Thermistor bead

VPO gives suitable results for Mn if adequate thermistor beads are used. Reliable Mw values are obtained by LALLS if anisotropy, fluorescence and light absorption are taken into account. Preliminary experiments by on-line SEC-LALLS are promising but need more investigation. 

Thermistor bead element. A thermal conductivity detection device in which a small glass-coated semiconductor sphere is used as the variable resistive element in the cell chamber. 

Apparatus Use a suitable vapor pressure osmometer, such as the Hewlett-Packard Model 302A, or equivalent, equipped with dual thermistor beads. 

Procedure Following the manufacturer s instructions, balance the osmometer to zero with o-dichlorobenzene on both thermistor beads, and establish the calibration constant, K, at 100°, using the four Calibration Standards. When the temperature within the osmometer has re-equilibrated to 100°, place an aliquot of the most concentrated Sample Preparation on the sample thermistor bead. After 4.0 min, balance the instrument to zero with the potentiometer, and record the AR value. Repeat this procedure with the same Sample Preparation two or three times, and average the AR values for that concentration. In a similar manner, obtain the average AR values for each of the other three concentrations of the Sample Preparation. Plot the four average AR values for the Sample Preparations as a function of AR/concentration, and extrapolate the line to zero to obtain the constant, Kjj, for the sample. Divide K by Kv to obtain the molecular weight of the sample tested. 

The effect of several operating parameters must be emphasized. They are the size of the drops on the thermistor beads, the response time, and the purity of both solvents and lignin samples. Furthermore, it must be noted that conflicting results have been reported on the constancy of the calibration factor (Bersted 1973, Brzezinski et al. 1973, Kamide et al. 1976, Burge 1979, Marx-Figini and Figini 1980, Kim 1985, Froment and Pla, unpubl. results, 1988). 

To minimize these effects, it is recommended therefore that medium size drops (b = 1.8-1.9mm) and solution concentrations in the range of 2-lOgkg-1 be employed. However, these effects are reduced or eliminated using the type of thermistor beads described in Section 8.3.3.1 which maintain the drop volume constant. 

The most interesting point of the study is the possibility of not needing thermo-stats due to direct contact between the enzyme and the thermistor bead. However, the insulation box is still very large - 300 x 260 x 175 mm. Moreover, direct enzyme immobilization on thermistors is very expensive because the whole sensing part has to be exchanged after the utilization of the enzymes. 

Thermistor Bead. disk. chip, or rod High Moderate -50 to 150... 

Despite the limitations of the Pennes bioheat equation, reasonable agreement between theory and experiment has been obtained for the measured temperature profiles in perfused tissue subject to various heating protocols. This equation is relatively easy to use, and it allows the manipulation of two blood-related parameters, the volumetric perfusion rate and the local arterial temperature, to modify the results. Pennes performed a series of experimental studies to validate his model. Over the years, the validity of the Pennes bioheat equation has been largely based on macroscopic thermal clearance measurements in which the adjustable free parameter in the theory, the blood perfusion rate [Xu and Anderson, 1999] was chosen to provide reasonable agreement with experiments for the temperature decay in the vicinity of the thermistor bead probe. Indeed, if the limitation of Pennes bioheat equation is an inaccurate estimation of the strength of the perfusion source term, an adjustable blood perfusion rate will overcome its limitations and provide reasonable agreement between experiment and theory. 

This technique employs a single thermistor serving as both a temperature sensor and a heater. Typically in this technique, either a thermistor is inserted through the lumen of a hypodermic needle, which is in turn inserted into the tissue, or the thermistor is embedded in a glass-fiber-reinforced epoxy shaft. Figure 2.4 shows the structure of a thermistor bead probe embedded in an epoxy shaft. Each probe can consist of one or two small thermistor beads situated at the end or near the middle of the epoxy shaft. The diameter of the finished probe is typically 0.3 mm, and the length can vary as desired. Because the end can be sharpened to a point, it is capable of piercing most tissues with very minimal trauma. 

FIGURE 2.4 Sketch of a thermistor bead probe. [From Xu et al. (1998), with permission.]... 

Typically, the Pennes bioheat transfer equation is used to predict the temperature transient. It is assumed fiiat the thermistor bead is small enough to be considered a point source inserted into the center of an infinitively large medium. The governing equation and initial condition for this thermal process are described as... 

The temperature pulse decay technique has been used to measure both the in vivo and in vitro thermal conductivity and blood flow rate in various tissues (Xu et al., 1991 1998). The specimen does not need to be cut from the body, and this method minimizes the trauma by sensing the temperature with a very small thermistor bead. For the in vitro experimental measurement, the measurement of thermal conductivity is simple and relatively accurate. The infinitively large tissue area surrounding the probe implies that the area affected by the pulse heating is very small in comparison with the tissue region. This technique also requires that the temperature distribution before the pulse heating should reach steady state in the surrounding area of the probe. 

Temperature Pulse Decay Technique. As described in Sec. 2.4 under Temperature Pulse Decay (TPD) Technique, local blood perfusion rate can be derived from the comparison between the dieoretically predicted and experimentally measured temperature decay of a thermistor bead probe. The details of the measurement mechanism have been described in that section. The temperature pulse decay technique has been used to measure the in vivo blood perfusion rates of different physical or physiological conditions in varimis tissues (Xu et al., 1991 1998). The advantages of this technique are that it is fast and induces little trauma. Using the Pennes bioheat transfer equation, the intrinsic thermal conductivity and blood perfusion rate can be simultaneously measured. In some of the applications, a two-parameter least-square residual fit was first performed to obtain the intrinsic therm conductivity of the tissue. This calculated value of thermal conductivity was then used to perform a one-parameter curve fit for the TPD measurements to obtain the local blood perfusion... 

Thermal conductivity detectors in PGC are universal detectors in that they can be used to measure any component (Figure 2). Typically, TCDs are used to measure percent level components, but because it is a concentration-sensitive detector, the TCD can be designed for lower level analyses by optimizing the detector volume and sensing elements. TCDs used in PGC utilize either filaments or thermistor beads as their sensing elements. Filaments have the advantage that they can be operated at higher temperatures, but they are more prone to failure than are thermistors. The sensitivity is mainly dependent on the detector temperature and the thermal conductivity difference between the carrier gas and the component of interest. 

Another problem is intolerance of core temperature sensors by the wearer. Oral temperature is not very accurate and reliable for core temperature assessments. Rectal probes provide more accurate readings, but are less tolerable. A more acceptable sensor is a tympanic sensor. It has a thermistor bead or small thermocouple placed in the ear canal against or very near the eardrum and held in place by a custom-molded ear plug. Tympanic temperature closely follows changes in core temperature. The preferred readout display is a digital one. 

Except for the Corona model 117, two carefully matched thermistors, on which the solution and solvent drops are placed, are installed in a chamber saturated with solvent vapour. The temperature difference (T-Tq) is detected as a change in the resistance of the thermistors. A large factor governing the accuracy of the temperature measurement of the drops by the thermistors in VPO is the unmatched resistance of the thermistor bead-temperature relations between two thermistors. Even if an aged and well-matched pair of thermistors are chosen at a specific temperature, these do not always match at a different temperature. That is, even if the temperature of the surroundings fluctuates only to a small extent, the conventional Wheatstone bridge is kept in a balanced state only when [14]... 

Fig. 3.12 Schematic illustration of a vapour pressure osmometer showing the solution and solvent drops on matched thermistor beads. The temperature difference between the solution and solvent is measured as the resistance difference, AR, between the thermistor beads.

In practice, ATg is not attained due to heat losses from the solution drop to the thermistor bead and its stem, and to the saturated vapour. Instead, a steady-state value ATs is achieved and is given by... 

Vapour pressure osmometers measure the resistance difference between the two thermistor beads and this is assumed to be proportional to the temperature difference, i.e. ARs = knATs. Thus, in order to be of practical use. Equation (3.109) must be modified to the form... 

Measurements of the equilibrium resistance difference (AT ) between the solution and solvent thermistor beads in a vapour pressure osmometer were made... 

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