should be stored at a stiH lower temperature for at least 3 days prior to use to assure recovery of bulb dimensions. 

Thermocouples Temperature measurements using thermocouples are based on the discovery by Seebeck in 1821 that an electric current flows in a continuous circuit of two different metalhc wires if the two junctions are at different temperatures. The thermocouple may be represented diagrammaticaUy as shown in Fig. 8-60. A and B are the two metals, and T and To are the temperatures of the junctions. Let T and To be the reference junction (cold junction) and the measuring junc tion, respectively. If the thermoelectric current i flows in the direc tion indicated in Fig. 8-60, metal A is customarily referred to as thermoelectricaUy positive to metal B. Metal pairs used for thermocouples include platinum-rhodium (the most popular and accurate), cmromel-alumel, copper-constantan, and iron-constantan. The thermal emf is a measure of the difference in temperature between To and T. In control systems the reference junction is usually located at... 

An evaluation of the HVAC system may include limited measurements of temperature, humidity, air flow, as well as smoke tube observations. Complex investigations may require more extensive or sophisticated measurements of the same variables (e.g., repeated COj measurements taken at the same location under different operating conditions, continuous temperature and relative humidity measurements recorded with a data logger). 

Thermocouple An instrument for the measurement of temperature consisting of two wires of different metals joined at each end. An electrical electromotive force is generated, the magnitude of which allows the temperature to be measured. 

The temperature differences found experimentally are less than expected theoretically because of heat losses within the apparatus. As indicated in the earlier part of this chapter, the experimental approach is to measure these temperature differences at a number of different concentrations and extrapolate to c = 0. The apparatus is calibrated using standard solutes of low relative molar mass, but despite this, the technique can be used on polymers up to of about 40 000. 

The heat transfer coefficient (/i) has been determined by measuring the temperature difference between the immersed heater and the bed. The h value increases with increasing Ug (Fig. 1(a)), but exhibits a maximum value with increasing (Fig. 2(a)). The effects of Ug on h is dominant, since the bubbling phenomena become more vigorous due to the... 

Seebeck used antimony and copper wires and found the current to be affected by the measuring instrument (ammeter). But, he also found that the voltage generated (EMF) was directly proportional to the difference in temperature of the two junctions. Peltier, in 1834, then demonstrated that if a current was induced in the circuit of 7.1.3., it generated heat at the junctions. In other words, the SEEBECK EFFECT was found to be reversible. Further work led to the development of the thermocouple, which today remains the primary method for measurement of temperature. Nowadays, we know that the SEEBECK EFFECT arises because of a difference in the electronic band structure of the two metals at the junction. This is illustrated as follows ... 

For example, the final heat treatment temperatures In the manufacture will produce different electrochemical properties, even with the same surface treatments (2-4) since the structure and electrical property of glassy carbon depends on the temperature, as Indicated by the single crystal TEM patterns and by measurement of temperature dependent conductivity (5-6). On the other hand. It Is also well established that the electrochemical properties of carbon-based electrodes are markedly affected by surface treatments. 

Techniques for accurate and reproducible measurement of temperature and temperature differences are essential to all experimental studies of thermodynamic properties. Ideal gas thermometers give temperatures that correspond to the fundamental thermodynamic temperature scale. These, however, are not convenient in most applications and practical measurement of temperature is based on the definition of a temperature scale that describes the thermodynamic temperature as accurately as possible. The analytical equations describing the latest of the international temperature scales, the temperature scale of 1990 (ITS-90) [1, 2]... 

Whereas the heat flux DSC measures the temperature difference between the sample and the reference sample, power-compensated DSCs are based on compensation of the heat to be measured by electrical energy. Here the sample and the reference are contained in separate micro-furnaces, as shown in Figure 10.6(b). The time integral over the compensating heating power is proportional to the enthalpy absorbed by or released from the sample. 

The measurement of an enthalpy change is based either on the law of conservation of energy or on the Newton and Stefan-Boltzmann laws for the rate of heat transfer. In the latter case, the heat flow between a sample and a heat sink maintained at isothermal conditions is measured. Most of these isoperibol heat flux calorimeters are of the twin type with two sample chambers, each surrounded by a thermopile linking it to a constant temperature metal block or another type of heat reservoir. A reaction is initiated in one sample chamber after obtaining a stable stationary state defining the baseline from the thermopiles. The other sample chamber acts as a reference. As the reaction proceeds, the thermopile measures the temperature difference between the sample chamber and the reference cell. The rate of heat flow between the calorimeter and its surroundings is proportional to the temperature difference between the sample and the heat sink and the total heat effect is proportional to the integrated area under the calorimetric peak. A calibration is thus... 

Three different principles govern the design of bench-scale calorimetric units heat flow, heat balance, and power consumption. The RC1 [184], for example, is based on the heat-flow principle, by measuring the temperature difference between the reaction mixture and the heat transfer fluid in the reactor jacket. In order to determine the heat release rate, the heat transfer coefficient and area must be known. The Contalab [185], as originally marketed by Contraves, is based on the heat balance principle, by measuring the difference between the temperature of the heat transfer fluid at the jacket inlet and the outlet. Knowledge of the characteristics of the heat transfer fluid, such as mass flow rates and the specific heat, is required. ThermoMetric instruments, such as the CPA [188], are designed on the power compensation principle (i.e., the supply or removal of heat to or from the reactor vessel to maintain reactor contents at a prescribed temperature is measured). 

Figure 9.1 A thermocouple (a) and a thermopile (b) as devices for measuring a temperature difference or a heat flow. A and B are wires of different metals.

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