US3470355A - Filament protection for hot wire detectors - Google Patents

Filament protection for hot wire detectors Download PDF

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Publication number
US3470355A
US3470355A US568872A US3470355DA US3470355A US 3470355 A US3470355 A US 3470355A US 568872 A US568872 A US 568872A US 3470355D A US3470355D A US 3470355DA US 3470355 A US3470355 A US 3470355A
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United States
Prior art keywords
detector
filament
circuit
filaments
bridge
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Expired - Lifetime
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US568872A
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English (en)
Inventor
Wilson P Ralston
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Applied Biosystems Inc
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Perkin Elmer Corp
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
    • G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
    • G01N27/14—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of an electrically-heated body in dependence upon change of temperature
    • G01N27/18—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of an electrically-heated body in dependence upon change of temperature caused by changes in the thermal conductivity of a surrounding material to be tested
    • G01N27/185—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of an electrically-heated body in dependence upon change of temperature caused by changes in the thermal conductivity of a surrounding material to be tested using a catharometer

Definitions

  • a detector 4circuit arrangement includes a detector filament and a reference filament arranged in a balanced bridge circuit and means for causing a heating current to fiow in the filaments.
  • Circuit means are provided for sensing variations in the equivalent resistance (Re) of the bridge circuit and for automatically reducing heating current to the bridge circuit when the resistance (Re) exceeds a preselected limiting value (RL).
  • Circuit means are also provided for sensing resistance unbalance of detector and reference filaments and for automatically varying the amplitude of heating current fiowing in the filaments in a manner for limiting the resistance ratio of the filaments. Through this arrangement a limit on the resistance of each filament is established thus inhibiting detector filament burnout.
  • This invention relates to detector arrangements employed with chromatographic apparatus.
  • the invention relates more particularly to a thermal conductivity type of gas chromatographic detector.
  • Thermal conductivity detectors utilized with gas chromatographic apparatus generally include one or a plurality of detector wire filaments and one or a plurality of reference wire filaments mounted in a heated detector block and arranged in a balanced bridge circuit. Circuit means cause a heating current to flow in the filaments.
  • the component Upon separation by the column of a component of a sample being analyzed, the component is carried over the detector filaments and decreases thermal conductivity between the detector filaments and the detector wall.
  • the decrease in thermal conductivity increases the temperature and thus the resistance of the detector filaments by an amount proportional to the concentration and inversely proportional ⁇ to the thermal conductivity of the separated component.
  • the bridge circuit thereby becomes unbalanced and an indication as to the quantity of a separated component is provided.
  • the sensitivity of the bridge circuit is greater when the average heating current flowing in the laments is relatively large. However, a relatively large current can result in overheating and cause filament burnout. This is true even though the filaments are carefully wound and the resistance deviations between the filaments is held to practical manufacturing limits.
  • detector filaments such as thermistors which exhibit a negative-temperature coefiicient were employed.
  • the resistance of the filament decreased upon overheating and filament power dissipation was thereby limited.
  • filament materials having nega-l tive resistance-temperature coeicients are undesirably limited in their upper operating temperature. Since it is generally desirable to operate the detector block at a temfice perature equal to or greater than the temperature of the separating column, thermistor filaments find limited use.
  • a hot wire form of thermal conductivity detector capable -of operating at the desired elevated temperature and having filaments exhibiting positive resistance-temperature coefficients has been utilized.
  • Another object of the invention is to provide an arrangement for inhibiting filament burnout in a thermal .conductivity detector of the type employing filaments having positive resistance-temperature coefficients.
  • a detector circuit arrangement includes a detector filament and a reference filament arranged in a balanced bridge circuit and means for causing a heating current to flow in the filaments.
  • Circuit means are provided for sensing variations in the equivalent resistance (Re) of the bridge circuit and for automatically reducing heating current t0 the bridge circuit when the resistance (Re) exceeds a preselected limiting value (RL).
  • circuit means are provided for sensing resistance unbalance of detector and reference filaments and for automatically varying the amplitude of heating current fiowing in the filaments in a manner for limiting the resistance ratio of the filaments. Through this arrangement a limit on the resistance of each filament is established thus inhibiting detector filament burnout.
  • FIGURE l is a diagram illustrating the general ar rangement of a gas chromatographic apparatus employing a thermal conductivity detector
  • FIGURE 2 is a diagram, partly in block and partly in schematic form, of a thermal conductivity detector and burnout inhibiting circuit arrangement constructed in accoi'dance with features of the present invention.
  • FIGURE 3 is a detailed schematic diagram of the circuit arrangement of FIGURE 2.
  • the apparatus is shown to include a separating column 10, a source of carrier gas 12, and a means comprising an injector block 14 for injecting a sample having components which are to be separated into a carrier gas stream.
  • the sample is carried through the column 10 by the carrier gas and flows to a detector means 16 for detecting the quantity of the separated components.
  • the carrier gas also fiows directly to the detector means 16 through suitable piping 17.
  • Means including a heater 18 and a source of heater power 20 are provided for causing programmed heating of the column 1G in a conventional manner.
  • the detector means indicated generally as 16 comprises a thermal conductivity detector having a lament mounting block, not specifically shown.
  • the block is heated by heater coil 22 which is energized by a heater supply 24 for causing the block to operate at a desired temperature, generally higher than the highest temperature of the programmed column and on the order of 400 C.
  • Reference and detector filaments are mounted in the block of the detector 16 and are indicated generally by the bridge circuit 26. In an alternative arrangement, single hot wire reference and detector filaments are arranged in a bridge circuit along with resistors of fixed value. The resistors do not perform temperature sensing functions.
  • Output terminals 32 and 34 of the bridge are coupled to a chart recorder 36 or other desired recording means for recording in a desired manner the relative magnitudes of the detected peaks.
  • the carrier gas is passed directly over reference laments of the detector 16 while the carrier gas and separated component of a sample are coupled to the block f6 via a tubulation 30 and passed over the detector filaments.
  • the detector bridge circuit including the detector and reference filaments is illustrated in greater detail in a circuit arrangement adapted for inhibiting burnout of the filaments.
  • the detector bridge circuit comprises the detector filaments 40 and 42 forming opposite arms of a balanced bridge circuit and the reference filaments 44 and 46 also forming opposite arms of the bridge circuit.
  • circuit means are provided for sensing an increase in the equivalent resistance (Re) of this bridge circuit, and for reducing the current to the bridge circuit when (Re) exceeds a pre-established limiting value (RL).
  • the detector bridge circuit itself is coupled as one arm of another bridge circuit, referred to hereinafter as the limiting bridge circuit.
  • the limiting bridge includes the impedance arms comprising resistances 48, 50 and 52.
  • a filament heating current is derived from a voltage source 53 and is applied to the limiting bridge via a constant current regulator 54 and the terminals 55 and 56.
  • the impedance values of resistances 48, 50 and 52 establish an initially first unbalanced limiting bridge condition when (Re) is less than (RL), a balanced limiting bridge condition when (Re) equals (RL), and a second unbalanced limiting bridge condition when (Re) exceeds (RL).
  • the first unbalanced condition for example, establishes output sensing terminal 62 relatively positive with respect to terminal 60 while terminal 62 is relatively negative with respect to terminal 60 when the second unbalance condition occurs.
  • the relationships between the bridge impedances for a balanced condition are expressed by:
  • a differential amplifier 58 is coupled between the output sensing terminals 60 and 62 of the limiting bridge and is arranged to provide an output voltage indication when (Re) exceeds the pre-established limiting value (RL).
  • the detector and reference filaments are carefully wound during manufacture and closely matched within reasonable manufacturing tolerances. 'Their cold resistance is generally on the order of 30 ohms.
  • the flow of filament heating current causes the filament resistance to i11- crease and the power dissipated by these filaments increases accordingly.
  • An increase in resistance beyond some safe value increases the likelihood of burnout.
  • a safe equivalent resistance (Re) equal to (RL) for the detector bridge can thus be determined.
  • the filaments may exhibit hot resistances providing a safe equivalent resistance (Re) up to 80 ohms at a temperature of 550 C.
  • the equivalent limiting resistance (Re) of the detector bridge is thus established and the resistances 48, 50 and 52 can be selected to satisfy the Equation 1 and to satisfy other circuit requirements.
  • the limiting bridge becomes balanced, and the balance is sensed by the differential amplifier 58.
  • Power to the detector bridge circuit is automtaically reduced as (Re) exceeds (RL).
  • RL the constant current regulator circuit 54.
  • This regulator operates in a conventional current regulating mode when (Re) is less than (RL).
  • the regulator current amplif tude is controlled by a control circuit 68.
  • the differential amplifier 58 provides an output Voltage indication which is applied to an amplifier and switch circuit 70.
  • the amplifier and switch circuit cause the regulator control 68 to operate on the constant current regulator 54 and interrupt current to the bridge circuit.
  • the amplifier and switch circuit 70 operates as a manually resettable bi-stable device for controlling the regulator.
  • a reset button 72 is provided for reapplying power to the bridge circuit after power has been interrupted.
  • circuit means are provided for sensing the magnitude of detector bridge unbalance in excess of the magnitude of unbalance for a pre-established sample concentration and for varying the current' fiow to the detector bridge circuit in a manner for limiting the excessive unbalance.
  • This .circuit means includes a differential amplifier 74 and means comprising resistances 84 and 86, the impedances of which are selected for establishing a limit for filament unbalance.
  • An exemplary value for the ratio RSG/R84 is 1.06 which provides limiting for concentrations greater than 10% for an air 'sample in a helium carrier gas.
  • the differential amplifier 74 then functions to reduce the current flow to the detector bridge by an amount suiiicient to reduce filament heating and reduce the detector filament-reference filament resistance ratio to within the limiting range. Output voltages of the differential amplifier 74 are coupled to the regulator control 68 for effecting the desired reduction in heating current.
  • FIGURE 3 a particular circuit arrangement of the filament burnout inhibiting means of FIGURE 3 4is illustrated. Elements in FIGURE 3 which perform similar functions as elements in FIGURE 2 bear similar reference numerals.
  • the limiting bridge differential amplifier 58 is shown to comprise NPN transistors 100 and 102 having emitter electrodes connected to a collector electrode of a transistor 88 operating as a constant current source. Operating voltages are established for maintaining transistor 102 in a collector current cutoff state when (Re) is less than (RL). An increase in the equivalent resistance (Re) of the detector bridge circuit to and in excess of the value (RL) causes a voltage at terminal 60 to increase in a positive sense and results in a corresponding liow of collector current in the transistor 102.
  • the varying voltage at collector electrode of transistor 102 is applied via an output terminal 107 to the amplifier and switch stage 70. This stage, described in greater detail hereinafter, is indicated in FIGURE 3 by the circuit within the dotted rectangle 70.
  • the detector bridge differential amplifier 74 employs a common reference transistor, 164.
  • One differential amplifier including transistors 152 and 164 senses the voltage between output terminals 32 and 33 while the other differential amplifier including transistors 154 and 164 sense the voltage between terminals 34 and 33.
  • the collector current of the differential amplifier increases and a limiting action is initiated.
  • Voltages occurring at output terminals 170 and 172 of these amplifiers are applied to the regulator control circuit 68 and the detector bridge current is reduced by an amount sufficient for maintaining the pre-established filament resistance ratio.
  • transistors 152 and 164 function together as a differential amplifier and collecto-r current fiows in transistor 152.
  • Collector 'voltage is applied via output terminal 170 and isolating diode 148 to the base of transistor 144 of the regulator control circuit.
  • the transistors 154 and 164 function as a differential amplifier and an increase in the collector current of transistor 154 results.
  • the collector output voltage is applied via terminal 172 and isolating diode 150 to the regulator control circuit 68.
  • the magnitudes of collector voltage variations of transistors 152 and 154 correspond to the degree of bridge unbalance.
  • a first regulator for difference amplifier 58 comprises the ground base transistor 88 which is returned to a relatively negative voltage E through a resistor 9d.
  • a second regulator for difference amplifier 74 comprises a grounded base transistor 92 which is also returned to a relatively negative voltage by a resistor 94.
  • Adjustable impedances in the circuit are provided for bridge balancing and transistor compensation.
  • a detector bridge balancing means comprises the fine and course adjusting potentiometers 174 and 176 respectively.
  • the adjustable resistances 160 and 162 are provided for compensating 4for variations of the transistors.
  • the amplifier of the switch circuit arrangement 70 comprises a PNP transistor 108 arranged as a common emitter circuit for inverting the signal polarity.
  • the collector resistor 112 is returned to a source of potential -E and is by-passed by a capacitor 114.
  • Collector current flow in transistor 102 indicating a limiting condition causes collector current fiow in transistor 168.
  • E When a voltage developed across resistor 112 reaches the value of E, the gate terminal of a silicon controlled switch 116 is driven positive and the switch is triggered on. A negative going voltage occurs across resistor 123 and is applied to the regulator control 68 through an isolating diode 141.
  • the regulator control circuit 68 responds in a manner for controlling current flow to the bridge.
  • the silicon control switch 116 is a bi-stable device and is required to be reset.
  • a switch 72 is provided which, through capacitor 120, momentarily clamps the anode of the SCS 116 to a potential -E. This arrangement turns ofr ⁇ current fiow in the SCS 116.
  • the SCS 116 then remains in an off condition until retriggered. It will be retriggered automatically if the bridge resistance remains above the limiting resistance (Re RL); ie., if the lament fault condition has not been corrected.
  • the constant current regulator 54 of FIGURE 2 is indicated in FIGURE 3 by the circuit arrangement within the dotted rectangle.
  • This regulator is conventional and includes the series regulating transistors 124 and 126.
  • a transistor 128 supplies a current to the base circuit of the transistor 126.
  • Zener diode 130 establishes a fixed bias voltage for the base of 128 and in addition is used to drop
  • a control amplifier indicated gen erally as and including a transistor and reference voltage Zener diode responds to this variation and alters the magnitude of base current being applied to the base of transistor 126 by the transistor 128 in order to aC- cornplish the current regulating function.
  • the regulator will maintain a nominal constant heating current output which is initially determined by an operator by adjusting the tap 132.
  • the tap is adjusted to include a desired number of resistances from the group including resistors 125, 127, 129, 131, 133, 135, 137 and 139 which are connected in series in the emitter circuit of transistor 124.
  • the regulator control circuit 68 of FIGURE 2 shown within the dotted rectangle of FIGURE 3 operates, responsive to a voltage from the amplifier and switch circuit 70 or the outputs of the detector bridge differential amplifier 74, to control the magnitude of heating current.
  • a transistor 142 of this regulator circuit is coupled to the base of transistor 126 of the regulator circuit, 54. This transistor operates to vary the base current to the regulator transistor 126, in accordance with a control voltage applied to its base from transistor amplifier 144.
  • a limiting condition indicator lamp 178 is also provided and is energized by transistor 144 when the regulator control circuit 68 causes an interruption or reduction of the current to the detector bridge circuit.
  • a detector bridge circuit arrangement including a detector filament and a reference filament each having a positive resistance-temperature coefficient
  • circuit means coupled to said bridge circuit for causing a current to fiow in said filaments
  • circuit means coupled to said bridge circuit for sensing when the equivalent resistance (Re) exceeds a preestablished limiting value and for causing a reduction in the amplitude of filament current fiow when the equivalent resistance (Re) exceeds the pre-established limiting value
  • circuit means coupled to said bridge circuit for sensing when a ratio of filament resistances exceeds a preestablished value and for causing a reduction in the amplitude of filament current flow when the ratio of filament resistances exceeds the pre-established value.
  • a detector bridge circuit arrangement including a detector filament and a reference filament each having a positive resistance-temperature coeflicient
  • circuit means for applying a heating current to said filaments
  • said bridge circuit having an equivalent resistance (Re) which increases as the resistance of a filament increases
  • circuit means coupled to said bridge circuit for sensing When the equivalent resistance (Re) exceeds a preestablished limiting value and for reducing the amplitude of current applied to the detector bridge circuit when the equivalent resistance (Re) exceeds the preselected value.
  • a balanced detector bridge circuit including a first resistive Idetector filament and a first resistive reference filament each having a positive temperature coefficient and arranged as arms of the bridge circuit,
  • said detector bridge circuit having an equivalent resistance (Re),
  • impedance means arranged as an initially unbalanced limiting bridge circuit and including said detector bridge circuit as an arm thereof,
  • said limiting bridge having sensing terminals providing voltages therebetween having relative amplitudes indicative of unbalance in a first sense when (Re) is less than a preestablished limiting value (RL) and unbalance in an opposite sense when (Re) is greater than (RL):
  • circuit means coupled between said sensing terminals and arranged for reducing filament heating current when said limiting bridge becomes unbalanced in the second sense.
  • a detector bridge circuit including a detector filament and a reference filament each having a positive resistance-temperature coefficient
  • circuit means coupled to said bridge circuit for causing a heating current to flow in said filaments
  • circuit means coupled to said bridge circuit for sensing when a ratio of detector and reference filament resistances exceeds a pre-established value and for controlling the heating current in a manner for limiting the ratio of detector and reference filament resistances to a pre-established value.
  • first and second resistive detector filaments and first and second resistive reference filaments arranged as opposite arms of a balanced detector bridge circuit
  • said bridge circuit having an equivalent resistance circuit means for applying a filament heating current to said filaments
  • circuit means including a differential amplifier coupled between arms of said detector bridge and arranged for detecting an unbalance of said detector bridge circuit and varying the heating current in a manner for limiting a ratio of detector to reference lament resistances to a pre-established value
  • circuit means including first, second and third resistances and said detector bridge circuit arranged as a limiting bridge circuit
  • circuit means including a differential amplifier coupled between arms of said limiting bridge circuit for detecting when (Re) exceeds a pre-established value (RL) and for interrupting filament heating current.

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  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Physics & Mathematics (AREA)
  • Health & Medical Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
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  • Investigating Or Analyzing Materials By The Use Of Electric Means (AREA)
  • Investigating Or Analyzing Materials Using Thermal Means (AREA)
US568872A 1966-07-29 1966-07-29 Filament protection for hot wire detectors Expired - Lifetime US3470355A (en)

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US56887266A 1966-07-29 1966-07-29

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US568872A Expired - Lifetime US3470355A (en) 1966-07-29 1966-07-29 Filament protection for hot wire detectors

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US (1) US3470355A (enExample)
CH (1) CH482188A (enExample)
DE (1) DE1648276A1 (enExample)
FR (1) FR1605449A (enExample)
GB (1) GB1189009A (enExample)
NL (1) NL6710467A (enExample)
SE (1) SE349865B (enExample)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2456950A1 (fr) * 1979-05-14 1980-12-12 Sereg Soc Catharometre

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
NL7906868A (nl) 1979-09-14 1981-03-17 Gould Godart Bv Gasanalysator.

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2545343A (en) * 1946-02-18 1951-03-13 Bendix Aviat Corp Electric motor control system
US3040157A (en) * 1959-09-30 1962-06-19 North American Aviation Inc Temperature control servo system
US3197616A (en) * 1963-03-04 1965-07-27 Donald W Gade Temperature regulation circuit
US3369107A (en) * 1965-04-07 1968-02-13 Neff Instr Corp Temperature regulating circuit for an ink reservoir

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2545343A (en) * 1946-02-18 1951-03-13 Bendix Aviat Corp Electric motor control system
US3040157A (en) * 1959-09-30 1962-06-19 North American Aviation Inc Temperature control servo system
US3197616A (en) * 1963-03-04 1965-07-27 Donald W Gade Temperature regulation circuit
US3369107A (en) * 1965-04-07 1968-02-13 Neff Instr Corp Temperature regulating circuit for an ink reservoir

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2456950A1 (fr) * 1979-05-14 1980-12-12 Sereg Soc Catharometre

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NL6710467A (enExample) 1968-01-30
DE1648276A1 (de) 1971-07-22
FR1605449A (en) 1976-01-09
CH482188A (de) 1969-11-30
SE349865B (enExample) 1972-10-09
GB1189009A (en) 1970-04-22

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