400- Series thermistor

Keywords— 400-series thermistor. Interfacing technique. Star topoiogy, Negative feedback, bio - sensor, Ethernet, LAN. 

The thermistor material is usually a metal oxide, eg, manganese oxide. Dopants, eg, nickel oxide or copper oxide, may be added to obtain a variety of resistance and slope characteristics. The material is usually skitered kito a disk or bead with kitegral or attached connecting wkes. Figure 4 shows a typical series of steps ki the production of a disk thermistor. 

In the framework of CUORICINO [41] and CUORE [42] experiments (see Section 16.5), Ge crystal wafers of natural isotopic composition have been doped by neutron irradiation, and the heavy doping led to materials close to the metal insulator transition. Several series of NTD wafers with different doping have been produced [43], After an implantation and metallization process on both sides of the wafers, thermistors of different sizes can be obtained by cutting the wafers and providing electrical contacts. 

Figure 15.8 shows the thermal scheme of one detector there are six lumped elements with three thermal nodes at Tu T2, r3, i.e. the temperatures of the electrons of Ge sensor, Te02 absorber and PTFE crystal supports respectively. C), C2 and C3 are the heat capacity of absorber, PTFE and NTD Ge sensor respectively. The resistors Rx and R2 take into account the contact resistances at the surfaces of PTFE supports and R3 represents the series contribution of contact and the electron-phonon decoupling resistances in the Ge thermistor (see Section 15.2.1.3). 

Thermal and mass flow-through sensors rely on differential measurements owing to the low selectivity of these types of detection. They use two flow-cells arranged in series (Fig. 2.9.B) or parallel (Fig. 2.9.C), each containing a sensitive microelement (a piezoelectric crystal or a thermistor). One of the cells houses the sensitive microzone, whereas the other is empty or accommodates an inert support containing no immobilized reagent (e.g. see [35]). 

Experimentally, AT is determined for approx, five different polymer concentrations. After several minutes, a constant temperature difference AT of the two drops is reached which is proportional to their initial difference in vapor pressure and thus proportional to the number of dissolved macromolecules in the solution drop. AT can then be determined by measuring the difference in electric resistance of the two thermistors. Then, ATIKc is plotted vs. c (thus the power law series is broken after the linear term in c) and the plotted values are extrapolated to c 0. Mj, is finally calculated from they axis intercept. 

One of the best features of thermal conductivity detectors with helium carrier gas is the ease of quantitative analysis. It has been shown experimentally that relative response factors, where sample weight is used, are independent of (a) type of detector (filament or thermistor), (b) cell and sensor temperature, (c) concentration of sample, (d) helium flowrate, and (e) detector current. In addition, relative response factors change only slightly within a series of homologous compounds. The first systematic study of TCD responses in helium was done by Rosie and Grob and are summarized in reference (6). 

P and I are shown as functions of U in Fig. 4.16 and R is shown as a function of U in Fig. 4.17. The curves represent equilibrium conditions, and it is evident that no equilibrium can exist above a certain maintained maximum voltage. If a higher maintained voltage is applied, the current will go on rising indefinitely until the accompanying high temperature destroys the unit. In practice there must always be a temperature-insensitive resistor in series with a thermistor if sufficient power to raise its temperature appreciably is to be applied. 

Some of the properties and uses of thermistors, other than temperature sensing, can be appreciated from the simple circuit shown in Fig. 4.18. A fixed voltage U is applied to an NTC thermistor of resistance R(T) in series with a load resistance R which is invariant with temperature. In this case there is the complication that, as the thermistor warms up and falls in resistance, the voltage across it also falls. The situation is analysed as follows ... 

Fig. 4.19 Resistance, current and temperature versus voltage for an NTC thermistor with series load P = (T— 7o)/30 W.

For most applications, an alternative is employed. Recall that, in measuring the resistance of a thermistor, a fixed resistor is normally connected in series with the sensor. If a constant-voltage source ( s) is used, the circuit current is inversely proportional to the total resistance. Then the relationship between the measured voltage drop across the fixed resistor and the thermistor temperature can be almost linear over a range of temperature. The linear part of this curve can be shifted along the temperature scale by changing the value of the fixed resistor. 

The most common detectors in IR are thermal, i.e. thermocouples, thermistors and bolometers. A thermocouple is based on the use of two different conductors connected by a junction. When a temperature difference is experienced at the junction, a potential difference can be measured. A series of thermocouples together is called a thermopile. Thermistors and bolometers are based on a change in resistance with temperature. They have a faster response time than thermocouples. With a Fourier Transform IR (FTIR), where rapid response and improved sensitivity is key, lead sulflde and InGaAs detectors are used as for NIR. Some arrays are also used. 

The DTA of cellulose, cellulose nitrate, pentaerythritol, pentaerythrityl trinitrate, and other compounds of this type has been studied by Pakulak and Leonard (135). When a thermistorized DTA apparatus was used, the upper temperature limit of the instrument was only about 200°C hence, cellulose and cellulose acetate did not give any peaks, while cellulose nitrate gave an exothermic peak with a A7 of 180°C. Similar results were noted for the pentaerythritol series. 

The temperature-resistance characteristics of thermistors depend upon their method of fabrication as well as their chemical composition. One consequence of this is that they are less stable than metal resistors. This instability is a serious disadvantage if a series of measurements is to be made over an extended period of time. Stable temperature characteristics are particularly desirable when making cryoscopic studies with organic solvent systems which are readily contaminated, because a determination of the freezing temperature is often the most convenient check on the solvent purity. This problem of long term stability can be alleviated by frequently checking the calibration against a thermometer which has stable characteristics, e.g. a platinum resistance thermometer. The temperature-resistance relationship for a thermistor follows the equation... 

A micro-thermistor sensor array was used by Wu et al. [13] to characterize a microscale heat exchanger that utilized a series of impinging jets exiting from micronozzles. The temperature... 

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. 

A micro-thermistor sensor array was used by Wu et al. [13] to characterize a microscale heat exchanger that utilized a series of inpinging jets exiting from micronozzles. The temperature sensor chip consisted of 64 thermistor-type polysilicon sensors, each of which were 4 pm x 4 pm in dimension and were spaced 500 p,m apart from one-another to form an 8x8 array in a 4mm x 4mm area at the center of the device. The sensors had a nominal resistance of 20 kS2 at room temperature and a temperature resolution of 0.1 °C. A polysilicon heater having a nominal resistance of 55 S2 was fabricated on the backside of the sensor chip in a manner similar to that utilized to fabricate the micro-thermistor array. Using this experimental setup, Wu et al. [13] showed that a 500p.m jet placed 750 xm above the heated surface cooled the chip from over 70 C to nearly 35 °C. 

Instrumented tripods with flowmeters, transmissiometers, optical backscatter sensors (OBS), in situ settling cylinders, and programmable camera systems have often been used in marine environments, for example, oceanographic studies of flow conditions and suspended particle movements in the bottom nepheloid layer [37,38]. These instruments were deployed to study suspended-sediment dynamics in the benthic boundary layer and were able to collect small water samples (1-2 L) at given distances from the seafloor. An instrumented tripod system (Bioprobe), which collects water samples and time-series data on physical and geological parameters within the benthic layer in the deep sea at a maximum depth of 4000 m, has been described [39]. For biogeochemical studies, four water samples of 15 L each can be collected between 5 and 60 cm above the seafloor. Bioprobe contains three thermistor flowmeters, three temperature sensors, a transmissiometer, a compass with current direction indicator, and a bottom camera system. 

Main motors need starter overload and short circuit protection. High rupture fuses (HRC) will protect the motor against short circuit conditions, and will interrupt the electrical supply in milliseconds of the fault occurring. It is essential that fuses of this type are always fitted. Conventional overload protection, thermal or magnetic, can offer no protection to a motor with an extended acceleration time. Thermistor overload protection is the only true protection for a motor under these conditions. A thermistor is embedded in each of the motor s three windings and connected in series. The resistance of these thermistors is designed to increase rapidly at a set temperature, depending upon the insulation class of the motor. The thermistors are connected to an electronic amplifier control unit in the starter enclosure, and will trip the starter contacts when required. The device will not reset until the motor has sufficiently cooled. 

In this proposed model of the bio-sensor system the temperature measurements obtained are much more reliable and the hardware section is reduced as con are to the conventional thermistors available in the market The thermistor is connected in negative feedback in the amplifier circuit, thus effecting/changing the voltage gain with respective change in temperature. With the help of the direct calibration table of 400 series probe, the number of temperature values is calibrated to the voltages obtained... 

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