Abstract:
This specification discloses a fast response temperature sensor assembly wherein a surrounding substrate having a low thermal conductivity and low heat capacity is coupled to a fast response temperature sensor. Foam glass is an advantageous material for use as the support substrate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to fast responding temperature sensors having a structural support. 
     2. Prior Art 
     Temperature sensing is becoming increasingly important for advanced automobile engine control. For example, operation of an air fuel ratio control system may require determination of the temperature of the incoming air. For an idealized temperature probe, the temperature change of the probe is a function of the amount of heat going into the probe which is dependent upon the specific heat of the probe and the probe mass. Known temperature sensors include a thermocouple having a relatively sturdy structure and a relatively long delay in response to temperature change. Faster responding temperature sensors are also known, but they are generally too fragile or are otherwise impractical for in car use. 
     The prior art indicates that there is tradeoff between speed of response and the rigidity of the temperature sensor. For example, attaching a substrate to a fast responding fragile sensor in order to render it more durable results in an increased response time. Typically, the increase in response time made the &#34;fast&#34; response sensor comparable to the sturdier sensor with a slower response time. These are some of the problems this invention overcomes. 
     SUMMARY OF THE INVENTION 
     In order to reduce heat leakage from a fast response temperature sensor this invention teaches using a mechanical support of a material having low density and low thermoconductivity. For example, a foam glass material can improve temperature sensor response times by factor of about 400 in comparison to glass. 
     In accordance with an embodiment of this invention, a fast response temperature sensor assembly includes a support substrate, a fast response temperature sensor and a coupling means. The support substrate has a low thermal conductivity, a low heat capacity, and a low density. The coupling means secures the temperature sensor to the substrate thereby strengthening the mechanical integrity of the temperature sensor. Further, if desired, additional support material forming a second substrate can be attached to the support substrate. The second substrate can have increased structural length and have a thermal conductivity higher than the support substrate. The higher thermal conductivity does not substantially affect the response time of the temperature sensor because the second substrate is thermally isolated from the temperature sensor by the support substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a temperature sensor in accordance with an embodiment of this invention; 
     FIG. 2 is a cross sectional view of a temperature sensor assembly in accordance with an embodiment of this invention; 
     FIG. 3 is a graphical representation of the response time of a sensor in accordance with an embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, a temperature sensor assembly 10 includes a sensor 11 which is attached to a foam glass substrate 12 by frit seals 13. Additionally, foam glass substrate 12 is attached to a quartz plate 14 by ceramic cement 15. 
     Such a temperature sensor assembly 10 is particularly advantageous for use at temperatures less than about 300° C. The temperature sensor has a response time substantially less than the seven second response time of temperature sensors durable enough to withstand automobile operating conditions. Advantageously, foam glass substrate 12 is selected to have a low thermal conductivity, a low heat capacity and a low density. 
     Sensors fabricated in accordance with an embodiment of this invention have been found to have response times in the 300 to 600 millisecond range. An unsupported sensor may have a response time in the range of about 50 to 150 millisecond. Accordingly, the degradation provided by the additional structural support in accordance with an embodiment of this invention is relatively small. 
     Referring to FIG. 3, the response of a test sensor assembly in accordance with an embodiment of this invention is shown. The response is of a small thermistor fritted to a foam glass substrate responding to a step function change in temperature. In this case, the response time is defined to be the time required for the sensor signal to reach 70% of its final value and is about 400 milliseconds. 
     Frit seals 13 have been found to be quite durable and showed no damage during testing periods of several hours and assembly 10 was repeatedly cycled between 100° C. and 350° C. The duration of a typical complete cycle is less than about one minute. 
     A typical material for foam glass substrate 12 is a foam glass product manufactured by Corning Glass Works. This material has a density of approximately 0.112 grams per cubic centimeter and thermal conductivity of less than 5.5×10 10  watt per centimeter degree giving a leakage of about 3.08 10 -5  watt 2  sec/cm 4  deg 2 . For comparison, the corresponding figure for solid glass is 1.25×10 -2  watt 2  sec/cm 4  deg  2 . 
     Heat conduction theory shows that the condition for the substrate not to perturb sensor response is that (K s  P s  C s  /K F  P F  C F ) 1/2  is small. Here K, P, C are the thermal conductivity, density and specific heat, respectively, and S and F subscripts refer to substrate and sensor, respectively. 
     Following is a table comparing glass and glass foam substrates. 
     
                       TABLE 1______________________________________  MaterialSubstrate       Device Response Time                       ##STR1##______________________________________solid glass      8 sec.          7.4 10.sup.-2glass foam 21 m sec.       4.1 10.sup.-3______________________________________ *Calculated for Pt metal film 
    
     Various modifications and variations will no doubt occur to those skilled in the various arts to which this invention pertains. For example, the particular temperature sensor or additional substrate may be varied from that disclosed herein. These and all other variations which basically rely on the teachings by which this disclosure has advanced the art are properly considered within the scope of this invention as defined by the appended claims.