Specific heats, selected elements

Flammable atmospheres can be assessed using portable gas chromatographs or, for selected compounds, by colour indicator tubes. More commonly, use is made of explos-imeters fitted with Pellistors (e.g. platinum wire encased in beads of refractory material). The beads are arranged in a Wheatstone bridge circuit. The flammable gas is oxidized on the heated catalytic element, causing the electrical resistance to alter relative to the reference. Instruments are calibrated for specific compounds in terms of 0—100% of their lower flammable limit. Recalibration or application of correction factors is required for different gases. Points to consider are listed in Table 9.10. 

The specific heat capacities of the metals nickel, zinc, rhodium, tungsten, gold, and uranium at 25°C are 0.444, 0.388, 0.243, 0.132, 0.129, and 0.116 J K g respectively. Calculate the molar heat capacities of these six metals. Note how closely the molar heat capacities for these metals, which were selected at random, cluster about a value of 25 J K mol b The rule of Dulong and Petit states that the molar heat capacities of the metallic elements are approximately 25 J K moUb... 

The specific heat increases rapidly with temperature, up to 1500 K where it levels off at approximately 2.2 kJ/kg-K as shown in Fig. It is believed to be relatively insensitive to the differences between the various grades of synthetic graphite, and the spread of values found in the literature may be attributed to experimental variations. The average value Is compared to that of selected other elements in Table 3.5. As can be seen, it is higher than most metals. 

R. J. Corruccini and J. J. Gniewek, Specific Heats and Enthalpies of Technical Solids at Low Temperatures , National Bureau of Standards Monograph 21, 1960. Preliminary Report on the Thermodynamic Properties of Selected Light-element Compounds, National Bureau of Standards Report 6928, 1960. 

Once the sensing element has been selected, its area, weight, and specific heat are fixed. Therefore, r is only a function of /z. If the manufacturer provides a time constant T for a given set of conditions, other r can be estimated based on the new conditions. If the system properties are about the same as those the manufacturer used during the step tests, then h will primarily be a function of fluid velocity. Also, the bulb has a time... 

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