This invention relates generally to apparatus and methods related to sensing a detectable characteristic, such as pressure, and providing signals from which measurements of the characteristic can be derived.  More particularly, but not by way of limitation, this invention relates to sensor apparatus, used within a transducer, for sensing pressure and temperature in such a manner that responses altered by temperature changes, whether caused by an external source or by pressure-volume heating, are compensated. 
Sensing characteristics is a well-known need in all types of industries and processes.  For example, in the oil and gas industry, pressure in a well bore needs to be sensed and measured because pressure is a characteristic used to evaluate the ability of a well to produce.  Such measurements are obtained with a transducer having one or more characteristic sensitive devices, referred to herein as sensors, which respond to the desired characteristic or characteristics (e.g., pressure).  A transducer also has electronics by which the responsiveness of each sensor is converted into an electrical signal from which the end measurement is ultimately obtained.  Sometimes associated with one or more of these sensors is a reference device which provides a reference signal against which the characteristic responsive signals are compared for obtaining a measurement of enhanced accuracy.  The use of such a reference device is disclosed in U.S. Pat.  No.  3,355,949 to Elwood et al. 
A sensor can be sensitive not only to the primary or desired characteristic, but also to some other, ancillary characteristic which coexists with the primary characteristic.  For example, a pressure sensor may have a response which is affected not only by pressure, but also by temperature.  Such sensitivity or responsiveness to the ancillary characteristic distorts the accuracy with which the primary characteristic can be measured unless the ultimate measurement is compensated for the effect of the ancillary characteristic.  For example, if the response of a pressure sensor is also affected by temperature, such a sensor used alone produces a signal, read as a pressure change, when the pressure remains constant, but the temperature changes.  Such a distortion also exists even when both a sensor and a reference device are used if both are affected by the ancillary characteristic, but to different extents or at different times. 
For purposes of simplicity, the remaining explanation will be given only with specific reference to a pressure sensor which is affected by temperature; however, it is contemplated that this problem can exist between other characteristics. 
In the specific example of a pressure sensor which has a pressure response affected by temperature, this general problem has been recognized.  In U.S. Pat.  No.  4,455,875 to Guimard et al.,  a reference detector is located in a separate compartment from a pressure sensor, both of which are affected by the ancillary characteristic of temperature.  A temperature sensor is located in the same compartment as the reference detector.  With these three devices, signals can be obtained and used with known equations to correct, to some degree, for the effect of temperature on the pressure sensor. 
The Guimard et al.  patent also recognizes a more specific aspect of the general problem.  This aspect is that the heat exchange rates of both the pressure sensor and the reference detector should be balanced relative to an external temperature change so that the pressure sensor and the reference detector will respond to the external temperature change at the same time.  That is, under static or equilibrium conditions wherein the temperature of the pressure sensor and the temperature of the reference detector are the same, the pressure sensor and the reference detector are presumably affected the same by the same constant temperature existing at both.  When an external temperature change occurs, however, the pressure sensor and the reference detector will have their responses affected at different times if the temperature change does not affect both at the same time.  To prevent such different, error-producing responses, the Guimard et al.  patent proposes to mount the separate reference detector within a heat sink arrangement to try to balance the heat exchange rates of the pressure sensor and the reference detector with the external environment. 
Although the Guimard et al.  patent recognizes the general problem and this specific aspect relating to temperature changes occurring outside the transducer, it fails to disclose another specific aspect of the general problem as it relates to the specific pressure/temperature example.  This other aspect is that temperature gradients can also result within the medium immediately surrounding the pressure sensor which is disposed within a fixed volume of the transducer.  As this medium is subjected to pressure changes, temperature changes result.  This is referred to as pressure-volume, or PV, heating.  U.S. Pat.  No.  4,607,530 to Chow recognizes this other specific aspect. 
The Chow patent discloses that a temperature sensor is to be located in the same environment as the pressure sensor so that the temperature at the pressure sensor can be obtained.  With this information, a dynamic model, based on preliminary data obtained during calibration procedures, is run in a processor to generate temperature-compensated pressure readings. 
The Chow patent does not disclose an apparatus wherein both the pressure sensor and the reference detector have a similar temperature time response to thermal transients; therefore, errors can still be produced from the different thermal responses between the pressure sensor and the reference detector unless the Chow modeling makes some attempt to resolve the difference by means of the software, which requires computing time such as in differentiating the temperature response.  Differentiation can produce errors by emphasizing normal random temperature errors.  Differentiating would require many samples to be taken over a period of time to derive a compensated temperature of the reference.  This means that a delay at least equivalent to the temperature time response between the reference detector and the pressure sensor occurs before a corrected measurement can be calculated. 
Although these problems related to characteristic sensors being affected by an ancillary characteristic have been recognized, there is still the need for an improved apparatus and method by which measurements of a primary characteristic can be obtained, which measurements are relatively simply compensated, in real time, for ancillary characteristics affecting the response to the primary characteristic.  More specifically, there is the need for an improved sensor apparatus having both a pressure sensor and a reference device which compensate in real time for temperature gradients arising from either or both external temperature changes or pressure-volume heating.  Such compensation should be simply, but rapidly, obtained to facilitate manufacturing and maintenance and to provide accurate, real time outputs of the desired characteristic.