Thermal measuring and testing – Temperature measurement – In spaced noncontact relationship to specimen
Reexamination Certificate
1998-11-16
2001-03-20
Gutierrez, Diego (Department: 2859)
Thermal measuring and testing
Temperature measurement
In spaced noncontact relationship to specimen
C374S132000, C374S181000, C136S227000, C136S230000, C250S338100
Reexamination Certificate
active
06203194
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a thermopile sensor and in particular to a radiation thermometer or a motion detector with a thermopile sensor.
From U.S. Pat. No. 4,722,612 an infrared thermometer having thermopile devices is known. The thermopile device is mounted on a disk-shaped insulative sheet, with the hot junctions being arranged in the center of the sheet and being surrounded by the cold junctions in the form of a circle. The sheet is stretched across a thermally conductive substrate of toroidal shape, with the thermopile being located on the upper side, and the substrate on the underside, of the sheet. The hot junctions of the thermopile lie in the area of the central aperture of the toroidal substrate, whilst the cold junctions of the thermopile lie in that area of the sheet that is supported by the substrate. As a consequence, in the known thermopile the cold junctions are thermally coupled to the substrate much better than the hot junctions and hence have a thermal capacity exceeding that of the hot junctions by a multiple.
When the known infrared thermometer is used under variable ambient temperature conditions, a temperature difference occurs between the housing of the infrared thermometer and the thermopile sensor, causing the heat radiation of the thermometer housing to superimpose itself upon the heat radiation to be measured, corrupting the measurement result. In addition, a temperature difference then occurs also in the interior of the thermopile sensor because, naturally, first the housing of the thermopile sensor experiences a temperature variation due to thermal conduction and/or convection which then propagates inwardly to the thermopile. For example, when the housing of the thermopile sensor is heated by the housing of the infrared thermometer, the sensor housing heats the air in the interior of the thermopile sensor very rapidly, which however in turn heats only the hot junctions of the thermopile by convection. In the known thermopile sensor the cold junctions are strongly thermally coupled to the substrate which, by virtue of its high thermal capacity, is practically not heated by the air. Naturally, however, also the substrate and with it the cold junctions are heated, but this occurs comparatively slowly by heat conduction from the housing of the thermopile sensor. Therefore, the temperatures of the cold and hot junctions heat at different rates, that is, temperature gradients occur also within the thermopile, introducing errors in the temperature measurement process. To compensate for such errors, the known infrared thermometer includes two thermopile devices of like construction, whereof one is exposed to the radiation to be measured while the other is not, and which are connected in series opposition to each other.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a thermopile sensor which is insensitive to temperature gradients while yet being of simple construction. It is another object of the present invention to provide a radiation thermometer having a thermopile sensor which is insensitive to ambient temperature variations while equally affording ease of construction.
According to the present invention, these objects are solved in that the thermopile sensor is configured such that the cold and hot junctions of the thermopile heat or cool at the same rate on changes in ambient temperature. This is accomplished in that differences in the thermal capacity (C) between the cold (C
K
) and hot (C
K
) junctions disposed on a supporting structure are compensated for by corresponding differences in the thermal insulation (2) of the cold (&lgr;
K
) and the hot (&lgr;
W
) junctions relative to the housing, that is, the ratio of thermal capacity to thermal conductivity in the region of the hot junctions corresponds approximately to the ratio of thermal capacity to thermal conductivity in the region of the cold junctions; that is, C
K
/C
W
≈&lgr;
K
/&lgr;
W
. Therefore, a heat transfer from the sensor housing effects a uniform temperature variation of the cold and hot junctions, preventing temperature gradients from developing within the thermopile which are apt to corrupt the measurement process. The heat transfer between the housing and the thermopile takes place both by thermal conduction along the supporting structure of the thermopile and by convection and radiation. The thermal capacity of the hot and cold junctions is composed of a component determined by the mass and the materials of the respective junction on the one hand, and of a component related to the supporting structure on the other hand.
The thermopile sensor of the present invention has the advantage of obviating the necessity for a complex thermal stabilization by the coupling to isothermal bodies of high thermal capacity, nor does it require elaborate correcting devices in the form of, for instance, a second thermopile connected in series opposition to the first. As a result, this thermopile sensor enables very small and simple probe tips to be constructed, for instance for radiation thermometers for measuring a person's body temperature in the ear. The thermopile sensor of the present invention is also especially suitable for use in radiation thermometers without optical waveguide or with a short optical waveguide or one that is integrally formed with the thermometer housing.
Preferably, the quality of thermal insulation of the cold and hot junctions from the housing of the thermopile sensor is essentially equally good, the junctions exhibiting together with their supporting structure an essentially identical thermal capacity. By symmetrically arranging the cold and hot junctions in respect of the housing, the desired equality of thermal insulation results in a particularly simple manner.
In a preferred aspect of an embodiment of a thermopile sensor of the present invention, the supporting structure includes a membrane of low thermal conductivity with a preferably low thermal capacity, and a frame supporting the membrane. Disposed on the membrane are both the cold and the hot junctions, their location on the membrane being however in an area that is not in contact with the frame. The membrane thus also serves a thermal insulating function between the cold and hot junctions and the housing.
In the thermopile sensor of the present invention, the hot junctions are exposed to the radiation to be measured to a higher degree than the cold junctions, or vice versa. This may be accomplished, for instance, by a diaphragm shielding the cold junctions in the radiation path, or by providing the cold junctions with a heat reflective layer whilst the hot junctions are provided with a heat absorptive layer. In another aspect, the cold and hot junctions are asymmetrically arranged within the housing of the thermopile sensor so that the radiation incident on the sensor through a radiation entrance opening practically reaches only the hot junctions. Preferably, however, the cold and hot junctions are symmetrically arranged within the housing of the thermopile sensor, while the radiation entrance opening is asymmetrical.
A radiation thermometer of the present invention utilizes the thermopile sensor briefly described in the foregoing. The thermopile sensor is arranged in the radiation path of the radiation thermometer such that the hot junctions are exposed to the radiation to be measured to a greater degree than the cold junctions, or vice versa. In a preferred aspect of this embodiment, the radiation thermometer includes means shielding the cold junctions in the radiation path of the radiation thermometer. This may be accomplished by a diaphragm, for example. In other embodiments, the thermopile sensor is asymmetrically arranged in the radiation path of the radiation thermometer.
The invention will be explained in the following with reference to embodiments of thermopile sensors and an infrared thermometer illustrated in the accompanying drawing. Further embodiments and a motion detector are set forth in the description. In the drawing,
REF
Beerwerth Frank
Kraus Bernhard
Braun GmbH
Goodwin Jeanne-Marguerite
Gutierrez Diego
Hopgood, Calimafde Kalil & Judlowe, LLP
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