Thermometer, accuracy

In use, a mantle of ice is frozen onto the outer surface of the thermometer weU. A common way to do this is to fiU the weU with cmshed dry ice until the mantle achieves a good thickness. Descriptions of the technique for doing this are given in several pubHcations and in manufacturers Hterature. The temperature of the water triple point is 0.01°C, or 273.16 K, by definition. In practice, that temperature can be realized in the ceU within 0.00015 K of the definition. In contrast, a bath of ice and water for producing the temperature 0°C is difficult to estabHsh with an accuracy better than 0.002°C. 

The SPRTs are devices of superb accuracy and resolution, but they are fragile and can easily be broken. They can also be put out of caUbration by strain, iaduced by even slight mechanical shock or vibration. The principal use of SPRTs ia scieace and iadustry is to maintain the cahbrations of working thermometers. 

Industrial resistance thermometers are also the subject of a number of national and international standards, which describe both cahbration constants and classes of accuracy and interchangeabihty. lEC pubhcation 751 was revised in 1976 to conform to ITS-90, and national standards will be revised to conform to this document. lEC 751 uses the fixed-poiat values of ITS-90 with the simpler algorithm of IPTS-48 ... 

As normally used in the process industries, the sensitivity and percentage of span accuracy of these thermometers are generally the equal of those of other temperature-measuring instruments. Sensitivity and absolute accuracy are not the equal of those of short-span electrical instruments used in connection with resistance-thermometer bulbs. Also, the maximum temperature is somewhat limited. 

Thermocouples tend to be rehable, but dial thermometers may need to be puUed and verified for accuracy. 

The resistance must be measured with extreme care and accuracy, since a small error in measuring the resistance will cause a much larger error in determining the temperature rise. When the temperature of the winding is to be determined by the resistance, the temperature of the winding before the test, measured either by thermometer or by ETD, may be considered as the cold temperature for the resistance measured. The machine must be left cold for at least 12 to 24 hours, depending upon the size of the machine, to obtain a stable reading. 

Type of thermometer Temperature range ( C) Accuracy Speed of response... 

Liquid-in-glass thermometers measure the thermal expansion of a liquid, which is placed in a solid container, on a length scale. The mercury thermometer is one example of liquid thermometers. Alcohol is also used with this type of instrument. The temperature range is -80 to a-330 °C depending on the liquid. The quality, stability, and accuracy vary considerably. The advantages are a simple construction and low price. A disadvantage is that they are not compatible for connection to monitoring systems. 

In filled thermometers the thermal expansion of a gas or a liquid is transmitted through a thin capillary tube to a bellows or helix, where the deformation indicates the temperature. The temperature range of filled thermometers is very wide, approximately -200 to +700 °C. They are extremely robust but are not very high in accuracy. The application is mainly for process instrumentation and as stand-alone control devices. 

The measurement range is dependent on the instrument but can cover the range -50 to +500 °C. The accuracy is not as high as the best contact thermometers. One reason for this is that the emissivity of the surface has an effect on the measurement result, and an emissivity correction is necessary for most instruments. The positive features are noncontact measurement and very fast dynamics, which enable a rapid scan of surface temperatures from a distance this is convenient when carrying out, for example, thermal comfort measurements. 

Rodebush has also implied that the accuracy with which very low temperatures can be measured is restricted by the uncertainty principle and by the nature of the substance under investigation. However, the accuracy of a temperature measurement is not limited in a serious way by the uncertainty principle for energy, inasmuch as the relation between the uncertainty in temperature and the length of time involved in the measurement depends on the size of the thermometer, and the uncertainty in temperature can be made arbitrarily small by sufficiently increasing the size of the thermometer we assume as the temperature of the substance the temperature of the surrounding thermostat with which it is in either stable or metastable equilibrium, provided that thermal equilibrium effective for the time of the investigation is reached. 

An instrument for measuring temperatures, in the rubber industry the term is usually applied to an instrument for determining the surface temperature of mill and calender rolls, moulds, etc. The instrument is usually based on thermocouples or, where higher accuracy is required, platinum resistance thermometers. Infrared (IR) techniques are now used which have the advantage of non contact but require careful calibration for the emissivity of the surface. 

It is worth remarking that, in fact, such devices do not supply true reference fixed points as explained in Section 8.2, since these devices were calibrated one by one against primary thermometers. Their accuracy is 0.1-0.2 mK. 

The most interesting liquids for low-temperature thermometry are 3He and 4He, especially for the calibration of resistance thermometers in the range from 0.5 to 4.2 K. Vapour pressure of H2 is also interesting to realize vapour pressure-fixed points included in ITS-90. The measure of He vapour pressure has been carried out with great accuracy [42,43] to establish the ITS-90 (see Section 8.3). There are several experimental precautions to be observed in order to obtain reliable measurements [2],... 

The contact resistance may change when the thermometer is moved from a position to another. Hence the accuracy of resistance temperature measurements below about 25 mK... 

The CBT is in principle a primary thermometer, but its absolute accuracy is at present too low for metrological applications [115]. 

For measurement of the oaks we used, per force, a mass spectrometer of somewhat low accuracy, and achieved the accuracy to demonstrate that trees are thermometers by making many measurements on each sample. On the tree sequences which we measured later, we used high precision spectrometers with accuracies of 0.1 parts per thousand (ppt) for 180/160 and 13C/12C, and 2 ppt for D/H. The measurements are expressed in terms of 8D and 6 g. 

Temperature Recording. Use an accurate temperature-sensing device, such as a clinical thermometer or thermistor or similar probe, that has been calibrated to ensure an accuracy of 0.1° and has been tested to determine that a maximum reading is reached in less than 5 min. Insert the temperature-sensing probe into the rectum of the test rabbit to a depth of not less than 7.5 cm and, after a period of time not less than that previously determined as sufficient, record the rabbit s temperature. 

This latter point begs the question Who can verify the accuracy of a reference point if its value may vary In other words, who is the ultimate source of calibration materials, such as a thermometer In the U.S., it is the National Institute of Standards and Technology (NIST). This is the same organization that we cited as the source of accurate standardization materials in Chapters 3 and 4. Experiments 16 and 17 in this chapter are exercises in the calibration of a temperature sensor and how such a calibrated sensor can be used. 

Viscosity depends on temperature. The higher the temperature, the lower the viscosity Pancake syrup, for example, flows more freely when heated. For reasonable accuracy when measuring viscosity, the temperature must be very carefully controlled. This means that the viscometer and sample must be immersed in a constant temperature bath and the temperature given time to equilibrate before the measurement is recorded. A calibrated thermometer must be used to measure the temperature. 

The historical development of titration calorimetry has been addressed by Grime [197]. The technique is credited to have been born in 1913, when Bell and Cowell used an apparatus consisting of a 200 cm3 Dewar vessel, a platinum stirrer, a thermometer graduated to tenths of degrees, and a volumetric burette to determine the end point of the titration of citric acid with ammonia lfom a plot of the observed temperature change against the volume of ammonia added [208]. The capabilities of titration calorimetry have enormously evolved since then, and the accuracy limits of modern titration calorimeters are comparable to those obtained in conventional isoperibol (chapter 8) or heat-flow instruments (chapter 9) [195,198],... 

Resistance temperature detector. In the low-medium temperature range, thermometers based on resistometry are often used. A reference high-accuracy thermometer is the platinum resistance temperature detector which may be used as an interpolation standard in the temperature range from the triple point... 

To test for temperature accuracy of a column oven, one should place the temperature probe/sensor of a calibrated/verified electronic thermometer (with at least 0.5°C precision) into the oven. The probe/sensor should not make contact with anything inside the oven. With the oven door closed, allow the temperature to stabilize for at least 20 min at each tested temperature (e.g., 30, 45 and 60°C). The thermometer s temperature should then be recorded at each temperature. The difference between the actual and set temperature should typically be within 1°C. 

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