Abstract:
A control circuit for a thermal conductivity cell employs a constant resistance bridge drive circuit which automatically adjusts to maintain a measurement filament at a constant resistance. A reference filament provides a differential signal representative of the concentration of an analyte. A detection circuit utilizes digital/analog methods to significantly reduce 1/f noise of an amplifier providing at least a seven fold improvement in signal-to-noise ratio. The circuit also includes a bridge nulling method adjusted under microprocessor control to eliminate manual offset adjustments. A reference protection circuit is coupled to the reference filament and prevents the voltage applied to the filaments from overheating the filaments in the event there is a breach in the gas flow path.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a control circuit for a thermal conductivity cell and particularly one which provides linearity, lower noise, and filament protection. 
     A variety of constant resistance thermal conductivity detector circuits exist which employ a single filament in the leg of a Wheatstone bridge. A servo loop forces the bridge to balance, thereby holding the filament at a constant resistance. With such a system, however, the single filament is subject to significant drift since there is no reference filament and the output is sensitive to the temperature of the thermal conductivity cell block. Further, the signal can suffer from non-linearities inasmuch as the thermal conductivity is proportional to the power dissipated and the voltage across the filament. Sensitivity suffers inasmuch as a high common mode output signal to drive the bridge is necessary, and, therefore, gain cannot be applied to the output signal to increase the sensitivity of the cell. Finally, without a reference filament, the effects due to vibrations and pressure variations are apparent in the output signal. 
     Attempts have been made to control drift by altering the filament resistance between two temperatures resulting in a differential signal independent of block temperature. Other approaches have employed a bridge control signal alternated between constant resistance and constant voltage resulting in a similar differential control signal. In some approaches, drift has been compensated for by attaching a temperature sensor to the cell block itself. The linearity problem has also been addressed utilizing a closed-loop analog circuit in most cases. In some instances, a resistance loop is closed employing a digital signal and comparator to sense an imbalance in the microprocessor controlled D-to-A converter to drive the bridge back to balance. These configurations have the advantage of having a linear output verses concentration as the pulse width is directly related to power dissipated in the filament. 
     The sensitivity of a detection system utilizing a thermal conductivity cell has been addressed by various analog methods, typically including the utilization of a voltage divider and switch capacitor network to reference the voltage across each resistor to a common ground. The sensitivity of such a system is somewhat improved over other methods, however, the sensitivity is still limited to about 2 ppm (parts per million) detection. The problem with pressure disturbances in a thermal conductivity system has been addressed as well in which two measurement filaments and two reference filaments have been employed in a four filament bridge in an effort to control the average resistance of the bridge to compensate for such pressure disturbances. 
     Thus, although the prior art has attempted to address individually each of the various problems inherent in a thermal conductivity detection system, the prior art has not solved each of the problems adequately nor comprehensively addressed these problems in an overall system which provides improved linearity, low noise, and a protected thermal conductivity system in which the thermal conductivity resistance filament is protected from oxidation. There exists, therefore, a need for an improved thermal conductivity control circuit which provides these advantages. 
     SUMMARY OF THE INVENTION 
     The system of the present invention provides a control circuit for a thermal conductivity cell by employing a constant resistance bridge drive circuit which automatically adjusts to accommodate various carrier gases, such as helium and argon. Additionally, a reference filament is placed in a third leg of the bridge to provide a differential measurement signal. The detection circuit utilizes digital/analog methods to significantly reduce 1/f noise of an amplifier providing at least a seven fold improvement in signal-to-noise ratio. This also eliminates the thermocouple effects in the path from the bridge to the amplifier. 
     The circuit includes a bridge nulling method which is adjusted under microprocessor control to eliminate the need for manual offset adjustments. Thus, the typical necessity of matching the measurement and reference filaments can be relaxed, thereby reducing the cost of a thermal conductivity cell employing such a control circuit. The automatic monitoring of the bridge filament can warn the operator when filaments have aged and require replacement. The reference filament is monitored individually and protected against exceeding its oxidation temperature. This system compensates for fluctuation in cell block temperature, exhaust pressure variations, and improves the linearity more than ten fold over conventional detection bridges. 
     These features and advantages of the present invention are embodied in a system in which reference and measurement filaments are incorporated in a Wheatstone bridge and a constant resistance circuit compares the bridge drive voltage applied to the bridge divided in half in a closed-loop feedback circuit to maintain the measurement filament a constant resistance. Signals from the reference detector are modulated at a frequency above the significant 1/f noise frequency of an amplifier in a preferred embodiment of the invention, amplified, demodulated, and filtered to provide a low noise output signal representative of the concentration of a sample gas through the measurement cell. 
     According to another aspect of the invention, the reference filament drive is continually adjusted for changes in relative resistance between the reference filament and the measurement filament by a null adjustment circuit coupled to the reference filament and to the bridge drive circuit. Output signals are coupled to a microprocessor for controlling the null adjustment to null any differences between the resistive legs of the bridge prior to an analysis. According to another aspect of the present invention, a reference protection circuit is provided and is coupled to the reference filament and controls the voltage applied to the filament through a voltage divider and integrator circuit to prevent overheating of the reference electrode in the event there is a breach in the gas flow path or other event which otherwise would allow the filament to overheat. 
    
    
     In the most preferred embodiment of the invention, each of the circuits are integrated in an overall control system for a thermal conductivity cell including measurement and reference electrodes to provide a highly sensitive detection system which is extremely linear and reliable over time. These and other features, objects and advantages of the present invention will become apparent upon reading the following description thereof together with reference to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a representative graph showing the linearity of an existent constant voltage bridge circuit as opposed to the linearity of the constant resistance system of the present invention; 
     FIG. 2 is a graph of a sample with nitrogen having different sample weights showing the constant output results achieved by the system of the present invention for different weights of samples as compared to the variations which occur with systems of the prior art; 
     FIG. 3 is a circuit diagram partly in block and schematic form of the system of the present invention; 
     FIG. 4 is a detailed schematic diagram of part of the system shown in FIG. 3; 
     FIG. 5 is a detailed schematic view of the modulator/demodulator circuit shown in FIG. 3; 
     FIG. 6 is a detailed schematic view of the null adjustment circuit shown in FIG. 3; and 
     FIG. 7 is a detailed schematic view of the reference protection circuit shown in FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIG. 1, there is shown empirical test results showing the linearity errors between a prior art constant voltage bridge drive thermal conductivity cell with the concentration on the X axis and the measurement error from a thermal conductivity cell on the Y axis. As can be seen at lower concentrations, the output signal  1  has a 3% to 6% error. Line  2  in FIG. 1, on the other hand, shows the concentration of an analyte verses output signal obtained by the constant resistance thermal conductivity system of the present invention, showing that with varying concentrations of an analyte, the detected output signal is substantially error free. 
     FIG. 2 shows the linearity of the output shown by line  2  of the constant resistant system of the present invention as opposed to line  1  of the prior art constant voltage thermal conductivity cell system for varying sample weights having the same sample concentration of nitrogen. As can be seen by comparing line  2  with line  1 , the constant resistance system represented by the present invention provides a substantially flat sample concentration of 9.5% nitrogen for weights varying from 10 mg to nearly 3 g, whereas the prior art constant voltage thermal conductivity system provides significant variations of from 9.3% to 10% for the same sample concentration. Further, the sensitivity of the thermal conductivity system of the present invention and its noise immunity features provide a sensitivity for detecting an analyte with a concentration as low as 100 parts per billion with a linearity of better than a 0.1%. These dramatically improved results for the use of a thermal conductivity cell in an analyzer for analytes, such as nitrogen and hydrogen or other analytes combusted in a furnace, is achieved utilizing the thermal conductivity cell and control system discussed initially in connection with FIG.  3 . 
     In FIG. 3, there is shown a control circuit  5  for controlling the operation of a thermal conductivity cell including a reference filament  14  and a measurement filament  16  coupled in a convention Wheatstone bridge configuration with a precision resistor  10  and precision resistor  12 , respectively, having one node  15  coupled to a bridge drive circuit  40  and an opposed node  17  coupled to system ground. The reference and measurement filaments  14  and  16 , respectively, are platinum wires, in turn, mounted in a thermally controlled block heated to a constant temperature falling within the range of from about 45° C. to about 50° C. Resistors  10 ,  12  are 90 ohm 0.1% precision resistors and the target resistance of the platinum filaments  14 ,  16  of the reference and measurement filaments, respectively, is 90 ohms at the operating temperature. 
     During an analysis, a combustion furnace is employed and a carrier gas, such as helium, is supplied to the reference cell  14  and the combined carrier gas and analyte flows through the measurement cell  16  such that the voltage across the reference filament  14 , which is coupled to the same voltage node  15  as filament  16  through the precision resistors  10 ,  12 , typically will be somewhat less than the voltage across the measurement cell  16 , with an analyte present due to the greater cooling of the reference filament  14  by helium. The measurement filament  16  resistance is held substantially constant by constant resistance circuit  30  which is coupled to the bridge drive in a closed-loop feedback circuit, described in detail in connection with FIG. 4 below, to maintain the resistance and, therefore, temperature of the filament  16  constant with varying analytes. The respective voltages V r  across reference filament  14  and V m  across measurement filament  16  are then applied to a noise reducing modulator/demodulator circuit  50  resulting in an output signal V out  applied to an analog to digital converter circuit  60 . Circuit  60  is a dual channel A/D converter which receives an input V b  (from the bridge drive circuit) and converts both signals to digital output signals at conductor  62  for V out  and conductor  64  for V b  to a microprocessor  70 , which as described in greater detail below, provides an output signal at output  71  coupled to the instrument which controls the furnace, display and printed output of the sample analysis information detected by circuit  5 . Microprocessor  70  also provides parallel digital control signals on conductors  72 - 79  to a null adjustment circuit  80  for balancing any resistance differences between filaments  14  and  16  initially and due to aging and assuring that the V out  signal remains positive. The V r  signal is also applied to a reference protection circuit  90  which monitors the V r  signal to apply a control signal to bridge drive  40  via conductor  42  to protect the reference filament from oxidation due to overheating in the event the gas flow path is opened or the carrier gas supply is exhausted. 
     Thus, circuit  5  (FIG. 3) of the present invention provides multiple functions, namely, maintaining the platinum measurement filament  16  at a constant resistance during an analysis (to improve the linearity of the output signal); modulating and demodulating and detecting the difference signal between the reference filament and measurement filament in a circuit to improve the sensitivity and noise reduction of the resultant output signal; a null adjustment to control for initial differences in the platinum filament resistances as well as compensating for aging; and protection for the reference and measurement filaments in the event of gas flow interruption. The details of the operation of the circuit shown in FIG. 3 is best understood by reference to the remaining circuit diagrams beginning with the constant resistance circuit of FIG. 4, showing also the details of the bridge drive circuit  40 . 
     In FIG. 4, the resistor  12  and its associated measurement filament  16  of the Wheatstone bridge is shown. The junction at node  32  is coupled to one input of an operational amplifier  31  of constant resistance circuit  30  by means of an integrator with an input resistor  33  and capacitor  35 . The junction of  33 ,  35  is coupled to one input of operational amplifier  31  and capacitor  35  coupled between input  33  and output  39 . Amplifier  31  receives a second input V 1  from a voltage divider comprising resistors  36  and  38  coupled in series between the bridge drive node  15  and ground node  17 . Resistors  36 ,  38  are equal value precision resistors and, in the preferred embodiment, 1 KOhm 1% resistors. Thus, the voltage V 1  equals V b /2, while the single V m  will attempt to vary as analyte flows through the thermal conductivity cell including filament  16 . As V m  tends to change, itts signal is integrated and compared by amplifier  31  which provides a control output signal at its output terminal  39  through a resistive divider network including sesistor  41  and resistor  43  with a junction  34  thereof coupled to the input of an operational amplifier  44  forming the bridge drive circuit  40  with its remaining input coupled to its output as a unity gain amplifier. Thus, as V m  tends to change, the feedback loop signal on input terminal  34  to amplifier  44  will tend to lower or raise V b  to maintain the voltage at a level such that V m  equals V 1  due to the feedback including the integrator circuit. The bridge drive circuit amplifier  44  also receives a signal from reference protection circuit  90  on conductor  42  which, if necessary, will override the V m  signal in a situation where protection of the reference filament is necessary, as will be described in connection with FIG. 7 below. 
     Typically, V 1  for a helium carrier in an analyte will be approximately 5 volts. In the event V m  drops, the drive applied to the bridge drive circuit  40  will be increased through the feedback loop to boost the voltage, keeping V b  at a level which maintains the temperature and, therefore, resistance of filament  16  at 90 Ohms, equaling that of resistance  12 . As seen in FIG. 3, V m , therefore, equals V b /2, which tends to be relatively constant. The signal V m  is constant for a given concentration of an analyte supplied to the thermal conductivity cell. The reference filament will have a different voltage V r , typically lower than V m , due to more cooling by being exposed to the carrier gas, such as helium, and the respective signals V r  and V m  are applied to the modulator/demodulator circuit  50  for processing to amplify and detect the resultant output signal V out , which represents the concentration of an analyte to be detected. 
     The linearity of the signal V out , which is achieved by the constant resistance circuit  30 , is shown by the following equations: 
     Filament Resistance 
     The resistances of the measure and reference filaments are temperature dependent according to the following equation: 
     
       
         R=R 0 *(1+*(T R −T 0 )) 
       
     
     Where 
     R: Filament resistance 
     R 0 : Reference resistance (70 ohm) at T 0    
     T 0 : Reference temperature=25° C. 
     : Coefficient of resistance=0.0043 ohms/° C. 
     T R : Temperature of the filament 
     Heat Conduction 
     Heat is transferred away from the heated filament to the cell block at a rate described by the following equation: 
     
       
         Q R =V R   2 /R−K*(T R −T B ) 
       
     
     Where 
     Q R : Heat transfer (W) 
     V R : Voltage across filament (V) 
     K: Thermal conductivity of gas (W/° C.) 
     T B : Temperature of cell block (° C.) 
     Thermal Conductivity of a Binary Gas Mixture 
     When two gases of differing thermal conductivity are mixed, the resulting thermal conductivity is described by the following equation: 
     
       
         K M =K R *(1+E*C M ) 
       
     
     Where 
     K M : Thermal conductivity (TC) of the gas mixture (W/° C.) 
     K R : TC of the reference carrier gas (W/° C.) 
     E: Equivalency factor relating the TC of the reference gas to the TC measurement gas 
     C M : Relative concentration of measurement gas in gas mixture 
     Derivation of Linearity for Constant Resistance Bridge 
     Electrical Equations for Constant Resistance Bridge 
     The circuit of FIG. 4 adjusts the bridge voltage, V B , to maintain the resistance of the measurement TC filament constant at 90 ohms. The reference filament will change its resistance as the bridge voltage is varied. The voltage across the reference filament is then: 
      V Rr =V B *R R /(R R +R M ) 
     Where 
     R R : Resistance of the reference filament 
     R M : Resistance of the measurement filament=90 ohms 
     V Rr : Voltage across the reference filament 
     V B : Bridge voltage 
     Solving equation (4) for V B : 
     
       
         V B =V Rr *(R R +R M )/R R   
       
     
     Solving equation (1) for T R  and substituting into equation (2) yields the following equation for reference voltage: 
     
       
         V R =sqrt(K R *R R *(A*R R +B)) 
       
     
     Where 
     
       
         A=1/(*R 0 ) 
       
     
     
       
         B=1/+T 0 +T B   
       
     
     Equivalently, the equation for the measure side voltage, V Rm , is: 
     
       
         V Rm =V B /2=sqrt(K M *R M *(A*R M +B)) 
       
     
     Solving equation (7) for K M : 
     
       
         K M =V B   2 /(4*R M *(T Rm −T B )) 
       
     
     Solving equation (3) for C M : 
     
       
         C M =(K M /K R −1)/E 
       
     
     By combining equations (5), (6), (8), and (9), the concentration of the analyte, C M , can be expressed as a function of the reference resistance, R R   
     
       
         C M ={[K R *R R *(A*R R )*(R M +R R ) 2 ]/[4R R   2 *R M *K R *(T Rm −T B )]−1}/E 
       
     
     The output voltage from the cell is simply the difference between the voltages across the reference and measurement filaments: 
     
       
         V 0 =V Rm −V Rr   
       
     
     From these equations, FIG. 1 plots the linearity error in V 0  vs. C M  as compared to the linearity error in a constant voltage bridge application. 
     In FIG. 5, the Wheatstone bridge circuit is shown and the V m  signal on conductor  32  is applied to an input terminal  51  of a low resistance solid state switch  52 , while signal V r  is applied to terminal  53  of the switch, which is schematically represented as a double pole, double throw switch in FIG.  5 . The switch  52  is a commercially available AGD433, which is coupled to a 1 kHz oscillator  55  also coupled to a second solid state switch  58  for synchronizing switches  52  and  58 . Switch  52  operates at a frequency of 1 kHz, thereby alternately chopping and applying V m  and V r  to a 100 gain amplifier  54  resulting in a square wave output signal shown as waveform  56  having a positive peak equal to V m −V r  and a negative peak equal to V r −V m , as amplified by amplifier  54 . The peaks typically will be a maximum of about 20 mV while the 1/f noise can be a few hundred nV. By chopping the signals V m  and V r  at 1 kHz, the 1/f noise inherent in the amplifier  54  can be eliminated by the demodulator  59  shown in FIG.  5 . 
     Signal  56  is applied to switch  58 , which alternately applies the positive and negative signals to one terminal of operational amplifier  61  having a feedback resistor  63  between its output and its remaining input and an input resistor  65  coupling signals  56  to such remaining input. The resultant signal at output terminal  66  constitutes a positive DC output signal, which includes some high frequency components filtered out by a low pass filter constituting resistors  57  and capacitor  58  which, in a preferred embodiment, was a 100 k resistor and a 2.2 microfarad capacitor. Thus, only signal frequency (0-2 Hz) signals representative of the analyte concentration are applied to the input of buffer amplifier  67 . Amplifier  67  provides essentially a DC output signal V out  at output terminal  68  which is applied to one input channel of the analog-to-digital converter  60  for providing a 24 bit digital output signal at output conductor  62  representative of the signal V out . 
     The A-to-D converter  60  operates at an approximately 100 ms sampling period to provide a 24 bit output signal on bus  62  applied to microprocessor  70 , which applies the V out  signal to the analytical instrument in a conventional fashion via bus  71 . In one embodiment, the thermal conductivity control circuit  5  can be used in an instrument such as a TC-436 instrument manufactured by Leco Corporation of St. Joseph, Mich. The A-to-D converter  60  also provides a 24 bit signal representative of V b  on conductor  64  which is detected by the microprocessor and employed to compensate for the temperature of the thermal conductivity block. Thus, if the block temperature changes, V b  will change and microprocessor  70  also receives temperature control information from the thermal conductivity cell block and is programmed to apply a correction factor to the V out  which varies as a function of the temperature of the block as represented by the V b  signal. 
     The null adjustment circuit  80  is employed to balance the initial differences between filament resistance  14  and measurement resistance  16  by injecting onto node  11  a voltage which is digitally selected by a network shown in FIG. 6 under the control of microprocessor  70 . Also, this adjustment assures that the V out  signal remains positive so it can be processed by A/D circuit  60  and typically will run between 0.2 and 2 volts in a normal system. The microprocessor  70  provides eight lines of output  72 - 79  which are coupled to solid state switches S 0 -S 7  of an R2R resistive ladder network circuit  80  as shown in FIG.  6 . Each of the eight bits drives a different one of the digital switches from the microprocessor  70 , which in a preferred embodiment is an Intel 8051 microprocessor. The R2R network  80  looks at the parallel bits and each of the switches (which are AGD433 devices), depending upon the voltage V out  detected by microprocessor  70 , maintains the ratio of resistance  10  over reference filament  14  the same as the ratio of resistance  12  over measurement filament  16 . The microprocessor  70  thus receives a V out  input signal on line  62  and through a conventional trapezoidal convergence function program provides parallel bit drive signals to the resistor ladder network  80  during the time when the carrier gas is flowing through both the measurement and reference filaments of the thermal conductivity cell system. If V out  is too high, the switches S 0 -S 7  are initially adjusted relatively full on to lower the injected signal to V r  on node  11  by coupling node  11  to ground and/or through a selected group of resistors. As the V out  signal decreases, the switches are gradually switched to the V b  state until such time as the signal is below 2 volts. If the signal is below 0.2 volts, the ladder network is switched under the microprocessor control to increase the signal injected to the node  11  by coupling more of the switches selectively in sequence to the V b  bus  46 . This process typically is conducted initially to compensate for the differences in resistance between filaments  14  and  16 . Once the resistive ladder has been programmed through the microprocessor, typically it need not be changed except with aging of the filaments  14  and/or  16  or in the event the V out  signal becomes too low. 
     In order to protect filaments  14 ,  16 , reference protection circuit  90  is provided which is shown in detail in FIG.  7 . Circuit  90  provides a signal to the bridge drive circuit  40  which, in the event the reference filament tends to overheat and, therefore, may oxidize, will reduce the drive voltage V b  until the cause of the overheating situation is corrected. Typically, this would occur in the event of an interruption of carrier gas flow or an inadvertent opening of the flow path. 
     Circuit  90  comprises a clamping diode  94  coupled to the output of an operational amplifier  91 , which receives an input signal along conductor  92  which is a voltage V b  divided by resistive voltage divider including resistors  93  and  95 . Resistor  95  has a slightly greater value than resistor  93 , such that a signal is, therefore, provided which is somewhat higher than V b /2 at input  92  to amplifier  91 . V r , on the other hand, is applied to the remaining input of the operational amplifier  91  through an integrator including resistor  97  and capacitor  99 . Typically, resistor  93  will be a 1 kOhm resistor and resistance  95  is selected to be a 1.2 kOhm resistor. In the event the carrier gas flow is interrupted and the resistance filament  14  increases, V r  exceeds the artificially increased V b /2 signal at conductor  92 , the output of amplifier  91  will drop, allowing diode  94  to conduct, thereby clamping the input signal at the positive input of operational amplifier  44  in the bridge drive circuit  40  to a lower voltage, thus reducing V b  to a level at which the temperature of the filaments  14  and  16  will not oxidize. Typically during normal operation, the voltage at the output of amplifier  91  will be higher than the signal at the positive input terminal of amplifier  44  and diode  94 , therefore, the reference protection circuit  90  will have no effect on the control of V b  by the constant resistance circuit  30 . If, however, the resistance of the reference filament  14  increases due to overheating, the bridge drive voltage V b  will be reduced to a level which prevents oxidation of the reference filaments  14  and  16 . 
     As can be seen by one skilled in the art, the various circuits can be employed individually or collectively in the overall circuit as shown, for example, in circuit  5  in FIG. 3 to provide an improved thermal conductivity cell control circuit which has extremely high linearity, low noise, and which controls for aging of the filaments as well as protecting filaments from oxidation. 
     It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.