The present invention relates to a method and system to carry out the calibration of an electrical instrument or circuit without the need for a potentiometer, and, more particularly, to the use of a method and a system with an application of a temperature sensor, such as a thermistor to read out a sensed temperature or to provide a signal indicative of a sensed temperature for inputting to a further control system. 
In the treatment of infants, and particularly those born prematurely, it is necessary to provide heat to the infant during the treatment of such infants and to minimize the heat loss of the infant.  Accordingly, there are various types of infant care apparatus to provide that heating, among such apparatus are infant warmers and infant incubators.  In general, the infant warmer comprises a flat, planar surface on which the infant rests while some procedure is being carried out on the infant.  There are normally protective guards that surround the infant to keep the infant contained within the apparatus and there is an overhead heater that directs radiant energy in the infrared spectrum toward the infant to impinge upon that infant to provide warmth. 
Infant incubators, on the other hand generally are of a more protective atmosphere that enclose the infant in a infant compartment where the temperature and perhaps humidity is also controlled and, in essence, the infant is contained in an enclosed chamber and provided with a controlled environment.  While the present invention can be used with an infant warmer, an infant incubator, or with various other types of equipment, the description hereinafter will be directed mainly to the use of that invention in the context of an infant warmer as a typical use of the invention. 
An infant warmer is shown and described in U.S. Pat.  No.  5,474,517 of Falk and is an example of the type of infant care apparatus that is used to provide warmth to an infant while carrying out some procedure on that infant. 
With the use of infant warmers, there is normally provided a means to sense a temperature and to display that temperature or use the signal representing that temperature in digital form for some further signal processing.  In general, one device normally used for such temperature sensors is a thermistor and such devices are certainly widely used in infant warmers.  The electrical circuits used to determine the temperatures sensed by thermistors are also well known and utilize the phenomena of a thermistor that the resistance of the device changes with the temperature of its environment.  Thus, as used herein, reference will be made to a signal from a thermistor representative of temperature however, it will be understood that the signal is a derived signal obtained by normal circuitry and in reality is measuring the change in resistance of the thermistor device. 
With infant warmers, the thermistor is normally placed on or affixed to the infant to detect the skin temperature of that infant.  The signal from the thermistor in analog form is used to display the infant temperature while that signal representative of the infant skin temperature is also used as an input to the overall control system that regulates the intensity of the heat directed toward the infant to provide the warmth to the infant.  One system used to control a radiant heater is shown and described in U.S. Pat.  No.  5,162,038 of Wilker.  In that patent, the control system uses a difference between the sensed patient temperature and a set point temperature inputted by the user to operate a proportional control loop and that proportional control technique adjusts the power to the heater. 
As a common component of temperature sensing systems using one or more thermistors, for example, there is a need to calibrate the system to ensure the accuracy of the thermistor signal.  In particular, the most common means of carrying out the calibration is by means of a potentiometer, or pot, and normally two pots are needed that can adjust the span and offset of the signal from the thermistor.  Thus, for each thermistor that is installed, it is necessary to calibrate the thermistor against some standard and carry out the fine tuning or adjustment of two pots to insure that each thermistor is providing an accurate signal for use by the control system. 
In general, such calibration is carried out initially at the facilities of the manufacturer of the apparatus and thereafter a re-calibration may be undertaken on a regular, established basis, in the range of about once a year.  In each re-calibration, the normal calibration using potentiometers is carried out by a biomedical engineer or biomedical technician that is professionally trained to carry out the procedure.  The conventional calibration procedure, for example, on an infant warmer utilizing a thermistor, is to remove the thermistor and to replace it with an extremely accurate precision resistor that is known to result in a predetermined temperature readout at the temperature display instrument. 
Thus, the engineer or technician must tweak or adjust two potentiometers for each thermistor.  Since the adjustment of one potentiometer, for example, the one adjusting the span calibration constant will have an effect on the other potentiometer, such as the offset calibration constant, the technician must continue make changes to both potentiometers as a balancing act to eventually arrive at the correct temperature indication at the temperature display instrument.  After that correct reading is achieved, a second extremely precise resistor is substituted into the circuit and with its resistance designed to result in a second known readout temperature to achieve two points on the calibration curve for the thermistor circuit and the same procedure repeated for the second precise resistor with the tweaking of the two potentiometers to get the correct readout temperature on the temperature display instrument.  In cases where the technician is highly skilled and/or fortunate, the procedure can take about 15 minutes per reading and at times, the procedure for one reading may take in excess of an hour.  Accordingly, as can be seen, the procedure of re-calibration is considerably time consuming and is a detailed procedure requiring a skilled engineer or technician. 
While the afore described re-calibration procedure is usable, it can be seen that it is quite difficult for the user to individually tweak or adjust various pots in a system, particularly where there may be a plurality of thermistors used, each having a pair of pots that need individual adjustment by a biomedical technician or biomedical engineer to carry out the calibration of such thermistors. 
Accordingly, it would be advantageous to have a system that eliminates entirely the use of the potentiometers in carrying out the calibration and re-calibration of thermistors so as to have an automatic electronic system of calibration that does not require the time consuming tweaking or adjusting of the individual potentiometers.