Shane, James H.
Sayir, Haluk
Shane, James H.
Sayir, Haluk
| 1. | A method for measuring temperature over a wide temperature range comprising the steps of: placing highly oriented pyrolytic graphite into a thermal environment within which a variation in temperature is to be measured, arranging the highly oriented pyrolytic graphite so that the orientation of its basal planes are aligned in a predetermined first direction, measuring the physical displacement of the highly oriented pyrolytic graphite material in a second direction transverse to the first direction in response to a change in temperature and converting to said change in temperature. |
| 2. | A method as defined in claim 1 wherein the physical displacement of the highly oriented pyrolytic graphite material is measured with a linear transducer or micrometer. |
| 3. | A temperature measuring device comprising: a hollow member composed of refractory material such as conventional graphite; highly oriented pyrolytic graphite disposed within said hollow member so that the orientation of its basal planes are aligned perpendicular to the longitudinal axis of the hollow member; and transducer means for responding to the expansion or contraction of the highly oriented pyrolytic graphite along the longitudinal axis of the hollow member. |
| 4. | A temperature measuring device as defined in claim 3 further comprising means responsive to said transducer means for providing measurement of temperature corresponding to the physical displacement of said highly oriented pyrolytic graphite. |
| 5. | A temperature measuring device as defined in claim 4 wherein said refractory hollow member is in the form of a tube having a closed and an open end with said highly oriented pyrolytic graphite disposed within said hollow member adjacent the closed end thereof and further comprising a refractory rod disposed in said hollow member in engagement between said highly oriented pyrolytic graphite and said transducer means. |
| 6. | A temperature measuring device as defined in claim 5 wherein said highly oriented pyrolytic graphite is in form of a stack comprising a plurality of disk shaped members arranged in tandem with each disk shaped member having a diameter substantially equal to the diameter of said hollow graphite tube. |
| 7. | A temperature measuring device as defined in claim 6 wherein the height of said stack of highly oriented pyrolytic graphite is tailored to provide a desired response characteristic of thermal expansion with change in temperature. |
| 8. | A temperature measuring device as defined in claim 7 wherein said device has a linear response characteristic of displacement with change in temperature. |
| 9. | 8 SUBSTTTUTE SHEET (RULE 26). |
FIELD OF INVENTION
The present invention relates to thermometric devices and methods and more particularly to a temperature measuring device and method for measuring temperature from as low as -200 °c up to 3000 °c with uniform accuracy over the entire temperature range.
BACKGROUND OF THE INVENTION
Current commercially available temperature measuring devices employ an electronic junction e.g. a thermocouple or an optical pyrometer to measure temperature change. Thermocouples are widely used to measure temperatures below 1700 °c. The drawback of the thermocouple is its inability to survive a temperature above the melting temperature of the junction materials. Therefore the high temperature end of the thermocouple is limited to certain temperature ranges and conditions. Moreover the thermocouple is not accurate at cold temperature and is limited in general to higher temperature measurement. Typical junction materials include Platinum and Rhodium. Some special alloys can be used as the junction material of a thermocouple to make somewhat higher temperature measurements but generally only under controlled environmental conditions and applications. Thermocouples are also susceptible to chemical reaction and composition change.
An optical pyrometer measures temperature by optically sensing the emitted waves at the hot surface which are converted into an electronic signal representative of a temperature measurement. The main
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SUBSTTTUTE SHEET (RULE 26)
problem with pyrometers and similar devices which measure temperature optically are their dependency on the emissivity of the material and the optical sensitivity of the optical device. Every material has its own emissivity characteristics and as the temperature changes the emissivity of the material changes. For accurate readings the characteristics need to be tuned into the system during the readout of the temperature measurement. Accordingly, optical pyrometers are susceptible to errors in temperature reading which can be as much as about 20 °c at temperatures over 1500 °c even under otherwise perfect conditions. Moreover, since pyrometers are optical devices, spot size, focus, optical axis, target surface, etc., complicate the measurement in addition to having to calibrate the pyrometer to account for the emissivity of the material and the change in resolution as the temperature increases or decreases.
SUMMARY OF THE INVENTION
The temperature measuring device of the present invention is based upon the use of highly oriented pyrolytic graphite (hereafter "HOPG") as the active temperature sensitive material for measuring temperature changes over a wide range of temperatures. It has been discovered in accordance with the present invention that the thermal expansion of highly oriented pyrolytic graphite is directly proportional to changes in temperature when measured in a specific direction, viz., along the "c axis" which for purposes of the present invention is in a direction perpendicular to the carbon layers or basal planes of the material. Highly oriented pyrolytic graphite has a high coefficient of thermal expansion of between 20 to 30x 10 '6 per °c. Detection of this expansion can be readily converted to a scaled temperature reading for high accuracy temperature measurement over a wide temperature range of e.g. between -200 °C to about 3000 °c. Moreover the maximum error in
temperature is less than 5 °c even at temperature readings well above 1500 °C.
The method of the present invention comprises the steps of: placing highly oriented pyrolytic graphite in an environment within which a variation in temperature is to be measured, arranging the highly oriented pyrolytic graphite in said environment so that the orientation of its basal planes are aligned in a first direction, measuring the physical displacement of the material in a second direction transverse to the first direction and converting this measurement into a scaled temperature reading corresponding to the variation in temperature.
The temperature measuring device of the present invention comprises: a hollow member composed of a refractory such as conventional graphite; highly oriented pyrolytic graphite disposed within said hollow member; and transducer means responsive to the expansion or contraction of the highly oriented pyrolytic graphite along the longitudinal axis of the hollow member for providing a scaled temperature reading responsive to temperature changes in the highly oriented pyrolytic graphite.
BRIEF DESCRIPTION OF THE DRAWINGS
Other advantages of the present invention will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings of which:
Fig. 1 is a diagrammatic view in cross section of the preferred embodiment of the temperature measuring instrument of the present invention;
SUBSTITUTE SHEET (RULE 26
Fig. 2 is a graph of physical displacement versus temperature for the temperature measuring instrument of the present invention over a temperature range from -200 °c to 500°C;
Fig. 3 is a graph similar to Figure 3 for HOPG disks over a temperature range from 800°c to 2200°C; and
Fig. 4 is a graph of the profile of the thermal expansion vs. temperature characteristic of the highly oriented pyrolytic graphite of the present invention measured by a NASA facility to verify its essentially linear characteristic.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The temperature measurement instrument illustrated in Figure 1 comprises a graphite tube 10 closed at one end 1 1 and filled with the highly oriented pyrolytic graphite preferably formed into a plurality of discrete wafer like disks 14 stacked in tandem adjacent the closed end of the tube 10. "Highly oriented pyrolytic graphite" (HOPG) for purposes of the present invention shall mean pyrolytic graphite which has been annealed at high temperature of substantially equal to or above 3000 °c. Graphite is made up of Iayer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonal arranged carbon atoms are substantially flat and are oriented so as to be substantially parallel and equidistant to one another. The substantially flat parallel layers of carbon atoms are referred to as basal planes and are linked or bonded together in groups arranged in crystallites. Conventional graphite has a random order to the crystallites. Highly ordered graphite has a high degree of preferred crystallite orientation. Accordingly, graphite may be characterized as laminated structures of carbon having two principal axes, to wit, the "c" axes which is generally identified as the axes or
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direction perpendicular to the carbon layers and the "a" axes or direction parallel to the carbon layers and transverse to the c axes. Pyrolytic graphite is essentially highly oriented polycrystalline graphite produced by high temperature pyrolysis. Briefly, the pyrolytic deposition process may be carried out in a heated furnace heated to above 1500 °C and up to 2500 °c and at a suitable pressure, wherein a hydrocarbon gas such as methane, natural gas, acetylene etc. is introduced into the heated furnace and is thermally decomposed at the surface of a substrate of suitable composition such as graphite having any desirable shape. The substrate may be removed or separated from the pyrolytic graphite . The pyrolytic graphite is then subjected to thermal annealing at high temperatures to form the highly oriented pyrolytic graphite "HOPG" material of the present invention.
In order to facilitate proper orientation of the HOPG material relative to the direction of its basal planes the highly oriented pyrolytic graphite HOPG material 12 is formed into wafer like disks 14 each having an outside diameter substantially equal to the diameter of the tube 10. This may be done simply by drilling out thin wafer like disks 14 of appropriate diameter from a sheet of CVD formed HOPG material. The disks 14 as shown in Figure 1 are arranged in tandem to form a stack positioned inside the tube 10 adjacent the closed end 11 thereof with the basal planes of the disks 14 aligned perpendicular to the longitudinal axis of the tube 10. A refractory bar 16 preferably of cylindrical geometry is inserted into the tube 10 engaging the disks 14. Both the bar 16 and tube 10 should be made of the refractory material to minimize distinguishing the CTE displacement of HOPG. A material such as conventional graphite has been used. A measuring device 20 is placed in contact with the rod 16 for measuring the physical displacement of the rod 16 in response to changes in temperature. The measuring device 20 may be conventional transducer such as a LVDT or a micrometer. The LVDT shown in figure 1 is a commercially available device which has a
spring assembly 23 extending from the device 20 to which a push rod 24 is connected. The push rod 24 is mounted to engage graphite rod 16. An indicator 25 provides a readout of the physical displacement of the transducer or a scaled temperature reading corresponding to the variation in temperature of the device. As shown in Figures 2 and 3 the thermal expansion characteristic for a stack of HOPG disks 14 in the "c" direction is essentially linear over a substantial temperature range extending as low -200°c to 2200°c and should be capable of measurement up to 3000 °C. The linearity of the HOPG material as is shown in Figure 4 was substantiated by NASA at its Lewis Research Center in Cleveland Ohio. Although its linearity has been substantiated this is not an essential or critical characteristic of the present invention. Instead its high CTE and high level of repeatability over an extended range is more significant even if it were proven that in a particular temperature region of interest its characteristic was not truly linear. The stack height i.e. the thickness of the combined HOPG disks 14 may be varied to tailor the expansion characteristic of the device 20 to improve accuracy for a particular application. This is demonstrated in Figure 3 showing two different size stacks of HOPG disks 14 with the 121.6 mm stack having a much larger slope than that of the 68.62 mm stack, the resolution of the device is a function of the stack height.
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