Reference points, temperature

The thermometer used in the large temperature interval between 13.8033 and 1234.93 K is a platinum resistance thermometer calibrated at fixed reference points. Temperatures are expressed in terms of W(T90), which is the ratio of the resistance R(T9q) of the thermometer at temperature T9o and the resistance at the triple point of water / (273.16), as seen in equation (A2.4) ... 

These graphs show that the temperature level is much higher along the radiant tube in the case of HRS. This is a preliminary conclusion for a higher level of heat flux. Although the temperature difference increases with the decreased reference point temperature for both systems, this is not significant in the case of HRS. 

It has to be noticed that although the profiles have been compared for the same maximum reference point temperature, the maximum temperature on the profiles, created for the right side of the tube, is not the same. It is due to the location of the TRP on the top of the tube. The real obtained maximum temperatures of the reference point are shown on graphs in Figure 24.13 and in Table 24.2. For all tests, the TRP point is situated on or very close to profile in the case of the HRS, while for the recuperative system this point usually is about 20°C higher than the maximum temperature of the right side profile. This is a result of nonuniform temperature distribution on the circumference, which will be explained further. 

In the case of the HRS, both peak temperatures are placed closer to the ends of the radiant tube, than the location of TRP (which was equal to 0.995 m). As a result the reference point temperature, which determines the life span of the tube, has to be moved to another place in the case of HRS. This temperature will be a few degrees higher on the top than on the right side, as will be described next. Temperatures around a cross section of the radiant tube at point 17, located in the middle of the tube (Figure 24.10), are shown in Figure 24.14. 

Longitudinal temperature profile of the radiant tube for recuperative and regenerative systems at different reference point temperature Ttrp (a) 840°C, (b) 880°C, (c) 950°C, and (d) 1000°C. 

If time and circumstances permit, data should be recorded when the compressor is run on process gas to establish a new base reference point. During operation, all monitoring equipment should be observed to establish a signature of vibration and temperature, and to be sure these data are all within permissible limits. 

T, = Temperature of the flowing gas at any section x-ft from the reference point, °F... 

Solution The first step is choosing a reference point. At the point 1 m right below the panel, the radiant temperature is 29 °C, and if we assurte that the temperature of air is 20 °C, then Tf, = 49 °C. 

The reference point of the weight was initial steady-state weight at 400 °C in air. In a few cases, the sample was kept for 3-72 h at the same temperature to see whether the weight changed further or not. It was found that the weight essentially reached in equilibrium at each temperature with 3 H. The number by the points (3, 11, 72) indicate the extra hours waited for this purpose. 

The fixed reference points for ITS-90 (temperatures at specified equilibrium states) are given in Table A2.1. These reference points are selected to calibrate thermometers over different temperature ranges as we describe later. 

The type of thermometer used to interpolate between the reference points depends on the temperature interval. 

Preliminary research has shown that Brillouin fiber-optic sensing systems provide a possible method to detect leaks and third-party intrusion on a pipeline over distances of 25 km or more. Their intrinsic response to both temperature and mechanical strain allows for the separation of these parameters and the detection of anomalies in the scan profiles. In addition, the same sensor could be integrated into the pipeline system to detect possible ground movement relative to fixed reference points. Limited test results on surface loads associated with the intrusion of vehicles and people on a pipeline have demonstrated the sensitivity of the system and its ability to discriminate loads at different soil depths. 

To obtain reference points at lower temperatures, as we shall see (Sections 8.4, 8.5), transitions other than (a)-(e) are necessary. 

The 3He melting pressure thermometer has been chosen to extend the ITS 90 for several reasons, such as the good sensitivity over three temperature decades, except around the pressure minimum at 315.24 mK. On the other hand, such a minimum is a reference point in the calibration of the pressure transducer in fact, the pressure must be measured in situ since, below 315.24 mK, the entrance of the measurement cell is blocked by solid 3He. 

Besides the pressure minimum, the 3He melting curve presents a few characteristics which can be used as temperature and pressure reference points the superfluid A transition, the A-B transition in the superfluid and N6el transition in the solid. The reference values are reported in Table 8.7 (see also Section 2.2.4). 

The next step is to determine the material factor (MF) for use in the form shown in Figure 10-3. Table 10-1 lists MFs for a number of important compounds. This list also includes data on heat of combustion and flash and boiling point temperatures. The additional data are also used in the computation of the Dow F EI. A procedure is provided in the complete index for computing the material factor for other compounds not listed in Table 10-1 or provided in the Dow reference. 

From this reference point, accurate absorbances can then be determined. This data is then plotted (absorbance vs. known concentrations) and the unknown concentration then extrapolated from the Beer s law plot. Factors that limit this technique include 1) sensitivity of the instrument being used — usually best between 10 and 90%T, 2) the magnitude of the molar absorptivity (e), 3) fluctuations due to pH changes, and 4) temperature changes. 

In view of the useful interpretations which can be given to the differences in E1/2 values, an obvious step is to select the E1/2 of one convenient ion as a zero or reference point. We follow Taft [14] and Breslow [15] in selecting E1/2 for the triphenylmethylium (trityl) cation, as this ion is easy to procure and is stable under acidic conditions. It seems useful to define a quantity Ef/2 which is 1/2 of the species in question minus EV2 of the triphenylmethylium ion measured under the same conditions of concentration, temperature, solvent, and nature and concentration of base electrolyte, and against the same reference electrode ... 

The most common reference points for temperature sensor calibration are the freezing and boiling points of water. The freezing point is a function of the purity of the water-ice system. When both pure ice and pure liquid water are present, the ice is melting and the temperature of a well-stirred mixture is, by definition, 0°C. It is the most accurate calibration point for that reason. If it is possible to use such a quantity in a calibration, then the calibration is true and without question. 

This latter point begs the question Who can verify the accuracy of a reference point if its value may vary In other words, who is the ultimate source of calibration materials, such as a thermometer In the U.S., it is the National Institute of Standards and Technology (NIST). This is the same organization that we cited as the source of accurate standardization materials in Chapters 3 and 4. Experiments 16 and 17 in this chapter are exercises in the calibration of a temperature sensor and how such a calibrated sensor can be used. 

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