Thermal measuring and testing – Thermal calibration system
Patent
1996-01-17
1998-01-13
Chilcot, Richard
Thermal measuring and testing
Thermal calibration system
374121, G01K 1500
Patent
active
057071462
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method and the apparatus for the implementation thereof for calibrating thermoelements in the temperature range from room temperature to ca. 2800.degree. C. with a preferred working range of about 700.degree.-1800.degree. C. according to the characterizing part of Claim 1.
The standard calibration methods previously used for industrial purposes are restricted to maximal temperatures up to about 1300.degree. C. and are characterized in that the thermoelements to be calibrated are heated homogeneously by contact in calibration furnaces, customarily by resistance heating. The term "homogeneously" signifies that the thermoelement must be heated not at its tip but over a region of about 200 mm; however, ordinarily that is a region in which during actual operation a temperature gradient is present. A special calibration method, suitable primarily for laboratory use, employs the melting of materials, as a rule in the form of wires, with accurately known melting point (Fritz Lieneweg: Handbook of Technical Temperature Measuring, Vieweg 1976). Another calibration method is known in which temperatures up to about 2000.degree. C. are determined by measuring the thermal noise resulting from thermal excitation of a comparison element kept at the same temperature level (H. Brixy: Combined Thermoelement-Noise Temperature Measuring, Nuclear Research Plant, Julich Gmbh; Institute for Reactor Design, Research Report '86 Jul-2051). However, this method requires extremely elaborate and cost-intensive signal processing.
The disadvantage of this state of the art is that because of the large heated region errors of the first and second kind cannot be detected in these regions. The method requires heating of a large region in order to ensure that the tip of the thermoelement is brought fully to the furnace temperature and does not remain at a -.DELTA.T due to effects of thermal conductance byway of the wires. Errors of the first kind derive primarily from diffusion processes and alloy-altering effects that change the composition of the material and hence its thermoelectric properties. Errors of the second kind are produced when thermoelements, the material of which has been altered so as to produce errors of the first kind, are employed in the range of other temperature gradients.
The object of the invention is thus to provide a thermoelement calibration method and the associated apparatus such that errors of the first and second kind can be demonstrated quantitatively with reference to a predetermined temperature gradient and calibrations can be carried out at temperatures up to at least 1800.degree. C.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of an apparatus, according to the present invention, for calibrating thermoelements; and
FIG. 2 is a schematic view similar to FIG. 1 of another embodiment of the present invention.
This object is achieved in accordance with the invention as follows: IR radiation emitted by a modulated (e.g. alternating-current operation) light source 1, preferably a halogen point radiator, can be adjusted to specific temperature levels in a simple manner by means of dimmer 2. The modulation of the light source allows the radiation emitted by the thermoelement tip 6 to be separated from the reflected part of the radiation from the light source. The IR radiation from the point radiator 1 is bundled by means of a reflector 3 in such a way that, depending on the configuration selected, parallel light is focussed by way of a lens system& (FIG. 1) or a parabolic reflector 5 (FIG. 2). In both embodiments the radiation is focussed onto the thermoelement tip 6. With this kind of heating only the tip 6 of the thermoelement 7 is heated to the desired calibration temperature. In this process a positioning device 12, preferably made of Al.sub.2 O.sub.3, serves to fix the thermoelement tip 6 reproducibly at the focal point. Because it is necessary that the emissivity be known exactly, in accordance with the invention at least 2 pyrometers are employed in such away that the fi
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Barber, et al., "Thermocouples above 600.degree.C: Infra-red thermometers and optical pyrometers", Measurement & Control, vol. 12, No. 5, May, 1979, pp. 207-213.
Fullin, et al., "A Centralized Temperature Control System for Diffusion Furnaces", Solid State Technology, Nov. 1972, pp. 40-43.
Schreck, et al., "Calibration of microsize thermocouples for measurements of Jan. surface temperature", Review of Scientific Instruments, vol. 64, No. 1, pp. 218-220, 1993, New York, U.S.
Bar Kai K. O.
Dienz Michael
Gaus Rainer
Amrozowicz Paul D.
Chilcot Richard
Industrieanlagen-Betriebsgesellschaft mbH
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