Thermistor bead probe

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.]... 

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... 

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. 

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. 

Bead thermistors are formed by placing two wires, commonly of platinum, in dose proximity and paralld to each other and bridging them with a drop of slurry, which is then sintered into a hard bead and encapsulated in protective glass. Such thermistors are quite stable, approaching, over narrow temperature limits, the stability of industrial metallic thermometers. However, the resistance tolerance may vary from unit to unit by as much as 20%, and matching or interchangeability is usually achieved by selection. Beads can be made quite small, which may allow application in, eg, temperature probes mounted in intravenous needles. 

These types of bead, disk, and chip thermistors are increasingly used in low-cost temperature probes in the transport, medical, food processing, and more recently in the renewable energy industries. Figure 3.172 illustrates the relative behavior of thermocouples (TCs), RTDs, and NTC thermistors. On the lower part of the figure, one can observe both the sensitivity and the nonlinearity of a particular thermistor. 

Miniature bead thermistors, which are particularly valuable as temperature sensors, can be made by arranging two sets of fine platinum wires at right angles with a separation of a fraction of a millimetre between the sets. The intersections of the wires are then enclosed by small beads of paste containing the thermistor material in powder form. The beads are dried out and sintered in the same way as bulk units or fused individually with a laser beam or an oxidizing flame and annealed. The beads are then separated with two platinum lead wires which can be attached to a probe. Their small mass enables them to reach a rapid thermal equilibrium with their surroundings. 

Fig. 8.3.1. Simplified representation of the Knauer Model 11.00 Vapor pressure osmometer / Thermostat 2 measurement cell 3 lid 4 porous wicks 5 thermostated heating block 6 hypodermic syringes 7 bead thermistors 8 thermistor probe...

The most stable and most useful type of thermistor is the glass-coated bead. In this design, a sintered oxide bead (0.075 to 1 mm in diameter) is sealed in glass, resulting in a probe bead with a diameter ranging from 0.125 to 1.5 mm. Important advantages of this small thermistor size are low probe heat capacity and rapid temperature response (0.1 to... 

A thermistor probe connected to a circuit to measure the temperature of the sample. (The thermistor is a glass bead attached to a metal stem whose resistance varies rapidly and predictably with temperature.)... 

The columns were attached to the end of the plastic tubes by which they are inserted into the calorimeter. Columns could therefore be readily changed with a minimum disturbance of the temperature equilibrium. Inside the plastic tube were the effluent tubing and the leads to the thermistor were fastened to a short piece of gold capillary with heat-conducting, electrically insulating epoxy resin. Veco Type A 395 thermistors (16 kQ at 25 °C, temperature coefficient 3.9%/°C) were used. These are very small, dual-bead isotherm thermistors with 1 % accuracy as such, they were interconvertible, comparatively well matched, and follow the same temperature response curve (within 1 %). An identical thermistor was also mounted in the reference probe. 

If uncertainties of 0.01°C or less, and/or if very small, fast-response thermometers are required, then thermistors must be used. Of course, they may be used also for less accurate work. Some bead-in-glass probe-type thermistors are stable and reproducible (Sappof, 1980), over a 1-year period, to the equivalent of 0.001°C, when used at temperatures below 100°C. Their response time can be as short as a few milliseconds. 

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