Precision platinum resistance thermometer

Experimental. A Parr model 1221 oxygen bomb calorimeter was modified for isothermal operation and to ensure solution of nitrogen oxides (2). The space between the water jacket and the case was filled with vermiculite (exploded mica) to improve insulation. A flexible 1000-watt heater (Cenco No. 16565-3) was bent in the form of a circle to fit just within the jacket about 1 cm. above the bottom. Heater ends were soldered through the orifices left by removing the hot and cold water valves. A copper-constantan thermocouple and a precision platinum resistance thermometer (Minco model S37-2) were calibrated by comparison with a National Bureau of Standards-calibrated Leeds and Northrup model 8164 platinum resistance thermometer. The thermometer was used to sense the temperature within the calorimeter bucket the thermocouple sensed the jacket temperature. A mercury-in-glass thermoregulator (Philadelphia Scientific Glass model CE-712) was used to control the jacket temperature. 

Accessories. Some forms of Wheatstone-bridge circuits are used for the measurement of temperature with base-metal or industrial platinum resistance thermometers, while the Mueller bridge is used with precision platinum resistance thermometers. 

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. 

Electrolytic type sensors Uxt thick film techniques, e.g. capacitor coated in gl bonded on to a ceramic disc mounted on a thermoelectric (Peltier effect) cooler. Control is by a platinum resistance thermometer which adjusts the temperature of the cooler to regain equilibrium after a change in capacitance due to moisture deposit. Range depends on technique. Capable of high precision. Limitations are similar to those for AIjO) sensor. Capable of being direct mounted. Relatively cheap. Suitable for on-line use. 

Practical difficulties arise in making very precise determinations of temperature on the thermodynamic scale the precision of the more refined thermometric techniques considerably exceeds the accuracy with which the experimental thermometer scale may be related to the thermodynamic scale. For this reason, a scale known as the International Temperature Scale has been devised, with several fixed points and with interpolation formulas based on practical thermometers (e.g., the platinum resistance thermometer between 13.803 K and 1234.93 K). This scale is intended to correspond as closely as possible to the thermodynamic scale but to permit more precision in the measurement of temperatures. Further details about this scale are given in Chapter XVII. 

Commercial Wheatstone bridges are available in a form in which RilRs, can be set by a step switch to accurate decimal ratios from 10 to 10, and R- is a precision four- or five-decade resistance box. The use of the Wheatstone bridge with platinum resistance thermometers is discussed in Chapter XVII. 

The provisional scale adopted by the Bureau of Standards may be expressed ia terms of the following fixed points. On the basis of the true thermodynamic scale these standard temperatures are known to an accuracy of possibly 0.5° at 500°C., and 3° at 1,200°C. On the basis of the platinum resistance thermometer scale defined as above, the temperatures below 1,000°C. can be determined with possibly 10 times this precision. The accuracy with which the platinum point is known on the thermodynamic scale is probably 10°C., and the accuracy of the tungsten point may be estimated as 50°C. 

Platinum resistance thermometers 1. Good linearity 2. Wide temperature range 3. Very good stability even at high temperature 4. Very good precision and accuracy 1. Slow response 2. Vibration and shock fragility 3. High cost... 

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 ITS-90 extends upward from 0.65 K, and temperatures on this scale are in much better agreement with thermodynamic values that are those on the IPTS-68 and the EPT-76. The new scale has subranges and alternative definitions in certain ranges that greatly facilitate its use. Furthermore, its continuity, precision, and reproducibility throughout its ranges are much improved over that of the present scales. The replacement of the thermocouple with the platinum resistance thermometer at temperatures below 961.78°C resulted in the biggest improvement in reproducibility. 

Platinum resistance thermometers - can be used over an extensive temperature range, typically 14-750 K. They are capable of a reproducibility better than 0.001 K, which is better than the accuracy with which the thermodynamic scale has been established. They have been used to measure temperature differences at temperatures close to 273 K with a precision of about 10" K. To achieve such reproducibility and precision the resistor must be of pure platinum and be mounted in a strain-free condition. Resistivity changes can occur when... 

Platinum resistance thermometers monitor the temperature, and a three-term tunable PID controller, which is RS232-linked to a PC, stabilizes the v ue at the set point to an accuracy of at least 0.1 °C by regulating the secondary heat transfer. The set point temperature can be prograimned, so that precise thermal cycling is available, or manually entered for step variation. The woridng temperature range is 5 C with a thermal response 0.2°C/s. 

The internationally agreed upon secondary temperature scale is described in Fig. 3.2. It is called the International Temperature Scale of 1990, in short — ITS 1990. It is based on the fact that secondary thermometers that must be calibrated, such as the platinum resistance thermometer are capable of higher precision than absolute thermometers. To make use of this higher precision, a number of fixed points have been agreed upon internationally. They are... 

The best-known resistance thermometer is the platinum resistance thermometer, mentioned in Fig. 3.2 as the instrument for the maintenance of the ITS 90. As a typical metal, its resistivity increases approximately linearly. Over a wide temperature the change is 0.4% of the resistance per degree. In order to make a platinum resistance thermometer, a wire is wound non-inductively, so that the total thermometer has a resistance of 25.5 n at 273.15 K. Under this condition, the resistance of the thermometer will change by about 0.1 n/K. The precision that has been achieved with platinum resistance thermometers is 0.04 K at 530 K and 0.0001 K at 273.15 K, and decreases to 0.1 K at 1700 K. Similar resistance thermometers have been built out of nickel, phosphor-bronze and copper. 

The heat losses as a function of the adiabatic deviation, Ta - 7 b. was measured (upper left in the diagram) and calibrated for each temperature. The sample temperature was determined with the platinum resistance thermometer, using a precision galvanometer and a Wheatstone bridge, indicated at the top right. The heat input into the sample, was... 

The sample temperature is measured by a miniaturized sensor, such as a thermocouple or a platinum resistance thermometer. The sensor voltage is then compared to a voltage set by the experimenter and an error voltage is used to drive current through the heater. For the best precision, three term control is used in which power is reduced as the set temperature is approached and optimum heating is provided to settle on the set temperature. 

For temperatures above about 20 K, the metallic resistance thermometers are more sensitive than the nonmetallic resistance thermometers. Temperatures above 20 K can be measured routinely with an industrial-type platinum resistance thermometer with an accuracy of better than 100 mK with time responses somewhat better than 1 s. Accuracy at the millidegree level requires a precision capsule type platinum resistance thermometer and careful calibration. 

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