referring first to fig1 there is shown a group of three amplifier circuits having a processor - addressable dual digital potentiometer 10 input circuit and a computer processor 20 output circuit , illustrating the essential circuit connections which form the heart of the present invention . the processor 20 output circuit is shown in fig1 as a display 21 , but this is representative of any type of computer output circuit or device which is known in the art . the addressable dual digital potentiometer 10 is a commercially available component , as for example , a type ds1803 manufactured by dallas semiconductor corporation . each potentiometer 11 , 12 represented within the box 10 consists of a resistor array having 256 selectable positions , formed by a series - connected plurality of semiconductor mosfet devices , each of which are switchable “ on ” or “ off ” by a binary voltage signal , and an 8 - bit register ( not shown ) to receive the binary voltage signals , and thereby to effectively set the “ wiper ” 13 , 14 position to any of the 256 positions , depending on the digital value of the binary voltage combination loaded into the register . the potentiometers 11 , 12 are connected together at their respective ends , and the top end connection is connected to an analog input voltage signal v in and the bottom end connection is connected to a voltage ground . the device is addressable and controlled by a two - wire serial interface over the lines “ data ” and “ clock ,” which are connected to the computer processor 20 serial output port . the computer processor 20 provides data for loading into the potentiometer registers , under control of a clock signal also provided by the processor 20 . each potentiometer wiper 13 , 14 is connected to a separate amplifier 15 , 16 having essentially identical transfer characteristics . the amplifier is a non - inverting , fixed gain operational amplifier . the output signal from amplifier 15 is connected to an amplifier 17 input via a 1000 - ohm ( 1k ) resistor , and the output signal from amplifier 16 is connected to the same amplifier 17 input via a 256 k resistor , thereby providing an additive input signal , where the two input signals are summed in the ratio of 256 / 1 to the amplifier 17 input from the respective amplifiers 15 , 16 . this provides an effective “ high byte ” and “ low byte ” contribution to the output voltage from amplifier 17 , where the output voltage is directly proportional to the input voltage , with a signal resolution of one part in 2 16 . the output signal from amplifier 17 , v out , is applied to an analog - to - digital input terminal of computer processor 20 , and therefore produces a digital value to the computer processor which is representative of the analog input voltage v in , to a resolution of one part in 65 , 536 ( 2 16 ). fig2 shows a block diagram of the present invention , illustrating the major circuit functions , balance , agc , and span , which are used to achieve the control desired herein , all in conjunction with the computer processor 20 . each of the circuit functions contains circuit components as shown in fig1 and each circuit function is operated under the control of the processor 20 . the balance circuit 30 has an input connected to receive the analog voltage signal v a from an infrared sensor , and an output connected to a switch terminal of switch 40 . another switch terminal of switch 40 is connected to a source of reference voltage v ref , which could come from a second infrared sensor , and the third terminal of switch 40 is connected to an input of agc circuit 50 . the output of agc circuit 50 is connected to the input of span circuit 60 , and the output of span circuit 60 is connected to an a / d input connection to processor 20 . if two infrared sensors are used to connect to the switch 40 , they are typically sensors which are responsive to different wavelengths of light . the v a signal is from a sensor monitoring the environment where a concentration of the gas desired for detection might be found , and the v ref signal is from a sensor monitoring the same environment but which is non - responsive to the gas desired for detection . fig3 shows a simplified circuit diagram of the several circuits described above ; it should be apparent that the circuit connections of each of the circuits is virtually identical to the other circuits , with the exception that the span circuit is referenced to a + 4 volt power supply rather than to circuit ground as are the agc and balance circuits . each circuit has an addressable dual digital potentiometer associated with an amplifier as described earlier ; namely , the balance circuit 30 has a dual digital potentiometer 31 associated therewith , the agc circuit 50 has a dual digital potentiometer 51 associated therewith , and the span circuit 60 has a dual digital potentiometer 61 associated therewith . in each case , the computer processor 20 is connected , via a “ data ” and “ clock ” signal line , to provide the register settings for the respective digital potentiometers . in operation , the circuits are first manipulated through a calibration procedure to select the initial settings for the agc , balance and span potentiometers . because of the particular circuits selected for the preferred embodiment , the “ zero ” voltage reference is selected to be + 4 . 0 volts , and the maximum gas concentration voltage is selected to be + 0 . 5 volts . since the infrared detector converts light intensity to voltage , this voltage selection translates into a beer &# 39 ; s law equation of : since the maximum , or full scale value of the beer &# 39 ; s law calculation is + 0 . 5 volts , we can calculate the value of the exponent at the maximum gas concentration level ( cmax ): 0 . 5 = 4 . 0 e − γlcmax ; taking the log of each side : − ln ( 0 . 5 / 4 . 0 )= γlcmax = 2 . 08 the foregoing calculations set the parameters for the beer &# 39 ; s law equation in the context of the actual circuit parameters used for this invention ; namely , a voltage indication of 4 . 0 volts when zero gas concentration exists in the sensor environment , and a voltage indication of 0 . 5 volts when a maximum gas concentration exists in the sensor environment . for all other concentrations of gas the following beer &# 39 ; s law formula can be solved for any measured voltage “ v ”: the calibration procedure can proceed according to the following steps . first , the processor selects the switch 40 and sets switch 40 to select the reference channel , which is the v ref position , and which corresponds to a zero gas concentration reference , and the agc potentiometer is selected by the processor . the processor sends binary digital feedback signals to the input registers of the agc digital potentiometer circuit to obtain an input voltage to the computer processor of 4 . 0 volts . thus , the gain is set by the processor to reflect the zero gas concentration value to the processor . next , the processor selects the switch 40 and sets switch 40 to select the analytical channel , which is the v a position , under conditions of zero gas concentration in the sensor environment . the processor sends binary digital feedback signals to the input registers of the balance potentiometer circuit to obtain an input voltage to the computer processor of 4 . 0 volts . this provides a sensor signal value to the processor corresponding to the zero gas concentration value set in the previous step . next , a known gas of known concentration is injected into the infrared detector environment ; the selected gas concentration is preferably ½ the maximum obtainable concentration level , or ½ cmax as derived above . this represents a gas concentration “ c ” equal to “ cmax / 2 ”. and the beer &# 39 ; s equation becomes : the processor sends binary digital feedback signals to the span potentiometer circuit to obtain an input voltage to the computer processor of 1 . 414 volts , which is the voltage associated with the target gas at ½ concentration ( see fig4 ). having determined the ½ - concentration voltage , any other voltage ( v m ) measured by the processor can be used to calculate any unknown gas concentration ( c u ) using beer &# 39 ; s law . the processor constructs the beer &# 39 ; s law curve of fig4 and prestores this curve in memory , either in the form of a table look - up or in any other convenient form useful to the processor , because three points along the exponential curve are known from the foregoing calibration steps , and therefore all other points along the curve can be plotted or calculated . any subsequent voltage received by the processor can be applied to the known exponential curve , and the gas concentration corresponding to that voltage value can be determined from the prestored table or curve . once the device is calibrated it can then be operated in real time , with unknown concentrations of target gas passing into the infrared sensor device . in real time operation , the switch 40 is switched four times per second to regularly measure the reference voltage at the computer , and to change the binary digital feedback value to the agc potentiometer if necessary to return the measured value to + 4 . 0 volts . this agc potentiometer change is effective for adjusting the reference sensor voltage value received by the processor , which also adjusts the analytical sensor voltage value , on the theory that whatever outside influence caused the drift in the “ zero ” voltage value would also cause an identical drift in the analytical sensor value . during the time the switch is connected to the sensor via the analytical channel , the actual gas concentration being measured is presented to the computer processor 20 input in the form of a voltage ranging between the zero concentration level (+ 4 . 0 volts ) and the full scale concentration level (+ 0 . 5 volts ); the actual measured voltage is applied to the curve of fig4 or to a curve - fitting process residing in computer software , to derive the relative gas concentration being measured . the balance potentiometer circuit and the span potentiometer circuit are not subsequently readjusted , unless a new calibration procedure is necessary . this operating procedure provides a sensor reading to the processor four times a second , and also provides a gain adjustment , if necessary , four times a second . the essential advantage of the invention is that the analog signals over the zero to full scale range are always identical regardless of the type of gas being measured , the range of concentrations of the gas , the amount of signal drift which occurs , and the signal degradation which may occur . it enables determination of gas concentrations using beer &# 39 ; s law a matter of trivial complexity , involving only a single response curve , where the analog signals can &# 39 ; t exceed the boundaries set for the device and the a / d resolution is constant . the present invention may be embodied in other forms without departing from the spirit or essential attributes thereof ; and it is , therefore , desired that the preferred embodiment be considered in all respects as illustrative and not restrictive , reference being made to the appended claims rather than to the foregoing description to indicate the invention scope .