Measuring and testing – Gas analysis – By thermal property
Reexamination Certificate
2001-03-13
2002-05-21
Williams, Hezron (Department: 2856)
Measuring and testing
Gas analysis
By thermal property
C436S147000
Reexamination Certificate
active
06389880
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to sensors and systems for measuring the concentration of oxygen present in a mixture of gases, wherein the sensor relies upon detection of thermal effects due to a wind formed in a magnetic field within the measuring device as a result of the paramagnetism of oxygen present in the mixture. A variety of such devices have been described for measuring oxygen concentration, and these magnetic wind oxygen sensing systems rely on the fact that when placed in a magnetic field, the paramagnetic oxygen will exert a pressure in a certain direction, while the remaining components of the mixture, which is substantially formed of diamagnetic gases, are unaffected by the local field. A mixture of oxygen with diamagnetic gases behaves, from the magnetic point of view, as a single component with a magnetic property (magnetic susceptibility) equal to the weighted average of the susceptibilities of each of the components.
One type of apparatus of this sort measures the concentration of oxygen by relying upon the inverse relationship between temperature and the magnetic susceptibility of oxygen, and provides a heater to raise the temperature of a portion of an oxygen-containing mixture in a local region of a magnetic field, producing a pressure differential that gives rise to a directional wind. The wind has a magnitude that depends on the local field, the local temperature differential or thermal gradient, and the level of oxygen. By arranging heating and heat sensing elements in close proximity so that movement of the wind carries heat or selectively cools one or another of the components, the magnitude of this wind, or its functional relationship to the oxygen concentration, may be calibrated.
A number of prior publications and patents describing instruments which exploit one or more of these effects are referenced in applicant's earlier U.S. Pat. No. 5,269,170 issued Dec. 14, 1993; U.S. Pat. No. 5,012,669 issued May 7, 1991; and U.S. Pat. No. 4,893,495 issued Jan. 16, 1990. These patents are hereby incorporated herein by reference in their entirety, and attention is particularly directed to their circuit diagrams illustrating sensing bridge and temperature or current control arrangements for balancing the bridge. Among the complicating factors which must be corrected are the problem of defining a layout or geometry such that the so-called chimney effect, the natural directional flow induced by the lesser density of heated gas, does not confound the response of the thermal elements, and the problem of compensating for the rate of cooling or heating due to specific heat or heat capacity of the background gases present in addition to the measurand. In the foregoing patents a number of constructions are proposed for addressing these factors. On a circuit level these may include the use of multiple heating or sensing elements arranged in bridges to balance or counterbalance certain effects that enjoy symmetry as a result of their spatial layout. Another useful technique involves electrically heating a portion of the assembly to a constant temperature and monitoring the current required to maintain that temperature. This current may then be used to develop a normalizing measurement to which other parameters are referenced.
However, one basic limitation of this technology resides in the fact that small heated sensing elements are employed to detect the wind. The temperature of these elements is affected not only by the magnitude of the magnetic wind induced by the heating and magnetic field structures, but also by the heat transfer characteristics of the background gases that are present. A change in background gases thus induces a shift in zero point (i.e., the output when oxygen concentration is zero) of the sensing circuitry.
Furthermore, while constructions as illustrated in the aforesaid patents have enhanced the accuracy of paramagnetic oxygen sensing systems, they rely on the use of multiple elements in bridge configurations. This typically requires that the response and characterizing parameters of the elements be quite similar, i.e., that the components be matched. Some matching of the circuit characteristics of components is generally feasible, and may initially be performed quite accurately, especially for certain thin film devices wherein hundreds or thousands of virtually identical units are fabricated in a single process on a single wafer. However, initial matching of the basic response characteristics may be insufficient to assure continued accuracy. As a practical matter, when discrete sensors such as thermistors or resistive heating elements are used, the very process of mounting and arranging their geometry within the sensing device may introduce asymmetries of response, or instabilities of location that result in asymmetries of response over time. For example, when a thermistor is mounted close to the wall of a massive magnet structure, the rate of cooling due to gas conduction between the thermistor and the wall will vary with the composition of the background gas and its thermal capacity. Further, for a given background gas, such conductive dissipation is markedly affected by even small changes in proximity to the wall, which may introduce disproportionately large conductive or radiative heat loss, or with an opposite effect, may give rise to boundary layer flow stagnation.
Various approaches have been presented in the prior art to address the dependence on background gas thermal characteristics. For example, the above-cited patents teach a method and circuitry for maintaining the bridge at a constant temperature, and carrying out adjustments to compensate for background gas effects. However, such bridge circuitry may augment the variations induced by background gas changes, and the correction circuitry may not fully correct for these background-induced variations. Moreover, facially identical components in a bridge may respond differently to the same drive current when placed in series, because their thermal dissipation characteristics are not well matched. Mismatch may occur either intrinsically in the response of the circuit elements, or because one unit of a pair is positioned fifty or a hundred micrometers differently with respect to nearby structures. The small heated elements are also inherently subject to thermal stresses and temperature cycling, causing wires to shift and local geometry to change over time, introducing asymmetric effects, such as those just described, even in sets of initially well-positioned and well-matched components.
These effects can imbalance or impair the practical effectiveness of a bridge circuit, and may result in loss of calibration.
The problem is compounded because, since extremely minute levels of force are engendered by the action of a paramagnetic gas within the magnetic field, it is necessary that the heat sensing and generating elements be sufficiently small to make the effect of the induced wind detectable. It is further desirable that the sensors be mounted in sufficiently small passages that high flux may be achieved and also that wind is effectively channeled to develop higher velocity. However, because the small thermal elements necessarily are mounted on small conductors, normal flexing, structural bending, vibration and thermal expansion effects result in migration or shifting of the actual position of the heating and sensing element. Thus their response to wind-induced thermal transfer, or the rate at which each dissipates heat, or the power required to maintain a constant temperature, resistance or signal in the element, will vary over time as well as changing with properties of the background gases. This is particularly true of constructions in which the elements are both heated to generate a wind, and also employed as sensing elements to respond to heat transferred to or from the heated element.
One approach to this problem might be to provide a strictly planar device incorporating otherwise conventional sensing bridge circuitry, i.e., to provide a small chip having
Iandiorio & Teska
Panametrics, Inc.
Politzer Jay L.
Williams Hezron
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