Thermometers defining standard

Primary thermometer Secondary thermometer Defining standard thermometer... 

Well-designed low-pressure gas thermometers can be used to determine (really approximate) the thermodynamic temperature. However, from a practical standpoint, where precision and simplicity in the implementation and transfer are the major considerations, secondary thermometers were chosen as the defining standard thermometers for a practical temperature scale. This scale was defined by the use of fixed reference points whose thermodynamic temperatures were determined from gas thermometer measurements. The International Committee of Weights and Measures (Comite International des Poids et Mesures, CIPM) is responsible for developing and maintaining the scale. 

National standard laboratories, such as the National Institute of Standards and Technology (NIST) in the United States, implement and maintain the practical temperature scale for their respective countries. They also help in the transfer of the scale by calibrating the defining standard thermometers These defining standard thermometers are costly to maintain and are primarily used in temperature calibration laboratories in industry or universities They are directly or indirectly used for calibration of thermometers used in actual applications. 

A thermometer used in the definition of ITS-90 calibration performed at defining fixed points or against another defining standard. 

An intermediate standard used to minimize the use and drift of a defining standard thermometer. 

A standard used in calibration that is the same type of thermometer as the thermometers to be calibrated. Calibration against a defining standard thermometer or a transfer standard thermometer is required at periodic intervals to ensure accuracy. 

As mentioned above, a special PRT that satisfies the specifications of ITS-90 is the defining standard thermometer (called the standard platinum resistance thermometer, SPRT) for the range of temperatures from 13.80 to 1234.93 K (961.78°C). According to ITS-90 [2], an acceptable SPRT must be made from sufficiently pure and strain-free platinum wire, and it must satisfy at least one of the following two relations ... 

Within its specified temperature range of operation, an SPRT has the best precision (used here interchangeably with repeatability—the ability to reproduce the same reading for the same conditions) as a temperature sensor. Thus, it is used as the defining standard thermometer for ITS-90. The precision of the best SPRT around room temperature is of the order of 0.01 mK. The ability of an SPRT to realize ITS-90 temperature (its accuracy) at the calibration points (defining fixed points) can be better than 1 mK. At temperatures other than the calibration points, there is an additional error due to the interpolation process this is also of the order of 1 mK under the best conditions. 

Transfer or working-standard thermocouples (including connecting wires—see Fig. 16.17) are individually calibrated by comparison calibration against a defining standard thermometer (such as an SPRT) or another transfer standard thermometer (usually a thermocouple). 

The type-S thermocouple, though no longer used as a defining standard for ITS-90, is still a reasonably accurate transfer standard thermometer. The precision of a type-S thermocouple at temperatures between 600 to 1000°C is about 0.02°C, and its accuracy is about 0.2 to 0.3°C. At lower temperatures (between about 0 and 200°C), a base-metal-type thermocouple (e.g., type T) is capable of a precision of about 0.01°C and an accuracy of 0.1°C. 

The defining standard thermometers are thermometers specified for ITS-90. They should be calibrated by NIST at regular time intervals, and are then used to calibrate transfer or working standard thermometers. Transfer standard thermometers serve as intermediate standards to reduce the use and drift of the defining standard thermometers. 

The temperature of exposed samples is dependent on both the air temperature in the cabinet and the absorbance of direct radiation. Temperature is usually measured with a black panel thermometer, which gives the surface temperature of a perfectly absorbing material. White panel thermometers are also commonly used which measure the other extreme. The actual temperature reached by a test piece depends on the material and its colour. It will also depend on the air temperature and velocity so that both the air and black panel temperatures should be controlled. ISO 11403-3 [23] defines three sets of conditions in air with the black standard temperature at 65 °C (ISO 4892-2 Method A [27]), behind glass at the same temperature (ISO 4892-2 Method B [27]), and behind glass at 100 °C. 

Lower Critical Solution Temperatures LCSTs were determined from plots of optical density at 600 nm versus temperature for 0.03% solutions of each polymer in PBS and were defined as the temperature at which Asoo = 0.1. Temperatures were raised at less than 0.3 C per minute and were measured with a thermometer that had been calibrated against an NBS primary standard thermometer. LCSTs for Figure 6 were determined from the cloud points of 0.01% solutions. 

The standard instrument used from —259.34 to 630.74°C is the platinum-resistance thermometer, and from 630.74 to 1064.43°C the platinum-10 percent rhodium/ platinum thermocouple is used. Above 1064.43°C the temperature is defined by Planck s radiation law. 

The International Practical Temperature Scale of 1968 (IPTS-68) has been replaced by the International Temperature Scale of 1990 (ITS-90). The ITS-90 scale is basically arbitraiy in its definition but is intended to approximate closely the thermodynamic temperature scale. It is based on assigned values of the temperatures of a number of defining fixed points and on interpolation formulas for standard instruments (practical thermometers) that have been cahbrated at those fixed points. The fixed points of ITS-90 are given in Table 1. 

The ITS-90 scale extends from 0.65 K to the highest temperature measurable with the Planck radiation law (—6000 K). Several defining ranges and subranges are used, and some of these overlap. Below —25 K, the measurements are based on vapor pressure or gas thermometry. Between 13.8 K and 1235 K, Tg is determined with a platinum resistance thermometer, and this is by far the most important standard thermometer used in physical chemistry. Above 1235 K, an optical pyrometer is the standard measrrremerrt instmment. The procedtrres used for different ranges are sttmmarized below. 

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. 

Using the following random numbers from 0 to 1, simulate the lifetime (time to thermometer failure) of the temperature recording system and estimate its mean and standard deviation. The lifetime is defined as the time (in weeks) for one of the thermometers to fail. ... 

The profiles of the time-averaged wind velocity U(z), temperatures on dry T(z) and wet Tw(z) thermometers both inside the drop layer 0 < z < h and over it up to 10 m height were obtained in 12 measurement series. Some results typical for windy weather can be seen in Fig. 1.14. The relative humidity

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Primary standards are those developed and maintained by national standards laboratories such as the National Bureau of Standards. These laboratories develop, maintain, and disseminate standards, such as the International Practical Temperature Scale. The IPTS-68 is disseminated to the users through secondary standards such as calibrated thermometers, fixed point references, and so on (see Table II). Some of these thermometers are calibrated directly against the defining fixed points and others are calibrated over the range of need by a comparison calibration against a standard interpolating thermometer. This ensures that the basis for temperature measurement, the IPTS-68, is the same everywhere throughout the world. 

Whatever thermometer is used, in either routine or in standardization applications, it must be calibrated periodically against a primary standard, or at least its calibration must be checked periodically at some temperature fixed points (defining fixed points or well-characterized secondary fixed points) in the range of the thermometer. 

From the zeroth law of thermodynamics, we know that two systems that are in thermal equilibrium with a third system are in thermal equilibrium with one another and, by definition, have the same temperature. The zeroth law is not only important in defining systems that have the same temperature, but it also provides the basic principle behind thermometry one measures temperatures of different systems by thermometers that are, in turn, compared to some standard temperature systems or standard thermometers. 

Thus far, we have related the measurement of uncertainty to the resolution—the scale divisions on a thermometer, the graduation marks on a beaker. It may also be defined by an engineer or a scientist with a significant amount of experience. In circumstances where measurements of the mean and standard deviation have been made, it is often simply expressed as the standard deviation a fi a. 

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