chap08 - PROBLEM 8.1 KNOWN/FIND Define and discuss the...

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PROBLEM 8.1 KNOWN/FIND: Define and discuss the significance of: a) temperature scale b) temperature standards c) fixed points d) interpolation SOLUTION: a) temperature scale - an established relationship for assigning numerical values to measures of temperature. The absolute temperature scales are: the Rankine scale, for U.S. customary units the Kelvin scale, for SI units b) temperature standards - a formally adopted and recognized means for practical realization of temperature measurement. Standards provide a means for the measurement of temperature which can be reproduced and agrees with the thermodynamic definition of temperature. c) fixed points - identifiable and experimentally reproducible conditions which are associated with a certain temperature (numerical value). See Table 8.1. d) interpolation - a method for determining temperatures other than those defined by fixed points on a temperature scale. For the majority of applications, the interpolating instrument is a platinum RTD.
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PROBLEM 8.2 KNOWN: An apparatus to produce phase equilibrium points is required. FIND: Describe the conditions necessary to establish phase equilibrium points. Identify the effects of elevation, weather and material purity. SOLUTION: Other than the vapor-pressure-temperature points for helium and hydrogen, the fixed points for ITS-90 are freezing points, melting points or triple points. triple point - The procedure for calibrating a thermometer at a triple point is: 1. completely freeze the sample (an appropriate mass of material) in a closed container 2. experimentally determine the energy required to melt the sample 3. re-freeze the sample 4. Add energy and record the thermometer output at 10, 20 40, 60, 70 and 80% melted. These readings should agree. This procedure demonstrates that a container capable of preventing contamination of a sample of material, while allowing the removal and addition of energy, is required to establish the triple point for a material. Similar requirements are needed for melting/freezing points, with the notable exception that containers must generally be flexible, to accommodate thermal expansion. Representative values of measured temperatures agree to 0.1 mK. A sample which is 99.9999% pure will produce measured temperatures over a phase change within 0.1 mK. Weather and elevation should be eliminated through appropriate design of the experimental apparatus.
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PROBLEM 8.3 KNOWN: A length of platinum wire having: length, l = 2 m diameter, D = 0.1 cm resistivity, ρ e = 9.83 × 10 -6 -cm FIND: The resistance of the wire, R SOLUTION: Since R l A e c = ρ and () 2 22 0.1 0.0079 cm 44 c AD π == = The resistance is found as 62 2 9.83 10 2 m 10 cm/m 0.25 0.0079 cm R × COMMENT: An RTD would normally have a reasonably large resistance, on the order of 25 . As such, a very small diameter wire or long length must be employed.
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PROBLEM 8.4 KNOWN: A Wheatstone bridge and RTD as shown in Figure 8.35, with α = 0.003925 ° C -1 R o = 25 at 0 ° C R 1 = 41.485 for balanced conditions FIND: a) The temperature of the RTD b) Compare the static sensitivity of this circuit to the circuit in Example 8.2 SOLUTION:
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This note was uploaded on 03/17/2008 for the course EML 3301C taught by Professor Chesney during the Spring '08 term at University of Florida.

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chap08 - PROBLEM 8.1 KNOWN/FIND Define and discuss the...

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