Platinum resistors

The voltage drop across the platinum temperature sensor is small since the platinum resistor has a nominal resistance of only 75 Q. The fully-differential LNA amplifies the minute voltage drop in order to provide an useful feedback signal to the differential-analog proportional controller. A simplified schematic of the fully-differential low-noise amplifier is shown in Fig. 5.18. 

The essential part of the thermal chemical sensor is the device that allows fast, sensitive, and precise detection of the temperature a thermometer. There are many thermometers available here we mention only three types, in the descending order of their sensitivity. They are thermocouples, platinum resistors, and thermistors. 

The coefficients for Pt are A = 4 x 10 3, B = 5.8 x 10 7, and po = 1 x 10-5 Q cm. With these parameters, the sensitivity, expressed as the temperature coefficient, is 0.4%°C 1 over a wide range of temperatures. Resistivities of other metals, as well as their temperature coefficients, are tabulated in standard reference tables (e.g., the CRC Handbook of Chemistry and Physics, 2006). Because the geometry of the resistor does not change with temperature, (3.8) is often written in terms of change of resistance R. Because of their chemical inertness and high temperature coefficient, platinum resistors are most common. They are the key part of the most successful thermal sensors, pellistors, which are discussed in Section 3.6.2. 

The complex electric permittivity, k = k + k , where k = C/C o is the real, and k = tan(8) / K is the complex part of the permittivity, was measured in the frequency interval 300 Hz - 1 MHz at different temperatures by a Solartron 1200 inq>edance gain analyser, using a parallel plate capacitor made of stainless steel. From the capacitance, C, and the tangent loss, tan(6), the values of k and k were calculated [2]. The temperature was controlled within O.IK using a platinum resistor Pt(lOO) as a sensor and a K30 Modinegen external cryostat coupled with a N-180 ultra-cryostat. 

For the temperature range up to 800 or even 1100°C sensors are not yet used in high volumes. There are several possible technologies. The first is to qualify special NTCs for these temperatures [3], the second is to use thermocouples, and the third is platinum resistors there are others. The battle has just started and so far there is no clear favorite in the market. 

Yellow Springs Instrument Co. 46TUC 0 to 51°C 0.05°C 0.15°C PLATINUM RESISTOR TYPE 618 25-100 6 Input channels, recorder output, analog meter... 

As resistive heating elements of this electrothermic interface thin film platinum resistors and surface mount technology (SMT) resistors are used (Richter et al. 2003, 2009a). The hydrogel components can be heated and cooled also by Peltier elements (Yu et al. 2003b Luo et al. 2003a). 

Resistive materials used in thermometry include platinum, copper, nickel, rhodium-iron, and certain semiconductors known as thermistors. Sensors made from platinum wires are called platinum resistance thermometers (PRTs) and, though expensive, are widely used. They have excellent stability and the potential for high-precision measurement. The temperature range of operation is from -260 to 1000°C. Other resistance thermometers are less expensive than PRTs and are useful in certain situations. Copper has a fairly linear resistance-temperature relationship, but its upper temperature limit is only about 150°C, and because of its low resistance, special measurements may be required. Nickel has an upper temperature limit of about 300°C, but it oxidizes easily at high temperature and is quite nonlinear. Rhodium-iron resistors are used in cryogenic temperature measurements below the range of platinum resistors [11]. Generally, these materials (except thermistors) have a positive temperature coefficient of resistance—the resistance increases with temperature. 

FIGURE 2.1.6 Temperature dependence of different types of thermistors in contrast to a platinum resistor. 

Velocity Sensors, Figure 3 Simulated gas temperature versus flow velocity at the position of platinum resistors. Nitrogen gas was used in this simulation [2]... 

Fig. 3.9 Lc/t High-temperature-high-pressure electrochemical wall-tube cell (A) inlet, (B) pre-cell, (C) mixing dishes, (D) platinum resistor, (E) reference electrode, (F) counter electrode, (G) zircaloy nozzle, (H) outlet, (I) working electrode, (J) cell and (K) zircaloy rings. Right Typical voltammograms in Fe /Fe 1 mM in 0.2 M Na2S04 (pH 1.5) on a platinum electrode at 85 °C. Sweep rate 50 mV s, H 0.264 cm, d 0.204 cm and = 0.05 cm. Flow rates (a) 4, (b) 8, (c) 12 and (d) 20 cm min. From [259], with permission...

Since specific heat at very low temperatures is minimal, even low heat input causes a large temperature increase. For measurements requiring exact temperature recording, care must be taken to minimize both the intrinsic heat generation of thermal sensors and the heat flux along their current leads. Carbon resistors and germanium diodes (heating power <10 W) are useful below 40 K. Above that temperature, platinum resistors suffice. 

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