Platinum temperature coefficient

These utilize the fact that the resistance of most materials changes with temperature. In order to be useful for this purpose this change must be linear over the range required and the thermal capacity must be low. Although the above implies a high resistivity and temperature coefficient, linearity and stability are the paramount considerations, and suitable materials are platinum, nickel and tungsten. 

Here R0 is the resistance at 0°C, and a and b are coefficients whose values are specified in internationally agreed standards covering platinum temperature sensors, for example DIN EN 60751. The coefficient b is so small that for most applications one can assume a linear relation between Rt and the temperature t. 

For each type of sensor, a classification framework sets precision tolerance ratings. Platinum temperature sensors have a positive temperature coefficient which is defined as ... 

Platinum is especially suitable for this application because even at high temperatures it has a good stability and a good resistance to contamination. However, different metals, all having a positive temperature coefficient, may be used, such as tungsten (for very high-temperature applications), nickel and nickel alloys and also (but rarely because of their low resistivity) gold and silver. 

The temperature coefficient of conductance is approximately 1-2 % per °C in aqueous 2> as well as nonaqueous solutions 27). This is due mainly to thetemper-ature coefficient of change in the solvent viscosity. Therefore temperature variations must be held well within 0.005 °C for precise data. In addition, the absolute temperature of the bath should be known to better than 0.01 °C by measurement with an accurate thermometer such as a calibrated platinum resistance thermometer. The thermostat bath medium should consist of a low dielectric constant material such as light paraffin oil. It has been shown 4) that errors of up to 0.5 % can be caused by use of water as a bath medium, probably because of capacitative leakage of current. 

High purity platinum wire is used in resistance thermometers because the temperature coefficient of resistance of pure platinum is linear over a wide temperature range. The platinum resistance thermometer is the recognized instrument for the interpolation of the international practical temperature scale from—259.35 to 630.74°C. Whereas such precision measurements require very high purity platinum, for most routine industrial measurements lower purity metal can be tolerated. Conventional wire-wound devices are quite fragile and this disadvantage has led to the introduction of printed resistance thermometers, which are cheap to produce and much more durable. They can be used as an inexpensive replacement for thermocouple applications in intermediate temperature applications. 

Platinum 200ft 70 to 870 0.4 temperature coefficient always positive). 

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. 

It is necessary to maintain close control over temperature because of its effect upon the base resistance of the semiconductor. With a platinum heater this can be effected to 0.1 °C through monitoring the resistance of the heater itself using a Wheatstone bridge. The relatively high temperature coefficient of resistivity of Ru02 complicates the control system. 

Palladium melts at 1549-2° C.4 and boils at approximately 2540° C. It can be distilled in the electric furnace. It is less volatile than iridium, but more so than platinum. Its coefficient of linear expansion with rise of temperature is 0-041176 between 0° and 100° C.7 its specific heat between the same temperatures is 0-05928,8 and between 0° and 1300° C. the value may be calculated from the equation 9 ... 

The filament wire is usually tungsten or platinum as both metals have reasonably high temperature coefficients of resistance and at the same time are very inert and unlikely to interact chemically with the... 

There was nothing to be altered in the experimental arrangement, for the method already described still gave, as was to be expected, any desired precision at these temperatures. The calibration of the platinum wire used occasioned some difficulties, but eventually we were able to fix with certainty a number of temperatures and to relate them by an equation, such that the very variable temperature coefficient for the region 20° to 40° abs which was what was really wanted, could be derived with sufficient accuracy (compare below). 

Measurement of Temperature.—Under favourable conditions the accuracy of the measurements is practically dependent only on the accuracy with which the platinum wire used has been calibrated. It is not sufficient to calibrate a sample of the wire in question once for all, but some control determinations at least must be made for every calorimeter used, for the temperature coefficient of the wire is slightly influenced by the manner in which it is embedded. The following methods were tried —... 

Now, as has already been remarked, this temperature coefficient varies at low temperatures in a manner not very favourable to accurate measurements there is a fairly sharp maximum in the neighbourhood of 8o° abs., and it falls very rapidly below 40° abs. In spite of the most careful calibration, which presents difficulties in the region from 25° to 45° abs. on account of the lack of fixed points, there are still uncertainties of a small amount. There is a further inconvenience in the fact that the platinum wire has its resistance considerably changed by the introduction of the electrical energy, and that, in consequence, the accurate determination of the latter, though quite possible, is nevertheless somewhat troublesome. 

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. 

The overall activation energies and frequency factors of overall catalytic rate expressions, and consequently their temperature coefficients, are less easy to constrain. For example, the relatively simple rate expression for the DAM model of the oxidation of carbon monoxide on a platinum on alumina catalyst (see Chapter 11) is thought to be ... 

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