Frequency Thermometry

The frequency response of the detection system is of low-pass type for characteristog-raphers and band-pass for bridges (see Section 10.4). In both types of measurements the narrowing of the bandwidth corresponds to a longer time of measurement. Depending on the chosen detection system, several problems (true traps) may be encountered in resistance thermometry. 

An interesting aspect of dielectric constant thermometry is the small influence of a magnetic field. On the other hand, measurements depend on both the measuring frequency and voltage (see Figs 9.12 and 9.13). Figure 9.13 shows an example of the dependence on frequency of both the dielectric constant and loss for Upilex S [93]. 

Magnetic thermometry has been developed chiefly to measure temperatures near absolute zero (below -458°F, or -272°C). These measurements are obtained by adiabatic demagnetization of a paramagnetic salt. Inductance can be measured with an AC bridge (as shown in Figure 3.164) whose balance is independent of frequency. The relationship between self-inductance and sus-... 

In principle, a resonance frequency or difference in frequencies can be employed to perform NMR thermometry. The accuracy of temperature measurement depends on the accuracy of the thermocouple (0.2°C) in the NMR spectrometer. Generally, a precision of better than 0.2°C has been reported with these NMR thermometry measurements. Individual calibration plots may not be required when using aqueous buffers. Proper calibration procedures should be adopted with nonaqueous buffers and in the presence of organic modifiers. 

Temperature information from CARS spectra derives from spectral shapes either of the 2-branches or of the pure rotational CARS spectra of the molecular constituents. In combustion research it is most common to perform thermometry from nitrogen since it is the dominant constituent and present everywhere in large concentration despite the extent of chemical reaction. The 2-branch of nitrogen changes its shape due to the increased contribution of higher rotational levels which become more populated when the temperature increases. Figure 6.1-21 displays a calculated temperature dependence of the N2 CARS spectrum for experimental parameters typically used in CARS thermometry (Hall and Eckbreth, 1984). Note that the wavenumber scale corresponds to the absolute wavenumber value for the 2320 cm 2-branch of N2 when excited with the frequency doubled Nd.YAG laser at 532 nm ( 18796 cm ), i. e. = 18796 -1- 2320 = 21116 cm. The bands lower than about 21100 cm are due to the rotational structure of the first vibrational hot band. 

Coherent Anti-Stokes Raman Scattering (CARS) Thermometry is a technique for temperature measurement in high temperature environments using a third-order nonlinear optical process involving a pump and a Stokes frequency laser beam that interacts with the sample and generates a coherent anti-Stokes frequency beam. 

Another resonant-frequency thermometer is the quartz crystal resonator (Benjaminson and Rowland, 1972), which, if the crystal is properly cut, is quite linear from about 190 to 525 K. Although this thermometer has excellent resolution, it does exhibit hysteresis and drift. The principle of quartz crystal thermometry is based on the temperature dependence of the piezoelectric resonant frequency of a quartz crystal wafer of a given dimension. The angle of cut of the quartz crystal is selected to give as nearly a linear and yet sensitive correspondence between resonant frequency and temperature as possible. This angle of cut is referred to as an LC (linear coefficient) cut. The temperature sensitivity of the quartz crystal thermometer is about 1000 Hz/°C. 

Other experimental aspects come into play for a correct application of DEWM. These refer to the relative comparison between the laser linewidths and the Doppler and collisional broadenings. As a matter of fact, the two limits of narrowband and broadband laser line have important consequences for the quantitative analysis of chemical species. The effect of absorption, caused by the frequency degeneracy, is also problematic in quantitative DEWM and has to be evaluated in some instances. To sum up, an all-embracing interpretation of DEWM measurements is not available as it is, by contrast, for CARS. Nevertheless, careful choice of the experimental conditions and rigorous analysis of the corresponding theoretical approximations can lead to reasonable results for both thermometry and concentration measurements, as... 

Some years ago D. Stuerga designed a microwave reactor, called the RAMO (reac-teur autoclave microonde), which is not a commercial device. The microwave applicator and the reactor are original. The resonant frequency of the cavity can be controlled by varying the position of a plunger. The effective cavity power can be increased by three orders of magnitude. The autoclave is made of polymeric materials, which are microwave transparent, chemically inert, and sufficiently strong to accommodate the pressures induced. The reactants are placed in a Teflon flask inserted within a polyetherimide flask. A fiber-optic thermometry system, a pressure transducer, and a manometer enable simultaneous measurement of temperature and pressure within the reactor. The system is controlled by pressure. The reactor is shown in Fig. 2.32. 

The most common method of temperature measurement is contact thermometry, as demonstfated in Fig. 4.1. One brings a thermometer, C, a system with a known thermal property, into intimate contact with the to be measured system, A. Next, thermal equilibration is awaited. When reached, the temperatures of A and C are equal. The use of C as a contact thermometer is based on the fact that if the two systems A and B are in thermal equilibrium with C they must also be in thermal equilibrium with each other. This statement is sometimes called the zeroth law of thermodynamics. It permits to use B with a known temperature to calibrate C, and then use C for measurement of the temperature of system A. A calibration with B can be made at a fixed temperature of a phase transition without degree of freedom, as given by the phase rule of Sect. 2.5.7. Less common are methods of temperature measurement without a separate thermometer system. They make use of the sample itself. For example, the temperature of the sample can be determined from its length, the speed of sound within the sample, or the frequency of light emitted. 

The method used is quite general and can be applied to other molecular species. Furthermore, in the near absence of background gas collisions it allows one to directly relate the rotational temperature of the ions to the temperature of the ambient black-body radiation. This feature (among others, see Refs. [101,105]) suggests the use of molecular ions, such as HD+ or CO" ", for BBR thermometry with possible applications in frequency metrology that is, it may help to improve the accuracy of frequency standards based on trapped ions [104]. 

Koelemeij, J.C.J., Roth, B., and Schiller, S., Cold molecular ions for blackbody thermometry and possible application to ion-based frequency standards, Phys. Rev. A, 76, 023413, 2007. 

An important development in recent years has been the advent of quartz crystal thermometry. When a quartz crystal is cut at a certain orientation to the axes of the crystal lattice the temperature dependence of the resonant frequency is large and nearly linear, and the Hewlett Packard Company has... 

The following is a thermally rather isolated copper container, controlled by resistance thermometers and restorative electrical heating approximately proportional to the square of temperature displacement from the set point. Manual reset is required for signilicant change of conditions. Resistance thermometry avoids the necessity for a precisely controlled reference temperature. The apparatus was constructed for investigation of piezoelectric frequency standards. It may be useful for other electronic elements, ... 

Proton Frequency Shift T1 Thermometry 320 Parenchymal Changes at MR Parenchymal Changes at CT Parenchymal Changes at US New Developments 324 Post-ablation Appearances and Follow-Up at Contrast Enhanced CT 325 Recognising Recurrence 326 Timing of Recurrence 327 CT vs MR for Detection of Recurrence 327 Conclusion 327 References 327... 

---