Abstract:
A sensor includes at least two microsensor chips in a housing. The chips may be arranged as (i) an absolute thermal conductivity sensor by exposing one chip to a sample fluid and the other chip to a sealed reference fluid, (ii) a differential thermal conductivity sensor by exposing the two chips to a sample fluid before and after it is modified, respectively, (iii) a one-axis rotation sensor by exposing both chips, positioned at an angle of 180° relative to one another, to a rotational flow in a toroidal chamber, (iv) a two or three axis rotation sensor by placing the two chips or three such chips on two or three orthogonal faces of a cube, (v) a one axis orientation/tilt/acceleration sensor by exposing the two chips to a fluid in a sealed toroidal chamber and by mounting the chips at an angle of substantially 90° relative to one another, (vi) a two axis orientation/tilt/acceleration sensor by placing the two chips at an angle of 90° relative to one another in a fluid filled chamber substantially without inertial flow, and (vii) a combined tilt/rotation sensor based on above (iii) by periodically adding and subtracting the signals from the two chips.

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention relates to the design and use of one type of thermal sensor building block for the creation of environmentally compensated sensor system embodiments of absolute and relative thermal conductivity, rotation and 1-to-3-axis tilt or acceleration. 
   BACKGROUND OF THE INVENTION 
   Thermal sensors that sense the thermal conductivity of a material such as a fluid have been used in a variety of applications including gas chromatography. 
   In gas chromatography, an unknown gas sample volume is injected into a carrier gas, the unknown gas is separated by the action of a separating column, and the separated sample gas components are transported by the carrier gas to and past a sensor, such as the thermal conductivity sensor, that senses changes in thermal conductivity inherent in the various components of the unknown gas. The thermal conductivity sensor responds to any component of the unknown gas whose thermal conductivity is different than that of the carrier gas. Helium is frequently used as the carrier gas because of its exceptionally high thermal conductivity. 
   The output of the thermal conductivity sensor peaks as each gas component passes by the thermal conductivity sensor and these peaks serve to identify each of the components of the gas by their elution timing and their concentration by their areas under the corresponding peaks. Thermal conductivity sensors available today for performing the above functions are bulky and expensive. 
   Present absolute thermal conductivity sensors that sense the thermal conductivity of fluids, such as gases, respond to changes in the chemical compositions of the fluids, which is generally the sensing objective. However, these present absolute thermal conductivity sensors also respond to changes in temperature, pressure, orientation, acceleration, vibration, rotation, and flow, which is generally an undesirable response of these absolute thermal conductivity sensors. 
   Flow disturbances, which result in an erroneously larger thermal conductivity being sensed, can be minimized by judicious sizing of the sensor housing in exchange for some loss in the speed of response. However reducing the undesirable influence of temperature, pressure, humidity, orientation, or rotation typically requires the use of additional temperature, pressure, and/or orientation sensors. 
   Moreover, differential thermal conductivity sensor assemblies have been used in Gas Chromatography systems where one thermal conductivity sensor is in contact with the outlet carrier gas stream carrying the separated components of an injected sample and the other thermal conductivity sensor is in contact with the inlet pure carrier gas stream. It is thought that, by placing the two thermal conductivity sensors in close proximity to one another, temperature differences experienced by the sensors are minimized and the sensors are exposed to the same flow rate. The sensors are typically oriented in the same direction. 
   However, the two thermal conductivity sensors in this system may be exposed to different, but steady pressures due to the pressure drop through the separation column of the gas chromatography system. Thus, such gas chromatography systems produce outputs that contain an undesired error component due to this pressure drop. Moreover, available differential thermal conductivity sensors used in these systems are typically manufactured in low volumes, with specially designed hot wire anemometers which result in the systems being very costly. 
   The present invention solves one or more of these or other problems. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention, an absolute thermal conductivity sensor comprises a housing and first and second microstructures. The housing has a channel exposed to a sample fluid whose absolute thermal conductivity is to be determined. The first microstructure sensor has a microheater and at least one temperature responsive microsensor, the first microstructure sensor is mounted to the housing so as to be exposed to the sample fluid in the channel. The second microstructure sensor has a microheater and at least one temperature responsive microsensor, the second microstructure sensor is mounted to the housing so as to be in contract only with a reference fluid and isolated from changes in composition of the sample fluid in the channel but not from common changes in temperature, pressure, orientation, and acceleration. 
   According to another aspect of the present invention, a rotation sensor comprises a housing and first and second microstructures. The housing has a a sealed axi-symmetric chamber containing a fluid and a rotation axis. The first microstructure sensor is mounted to the housing so as to be exposed to the fluid in the sealed chamber, and the first microstructure sensor includes a microheater and at least one temperature responsive microsensor. The second microstructure sensor is mounted to the housing so as to be exposed to the fluid in the sealed chamber, the second microstructure sensor includes a microheater and at least one temperature responsive microsensor, and the first and second microstructure sensors are mounted with respect to the axis so as sense a rotation of the housing about the axis, based on the sensors&#39; high sensitivity to flow of the fluid relative to the housing. 
   According to still another aspect of the present invention, a differential thermal conductivity sensor comprises a housing and first and second microstructures. The housing has a channel exposed to a fluid whose thermal conductivity is to be determined. The first microstructure sensor is mounted to the housing so as to be exposed to the fluid in the channel, and the first microstructure sensor includes a microheater and at least one temperature responsive microsensor. The second microstructure sensor is mounted to the housing so as to be exposed to the fluid in the channel, the second microstructure sensor includes a microheater and at least one temperature responsive microsensor, the two channels are precisely of the same shape, and the first and second microstructure sensors are mounted to the housing so that there is a minimum distance between the first and second microstructure sensors and the channel as well as a minimum dead volume around each individual sensor so as to maximize differential fluid property measurement sensitivity and time resolution, with least interference by uncontrolled differences in temperature, pressure, fluid velocity, orientation and acceleration. 
   Accordingly to a further aspect of the present invention, a differential thermal conductivity sensor comprises a housing and first and second sensors. The housing has a channel exposed to a unprocessed fluid whose thermal conductivity is to be determined before processing the fluid. The first sensor is mounted in a first housing so as to be exposed to the unprocessed fluid in a first part of a channel. The second sensor is mounted in a second, closely co-located housing so as to be exposed to the processed fluid in a second part of the same channel. 
   According to yet another aspect of the present invention, an orientation sensor comprises a housing and first and second microstructure sensors. The housing has a sealed chamber containing a fluid and a rotation axis. The first microstructure sensor is mounted to the housing so as to be exposed to the fluid in the sealed chamber, and the first microstructure sensor includes a microheater and at least one temperature responsive microsensor. The second microstructure sensor is mounted to the housing so as to be exposed to the fluid in the sealed chamber, the second microstructure sensor includes a microheater and at least one temperature responsive microsensor, and the first and second microstructure sensors are mounted at an angle of substantially 90° with respect to one another so as to sense two-axis orientation of the housing, or 180° with respect to each other so as to sense one-axis orientation of the housing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages will become more apparent from a detailed consideration of the invention when taken in conjunction with the drawings in which: 
       FIG. 1  illustrates an absolute thermal conductivity sensor according to a first embodiment of the present invention; 
       FIG. 2  illustrates a processor that can be used to process the outputs of two differential microstructure flow sensors; 
       FIG. 3  illustrates an orientation sensor according to a second embodiment of the present invention; 
       FIG. 4  illustrates a differential thermal conductivity sensor that can be used in gas chromatography according to a third embodiment of the present invention; and, 
       FIGS. 5 and 6  illustrate rotation sensor according to a fourth embodiment of the present invention. 
       FIG. 7  illustrates a three axis rotation sensor that incorporates three of the rotation sensors shown in  FIGS. 5 and 6 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows an absolute thermal conductivity sensor  10  according to a first embodiment of the present invention. As shown in  FIG. 1 , the absolute thermal conductivity sensor  10  includes a first microstructure sensor  12  and a second microstructure sensor  14  mounted to a housing  16 . 
   The first microstructure sensor  12 , for example, may comprise a first microstructure  18  supported by a first mounting block  20  secured to the housing  16 . A first O-ring seal  22  fitted in a recess of the first mounting block  20  seals against leakage between the housing  16  and the first mounting block  20 . 
   The second microstructure sensor  14 , for example, may comprise a second microstructure  24  supported by a second mounting block  26  secured to the housing  16 . A second O-ring seal  28  fitted in a recess of the second mounting block  26  seals against leakage between the housing  16  and the second mounting block  26 . 
   Each of the first and second microstructures  18  and  24 , for example, may be constructed in accordance with the microbridge disclosed in U.S. Pat. No. 4,683,159. Thus, each of the first and second microstructures  18  and  24 , for example, may be constructed as one or more temperature responsive microsensors that sense heat emitted by a microheater. More specifically, each of the first and second microstructures  18  and  24  may include a microheater between a pair of temperature responsive microsensors. 
   The housing  16  has a channel  30  that supports a fluid whose thermal conductivity is to be sensed. The first microstructure  18  is exposed to the fluid in the channel  30 . However, a diaphragm  32  seals the second microstructure  24  from the fluid in the channel  30 . A fill fluid fills the chamber formed between the diaphragm  32  and the second microstructure  24 . This fill fluid, for example, may be an inert gas or liquid, which ideally is close to the average composition of the fluid typically encountered as the sample fluid in contact with the first sensor. 
   A first shield  34  is provided to shield the first microstructure  18  from fluid convection that detrimentally affects the accuracy in sensing the thermal conductivity of the fluid in the channel  30 . A second shield  36  is provided between the second microstructure  24  and the diaphragm  32 . The first and second shields  34  and  36  may be similar to the shield  25  disclosed in U.S. Pat. No. 6,322,247 and may be incorporated as part of the housing  16 . 
   The absolute thermal conductivity sensor  10  enables the sensing of small changes in the composition of the monitored fluid in the channel  30 , and is useful in those cases where forced sample and/or reference streams are not available. 
   The second microstructure  24  is used to cancel changes in such environmental conditions as fluid pressure, temperature, acceleration, orientation, etc. from the output of the first microstructure  18 . Thus, the output of the first microstructure  18  experiences minimal interference from such environmental conditions. 
   Accordingly,  FIG. 2  shows an arrangement to cancel changes in environmental conditions from the output of the first microstructure  18 . As shown in  FIG. 2 , the temperature responsive microsensors (with arrows) of the first microstructure  18  are coupled in opposite legs of a Wheatstone bridge having two pairs of opposing terminals, one pair receiving a bridge input potential and the other pair forming a bridge output that is coupled to the input terminals of a first differential amplifier  38 . Similarly, the temperature responsive microsensors (with arrows) of the second microstructure  24  are coupled in opposite legs of a Wheatstone bridge having two pairs of opposing terminals, one pair receiving a bridge input potential and the other pair forming a bridge output that is coupled to the input terminals of a second differential amplifier  40 . 
   Thus, the output of the first differential amplifier  38  provides an output for the first microstructure  18 , and the output of the second differential amplifier  40  provides an output for the second microstructure  24 . A third differential amplifier  42  subtracts the output of the first differential amplifier  38  from the output of the second differential amplifier  40  so as to substantially cancel environmental effects, such as fluid pressure, temperature, orientation, etc, from the output of the first microstructure  18 . (The supplies to the microheaters of the first and second microstructures  18  and  24  are not shown but are conventional.) 
   If an air/fuel ratio is to be sensed, any sensing error caused by ambient humidity can be eliminated through use of the absolute thermal conductivity sensor  10 . Thus, the absolute thermal conductivity sensor  10  eliminates the need for an extra humidity sensor to sense ambient air. The extra humidity sensor is needed in prior art devices in order to increase the accuracy of sensing the air/fuel ratio at engine intakes when a reference sensor is exposed to ambient air. 
     FIG. 3  shows an orientation sensor  44  according to a second embodiment of the present invention. As shown in  FIG. 3 , the orientation sensor  44  includes a first microstructure sensor  46  and a second microstructure sensor  48  mounted to a housing  50 . 
   The first microstructure sensor  46 , for example, may comprise a first microstructure  52  supported by a first mounting block  54  secured to the housing  50 . A first O-ring seal  56  fitted in a recess of the first mounting block  54  seals against leakage between the housing  50  and the first mounting block  54 . 
   The second microstructure sensor  48 , for example, may comprise a second microstructure  58  supported by a second mounting block  60  secured to the housing  50 . A second O-ring seal  62  fitted in a recess of the second mounting block  60  seals against leakage between the housing  50  and the second mounting block  60 . 
   Each of the first and second microstructures  52  and  58 , for example, may be constructed in accordance with the microbridge disclosed in U.S. Pat. No. 4,683,159. Alternatively, each of the first and second microstructures  52  and  58  may be microbricks as discussed below. Thus, as shown in this patent, each of the first and second microstructures  52  and  58  may include a microheater element between a pair of temperature responsive microsensors. 
   Although not shown, shields such as shields  34  and  36  can be provided for the first and second microstructures  52  and  58  as discussed above. Alternatively, shields such as shields  34  and  36  can be provided with respect to  FIGS. 1 and 5 , and the devices shown in  FIGS. 3 ,  4 ,  6 , and  7  can be operated without such shields. 
   The housing  50  has a sealed chamber  64  containing a fluid that is common to both the first and second microstructures  52  and  58 . A diaphragm  66  seals the sealed chamber  64  such that the first and second microstructures  52  and  58  are exposed to the common fluid in the sealed chamber  64 . A fill fluid fills the sealed chamber  64 . This fill fluid, for example, may be an inert gas or liquid, which ideally is close to the average composition of the fluid typically encountered as the sample fluid in contact with the first sensor. 
   Because the first and second microstructures  52  and  58  are mounted at 90° relative to one another and a common liquid (or pressurized gas) is sealed into the sealed chamber  64 , the orientation sensor  44  with only the first and second microstructures  52  and  58  operates as a two-axis orientation sensor with intrinsic temperature and pressure compensation. That is, the gravity effects on the kinetics of the fluid within the sealed chamber  64  produce different responses from the first and second microstructures  52  and  58  and can be used as an indication of the orientation of the orientation sensor  44 . 
   The circuit of  FIG. 2  can be used to process the outputs of the first and second microstructures  52  and  58  so as to provide an output from the third differential amplifier  42  that indicates the two axis orientation of the differential thermal conductivity sensor  44  and that is compensated for temperature and pressure. Alternatively, a third microstructure could be provided for sensing the fluid in the chamber  64 . In this case, a first circuit as in  FIG. 2  could be provided for the first and third microstructures, a second circuit as in  FIG. 2  could be provided for the second and third microstructures, and a third circuit as in  FIG. 2  could be provided for the first and second microstructure. Thus, each pair of microstructures is used to cancel out the effects of environmental conditions such as fluid pressure, temperature, orientation, etc. from the orientation outputs. All together, each pair of sensors represents a tilt or acceleration sensor for one of the three spatial axes. 
   As further shown in  FIG. 3 , a third microstructure  68  with mounting block and O-ring may be provided so that the first, second, and third microstructures  52 ,  58 , and  68  are orthogonal to one another and so that the third microstructure  68  is also exposed to the fluid in the fluid chamber  64 . With this third microstructure  68 , the orientation sensor  44  is a three axis orientation sensor. 
   Each pair of the first, second, and third microstructures  52 ,  58 , and  68  can be processed by a corresponding circuit (such as shown in  FIG. 2 ) to provide three separate outputs that collectively indicate three axis orientation. Alternatively, a fourth microstructure could be provided for sensing the fluid in the chamber  64 . In this case, a first circuit as in  FIG. 2  could be provided for the first and fourth microstructures, a second circuit as in  FIG. 2  could be provided for the second and fourth microstructures, and a first circuit as in  FIG. 2  could be provided for the third and fourth microstructures to provide a three axis orientation output. The fourth microstructure, thus, is used to cancel out the effects of environmental conditions such as fluid pressure, temperature, orientation, etc. from the orientation outputs. 
     FIG. 4  shows a differential thermal conductivity sensor  70  according to a third embodiment of the present invention. For example, the differential thermal conductivity sensor  70  can be used in gas chromatography. As shown in  FIG. 4 , the differential thermal conductivity sensor  70  includes a first microstructure sensor  72  and a second microstructure sensor  74  mounted to a housing  76 . 
   The first microstructure sensor  72 , for example, may comprise a first microstructure  78  supported by a first mounting block  80  secured to the housing  76 . A first O-ring seal  82  fitted in a recess of the first mounting block  80  seals against leakage between the housing  76  and the first mounting block  80 . 
   The second microstructure sensor  74 , for example, may comprise a second microstructure  84  supported by a second mounting block  86  secured to the housing  76 . A second O-ring seal  88  fitted in a recess of the second mounting block  86  seals against leakage between the housing  76  and the second mounting block  86 . 
   Each of the first and second microstructures  78  and  84 , for example, may be constructed in accordance with the microbridge disclosed in U.S. Pat. No. 4,683,159. Thus, as shown in this patent, each of the first and second microstructures  78  and  84  may include a microheater between a pair of temperature responsive microsensors. 
   The housing  76  has a flow path  90  having a first portion containing a fluid that passes the first microstructure  78  in one direction and a second portion in which the fluid passes the second microstructure  84  in the opposite direction. In the example of  FIG. 4 , fluid in the flow path  90  flows from left to right through the first portion along the first microstructure  78  and from right to left through the second portion along the second microstructure  84 . As shown by way of example in  FIG. 4 , the first and second portions of the flow path  90  are parallel to one another. In gas chromatography, the first portion the flow path  90  (to which the first microstructure  78  is exposed) carries the reference gas, and the second portion the flow path  90  (to which the second microstructure  84  is exposed) carries the separated reference and sample gases. 
   The differential thermal conductivity sensor  70 , when used to implement a low cost gas chromatography differential thermal conductivity sensor, enables the sensing of small composition differences between the two equal flows forced past the first and second microstructures  78  and  84  that are closely spaced. A pressure drop such as across a restriction may be used to force the flow through the channel  90 . 
   The distance between each of the first and second microstructures  78  and  84  and the channel  90  should be minimized for good measurement time resolution. Additionally, a minimum dead volume, such as ≦5 nL, may be provided around the first and second microstructures  78  and  84  where the first and second microstructures interface with the channel  90 . For example, the volume of space between the first microstructure  78  and the housing  76  may be filled with a filler or plug so that, as viewed in  FIG. 4 , the lower surface of the first microstructure  78  and the lower surface of the filler or plug form a smooth and/or coplanar surface with the housing  76  along the first portion of the channel  90 . Similarly, the volume of space between the second microstructure  84  and the housing  76  may be filled with a filler or plug so that, as viewed in  FIG. 4 , the upper surface of the second microstructure  84  and the upper surface of the filler or plug form a smooth and/or coplanar surface with the housing  76  along the second portion of the channel  90 . The use of microbrick sensors as described below can be used to minimize this dead volume. 
   The circuit of  FIG. 2  can be used to process the outputs of the first and second microstructures  78  and  84  in the same manner as the outputs of the first and second microstructures  18  and  24  are processed such that the output from the first differential amplifier  38  can be used to identify the components of the fluid flowing through the channel  90 . 
     FIGS. 5 and 6  show a rotation sensor  100  according to a fourth embodiment of the present invention. As shown in  FIG. 5 , the rotation sensor  100  includes a first microstructure sensor  102  and a second microstructure sensor  104  mounted to a housing  106 . 
   The first microstructure sensor  102 , for example, may comprise a first microstructure  108  supported by a first mounting block  110  secured to the housing  106 . A first O-ring seal  112  fitted in a recess of the first mounting block  110  seals against leakage between the housing  106  and the first mounting block  110 . 
   The second microstructure sensor  104 , for example, may comprise a second microstructure  114  supported by a second mounting block  116  secured to the housing  106 . A second O-ring seal  118  fitted in a recess of the second mounting block  116  seals against leakage between the housing  106  and the second mounting block  116 . 
   Each of the first and second microstructures  108  and  114 , for example, may be constructed in accordance with the microbridge disclosed in U.S. Pat. No. 4,683,159. Thus, as shown in this patent, each of the first and second microstructures  108  and  114  may include a microheater between a pair of temperature responsive microsensors. 
   The housing  106  has a donut shaped chamber  120  containing a fluid. Accordingly, both of the first and second microstructures  108  and  114  are exposed to the fluid in the donut shaped chamber  120 . The fluid contained in the donut shaped chamber  120  can be a gas such as nitrogen, argon, etc. Alternatively, the fluid contained in the donut shaped chamber  120  can be a liquid such as water, heptane, oil, etc. 
   As shown by the cross-sectional side view of  FIG. 5  and the top view of  FIG. 6 , the housing  106  is able to rotate about an axis  122 . Because of inertia, the fluid within the donut shaped chamber  120  tends to remain motionless with respect to the first and second microstructure sensors  102  and  104  when the housing  106  begins to rotate. Thus, the microstructure sensors  102  and  104  will experience a flow that represents rotation, rotation rate, rotational acceleration, and/or tilt, while canceling disturbances such as those caused by changes in orientation, pressure, and ambient temperature. For such applications, the fill fluid in the donut shaped chamber  120  is preferably liquid for greater inertial effects. 
   Proper selection of liquid viscosity (damping), cavity wall smoothness, and chamber height and width/radius ratio enables finding a tradeoff between sensitivity and after-run error, which have characteristic exponential rise and decay times to enable deconvolution of the desired rotation rate for the specified angular range of interest. 
   By subtracting the opposed flow signals from the microsensors of the first and second microstructures  108  and  124 , the rotation effect is effectively summed, and linear acceleration and environmental effects are canceled. The circuit of  FIG. 2  could be used for this purpose. 
   Additionally, two rotation sensors  100  can be mounted orthogonally to one another so as to provide a two axis rotation sensor, or, as shown in  FIG. 7 , three rotation sensors  100  can be mounted orthogonally to one another so as to provide a three axis rotation sensor. 
   The absolute thermal conductivity sensor  10 , the orientation sensor  44 , the differential thermal conductivity sensor  70 , and the rotation sensor  100  simultaneously eliminate first-order temperature, pressure, and orientation effects which disturb the operation of individual or single thermal conductivity (TO) sensors. 
   The absolute thermal conductivity sensor  10  senses concentration changes in stationary fluids, when no pressurized reference fluid is made available by the system, whereby such stationary reference is built into the sensor. Thus, the absolute thermal conductivity sensor  10  can be used to sense small changes in the composition of stationary fluids (gases or liquids) such as in air, process streams, and/or gas or liquid in fluid storage tanks (aircraft tanks, fuel cell processing tanks, etc.) 
   The differential thermal conductivity sensor  70  senses concentration changes in flowing fluids (gases or liquids), when a pressurized reference stream is made available by a system, such as a Gas Chromatography system. Also, the differential thermal conductivity sensor  70  can be used to sense small changes in a carrier gas stream, before and after a sample volume is added and separated into components. Moreover, the differential thermal conductivity sensor  70  has a sub-millisecond response time. 
   The rotation sensor  100  senses (with a fluid sealed in with the microstructures as shown in  FIG. 5 ) either rotation or acceleration, separately, or sequentially, but not simultaneously, by taking advantage either of the inertia of the fluid fill in the chamber  120 , or of the thermal microconvection driven by the microheaters of the first and second microstructures  108  and  114  to sense orientation or acceleration. 
   Off-the-shelf, relatively high volume microbridge or micro-membrane sensor chips can be used as the microstructures  18 ,  24 ,  52 ,  58 ,  78 ,  84 ,  108 , and  114  of the sensors  10 ,  44 ,  70 , and  100  to implement a low cost differential thermal conductivity sensor. Accordingly, the sensors  10 ,  44 ,  70 , and  100  are more affordable than prior thermal conductivity sensors while providing good accuracy and insentivity to environmental conditions. 
   Certain modifications of the present invention have been discussed above. Other modifications will occur to those practicing in the art of the present invention. For example, as discussed above, off-the-shelf microbridge or micro-membrane sensor chips can be used as the microstructures  18 ,  24 ,  52 ,  58 ,  78 ,  84 ,  108 , and  114  of the sensors  10 ,  44 ,  70 , and  100 . 
   Moreover, microbridges are suggested as examples of the microstructures  18 ,  24 ,  52 ,  58 ,  78 ,  84 ,  108 , and  114 . Microbridges are microdevices that include a heater, an upstream temperature responsive microsensor, and a downstream temperature responsive microsensor formed as a bridge over a well, typically in a silicon substrate. Instead, each of the microstructures  18 ,  24 ,  52 ,  58 ,  78 ,  84 ,  108 , and  114  may be a corresponding Microbrick™ described in U.S. patent application Ser. Nos. 10/150,851 and 10/337,746. This type of device includes a microheater, an upstream temperature responsive microsensor, and a downstream temperature responsive microsensor formed on a substantially solid substrate such as a silicon substrate. 
   Furthermore,  FIG. 2  illustrates a processor that can be used to form a difference between the outputs of two microstructure sensors. The processor of  FIG. 2 , however, may take alternative forms such as a computer, logic gates, programmable logic arrays, and/or other circuits or arrangements to form a difference between the outputs of two or more microstructure sensors. 
   Accordingly, the description of the present invention is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appended claims is reserved.