Method and apparatus for the measurement of the thermal conductivity of gases

Abstract: New methods and new katharometer apparatus for the measurement of thermal conductivity of gases are provided. The apparatus employs a single katharometer element (10), that is maintained at a constant operating temperature. If the element (10) temperature changes, changing its resistance, owing to passage of a gas with a different thermal characteristic, an amplifier (32) changes the power supplied to the katharometer element (10) to restore it to the predetermined temperature, the amplifier (32) output being a measure of the gas thermal conductivity. Operation of the apparatus is therefore independent of the resistance/temperature characteristic of the element (10) and is relatively insensitive to ambient temperature changes. Preferably a thermistor is employed as the katharometer element (10). Various operating circuits for the katharometer are disclosed. The invention also provides new operating methods for katharometers to reduce the sensitivity of measurement to thermal leakage and temperture changes to the katharometer body, and to permit the comparison of voltages across a series connected katharometer element (10) and a resistor (18) at a common reference point.

Description: FIELD OF THE INVENTION This invention is concerned with new methods and new apparatus for the measurement of the thermal conductivity of gases. It is concerned particularly but not exclusively with such methods and apparatus for the measurement of the composition of gas mixtures by measurement of their thermal conductivity, as employed in the measurement of the gas content of molten metals. REVIEW OF THE PRIOR ART A commercially important application of the measurement of gas thermal conductivity is the determination of the amount of gas, particularly hydrogen, in a body of molten metal, particularly aluminium and its alloys. The presence of more than a predetermined small amount of hydrogen (e.g.0.1-0.15ml H₂ per 100g metal) can have deleterious effects on the properties of the metal, and accurate measurement is therefore necessary to ensure that the content is below this value. In practice a suitable porous probe, such as that disclosed in our European Patent Publication No. A1 0295 798, published 21st December 1988, is immersed in the molten metal and a carrier gas such as nitrogen is circulated in a closed loop between the probe and a katharometer. Gases dissolved in the metal are entrained in the carrier gas in proportion to their concentration in the metal, and if the thermal conductivities of the entrained and carrier gases are sufficiently different, then measurement by the katharometer of this parameter for the carrier gas alone, constituting the reference gas, and for the resultant mixture, constituting the test gas, can be used to determine the concentration of the gas dissolved in the metal. One type of katharometer apparatus commonly employed hitherto uses two cells electrically connected as two opposed arms of a resistance bridge, one of the cells receiving or containing a reference gas and constituting a reference cell, while the other receives a stream of the test gas to be measured and constitutes the measuring cell. Each cell contains a fine heated platinum wire whose resistance depends upon its temperature, the amount by which the wire is cooled upon passage of the gas through the cell depending upon the gas thermal conductivity, which will usually vary with the gas composition because of the different values for the different gases. The resultant change in the resistance of the measuring cell unbalances the bridge, and the value of the resulting unbalance voltage is a function of the thermal conductivity of the test gas. The manufacture and operation of katharometer apparatus to give consistent results presents a number of difficulties. It is difficult in the first place to produce commercially two katharometer cells with sufficiently similar static and dynamic characteristics to provide a bridge that can be balanced without the need for static and dynamic correcting circuit elements. The two cells should be kept as closely as possible at the same temperature, but this is difficult to achieve when the filament of the measurement cell inherently varies in temperature to provide the necessary unbalance. It is usual therefore to try to maintain the two cells at some standard temperature so as to match their responses as closely as possible. A typical range of hydrogen gas concentration in molten aluminium is 0.1 to 0.3 m1H₂/100g corresponding to 1%-9% by volume in the carrier gas, but it is possible for the percentage to be as high at 25%, and it is not unknown for this type of katharometer to be unable to measure values above 0.4, so that accurate measurement of these higher values becomes impossible. Attempts, have been made to avoid this problem by providing a katharometer using a single cell. U.S. Patent No. 4,685,325 discloses such a single cell katharometer in which the cell is supplied with current from a constant current source to heat its filament. A balancing circuit is connected across the cell to balance the current against this constant current source, so that the output voltage is zero when the carrier gas alone is passing through the cell, the voltage change developed across the filament being a function of the proportion of hydrogen in the carrier gas. A textbook by R. Bock, Methods of Analytical Chemistry, and Introduction, Part 2, published 1984 by Verlag Chemie GmbH, contains a description of apparatus and methods for the measurement of the thermal conductivity of gases employing a katharometer in which the temperature sensitive katharometer element is a thermistor. The circuit disclosed employs two detectors connected as parallel elements of a bridge. The use of a thermistor as katharometer element is also known from J.P. Bentley, Principles of measurement systems, published 1983 by Construction Press. In EP-A-0234251, there is described apparatus and methods for detecting combustible gases, the apparatus comprising a single sensor for carrying out a thermal conductive measurement that is supplied with variable current to maintain its temperature constant. The apparatus includes an additional feature of changing this constant temperature to another value dependent upon the composition of the gas under test. Thus the detector is switched between high and low temperature operation, the operation beginning with the low temperature setting and switching to the high temperature setting as the element is heated by the combustion of the detected gases. In EP-A-0264554, there is disclosed apparatus for the automatic calibration of a gas sensor employed in a system for the continuous injection of a gas, such as an anaesthetic gas, by an injector into a carrier gas. The injector and detector are connected in series and the apparatus employs a gas switching system which reverses the flow of gas through them for the calibration. US-A-4,316,382 discloses a katharometer comprising a chamber having a filament suspended therein that is connected to a filament drive circuit. A voltage that is required to keep the filament at a constant temperature is developed by the filament drive circuit. This output voltage thus depends on the thermal conductivity of the gas in the chamber. It is the principal object of the present invention to provide new methods for the measurement of gas thermal conductivity employing katharometers. It is another principle object to provide new katharometer apparatus for such measurement employing a single temperature sensitive katharometer element. In accordance with a first aspect of the present invention there is provided an apparatus for measurement of thermal conductivity of a test gas as defined in claim 1. In an embodiment the control means comprises a resistance connected in series with the katharometer thermistor element; means for supplying electrical power to the thermistor element and the resistance in series to produce voltages across each of them corresponding to their respective resistances; means for transferring at least one of the voltages thus produced as a first transfer voltage to a common reference point of the circuit with the other voltage; and means for comparing the two voltages having the common reference point and controlling the supply of electrical power to the thermistor element and resistance in series in accordance with the comparison of the two voltages. In accordance with a second aspect of the invention there is provided a method as defined in claim 7. Particular preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, wherein:- Figure 1 is a schematic circuit diagram of a first embodiment; Figure 2 is a perspective view of a katharometer illustrating the use of isothermal heat sink plates to stabilise the temperature of the leads of the katharometer element; Figure 3 is a schematic diagram of an apparatus for measuring the gas content of a molten metal; Figures 4A and 4B illustrate respectively the manner in which a thermistor katharometer element and a heated filament katharometer element are mounted in a katharometer body in the prior art; Figure 5 is a graph of a factor B for a thermistor katharometer element at different ambient temperatures; Figure 6 is a graph showing the response of a circuit of the invention to change of hydrogen content of a test gas; Figures 7 and 8 are respectively schematic circuit diagrams of second and third embodiments. The apparatus of the invention specifically described herein employs a thermistor 10 as a temperature sensitive katharometer element whose electrical resistance varies with its absolute temperature, with the absolute value of its resistance being sufficiently constant for a predetermined temperature value. In the embodiments illustrated the thermistor is mounted in the usual relatively heavy metal katharometer enclosure 12 having a gas inlet 14 and a gas outlet 16, so that respective streams of test and reference gases can be passed through the enclosure interior. In this embodiment the element is connected in series with a fixed resistor 18 of resistance R to provide a first potential divider having a junction 20 at which a voltage V₁ will appear, the divider being supplied with electric power from a source 22. Two series-connected fixed resistors 24 and 26, respectively of resistance R₁and R₂, constitute a second reference potential divider, also supplied from the source 22, and have their junction 28 connected to one input terminal 30 of a differential amplifier 32 so as to apply a reference voltage V₂ thereto, the junction 20 being connected to the other amplifier input terminal 34. The amplifier is supplied with power from terminals 36 and a start-up load resistor 38 is connected between the source 22 and the circuit to set the start-up voltage (positive or negative) that is applied to the cell. The thermistor 10 will be heated by the current passing through it and as its temperature increases its resistance decreases, increasing the value of V₁. If the potentials V₁ and V₂ are not equal the amplifier 32 produces a change in its output voltage proportional to their difference that heats the thermistor 10 further, and thus further decreases its resistance until balance is reached, at which point the element is at a steady temperature and corresponding steady resistance value. In this steady state a constant voltage V₀ will be produced at the output terminal 40 of the amplifier and can be measured between the output terminals 42. If a stream of a gas of higher thermal conductivity is now introduced into the katharometer the thermistor 10 cools resulting in an increase in its electrical resistance so that V₁ decreases, which results in an increase in the voltage V₀ and an increase in the current through the thermistor and resistor 18 to increase the electric power (energy per second) supplied to the thermistor until its temperature and resistance are restored to the predetermined values. The new value of the voltage V₀ is correlated with the gas thermal conductivity of the mixture of gases in the stream by the relation: where: R is the resistance of resistor 18; R₁ is the resistance of resistor 26; R₂ is the resistance of resistor 24; G is a geometrical constant of the katharometer based on the geometry of the katharometer cell and of the thermistor and its disposition in the cell; T t is the temperature of the thermistor 10 and is to be as constant as possible; T b is the ambient temperature of the katharometer body measured in degrees Kelvin; K L is an equivalent gas thermal conductivity corresponding to the heat loss (leakage) by the thermistor electrical leads and is to be maintained as small and constant as possible; K i is the equivalent gas thermal conductivity corresponding to the thermal resistivity of the thermistor 10 and is therefore a constant; and K m is the thermal conductivity of the test gas to be determined and is therefore a variable. This equation contains a thermal model describing the operation of the katharometer circuit just described and can be regarded as comprising three parts. The left hand part involving resistance values describes the electrical dependence of the configuration of the circuit. The centre part involving temperature values embodies an important consequence of the fact that the value of T t is constant, which means that either T b must be known accurately, using an independent measuring instrument such as a thermometer, or the ratio of two measurements taken very close together in time is used, when T b has not changed substantially if at all and this factor can be eliminated from the final result. The right hand part of the equation describes a specific simplified but sufficiently accurate thermal model of the operation of the katharometer. One of the parameters is the unknown value Km to be measured, and it is necessary therefore to obtain suitable values for K i and K L ; these are obtained from measurements of any three known gases, preferably nitrogen, argon and helium (or hydrogen) made at a given temperature, the values being obtained from the ratio of the resultant Vo values. Since the left hand part of the relation is dependent upon the specific electrical circuit of the katharometer it can itself be regarded as a circuit constant, when the relation will have the more general form below that is applicable also to other types of katharometer: The total electric power that is supplied to the element 10 can be determined by the relation: Power = V₀² R · R₁ · R₂ (R₁ + R₂)² Since it is the amount of power (energy per second) that is supplied to the thermistor 10 that maintains its temperature, either the voltage or the current can be measured to obtain a measurement representative of the gas thermal conductivity, since all of the resistances in the circuit are known and are of constant value, the measurement of the voltage usually being preferred. The value of K L can be kept at a substantially constant value by an arrangement such as that illustrated by Figure 2, in which the lead wires 44a and 44b to the thermistor are kept as short as possible and welded independently to respective isothermal heat sinks 46a and 46b, constituted in this embodiment by copper plates, the plates being cemented to a support block 48 attached to the enclosure 12 and electrically insulated from the enclosure and from one another. It will be noted that it is immaterial whether V₀ is positive or negative, and this has no effect on the measurement of Km, since the term V 2 / 0 appears in the relation (1). In a specific embodiment the thermistor 10 was obtained from Gow-Mac Corporation, having an internal resistance of 8 Kohm at 25°C. The value of R was 1K and that of R₁ and R₂ was 10K all three being metal film type of 1% tolerance with thermal coefficient of ± 50 ppm per °C. The value of resistor 38 was 22K. The amplifier was type LT1013AM and the values of V⁺ and V⁻ were respectively +15 and -15 volts. In using the katharometer to measure the percentage of hydrogen in molten aluminium it is connected in a closed circuit with a porous probe, such as that described in our above-identified Patent Publication. Referring now to Figure 3 there is shown therein a probe element 74, consisting of a monolithic body of gas-permeable, liquid-metal-impervious material, immersed in a body 76 of molten metal, specifically of molten aluminium or an alloy thereof. The body 74 may be stationary, as would be obtained in a ladle or a laboratory sample, or it may be a stream of metal, as would be obtained in a transfer trough leading from a casting furnace. A fine bore tube 78 extends from a gas inlet in the body of the probe element to a recirculation pump 80 via a non-return valve 82, and thence via another non-return valve 84 to the gas outlet 16 of the katharometer. Another fine bore tube 86 extends from a gas outlet from the body 74 to the gas inlet 14 to the katharometer, so as to complete a closed circuit including the probe element, the pump and the cell. The tube 30 includes a T-junction by which the gas circuit is connected to a controllable flushing valve 88 which when opened admits a carrier gas, usually nitrogen, into the circuit from a suitable source, usually a cylinder of the compresssed gas (not shown). The katharometer cell is connected to its control circuit 90, which in turn is connected to controller computer 92. A thermocouple 94 is mechanically connected to the probe element so that it is immersed therewith into the molten metal 76 and provides the necessary measurement of the metal temperature. The thermocouple 94, the pump 80, and the flushing valve 88 are also connected to the computer controller 92 which is arranged to automatically control the apparatus through each concentration determining cycle of operations, and to feed the results of the cycle to one or more display and/or recording devices which will be apparent to those skilled in the art. A typical measurement cycle will begin with the flushing valve 88 being opened by the controller 92, so that dry nitrogen under pressure circulates through the entire circuit, entering at both the probe gas inlet and the outlet and exiting through the porous body of the probe element; this circulation is maintained long enough to ensure that only nitrogen remains in the circuit. The flushing operation is maintained until the probe has been lowered into the melt when the valve 88 is closed and the pressure of the nitrogen in the circuit will quickly reach a steady value. The operation of the pump motor 80 causes the volume of carrier gas in the circuit to be constantly recirculated therein. A first reading of V₀ is taken while the dry nitrogen is circulating. As the gas is circulated continuously between the probe and katharometer hydrogen from the aluminium accumulates in the nitrogen carrier gas until equilibrium is reached, based on the respective partial pressures, this usually taking about ten minutes; a second reading of V₀ is then taken, from which the gas thermal conductivity is determined. This is also the operating procedure used with prior art apparatus. During this relatively long period the temperature of the block 48 and also of the enclosure 12 can change by several degrees, with the result that the first reading is no longer a valid zero reading. Accordingly in a method of the invention a first reading is taken after the carrier nitrogen has been circulated until the equilibrium with the entrained hydrogen is obtained; immediately after taking this reading pure nitrogen is again injected into the circuit by opening the valve 88 to purge the cell of the gas mixture and a second reading is taken about 10-30 seconds, preferably about 15-20 seconds, after the first reading. The katharometer body 12 is a relatively large heat sink and any changes in its temperature therefore take place very slowly, so that the effect of these changes is minimized. The hydrogen concentration is determined by calculation from the two closely-timed thermal conductivity readings that are obtained; since the temperature differences are minimized the precision of measurement is increased. The new apparatus thus uses a single thermal element and precision can be maintained even with relatively wide changes in the ambient temperature of the katharometer between 10°C and 60°C. Such a wide temperature variation is encountered in the field environment of an industrial operation, for example in an aluminium melting installation where the apparatus must be used close to the furnace of the metal transfer runner. This precision can be obtained by use of a single measurement provided the temperature of the katharometer body is known to within 0.01°C, or if two closely timed readings are compared as just described above. It will be seen in particular that the control of the temperature of the thermistor 10 is simple in that it is only necessary to control its electrical resistance; the device can be operated at any convenient temperature above the gas temperature within its normal temperature range and it is only necessary for it to be maintained constant at that temperature. The lower thermal conductivity values can be measured with absolute precision to within ± 0.03%. The corresponding precision of percentage of hydrogen in the hydrogen/nitrogen gas mixture is about 1% relative on a 1% hydrogen mixture. The signal level obtainable is dependent on the resistance values of the components, particularly that of the thermistor 10, and is independent of its resistance/temperature characteristic. It is also possible to measure the higher values (up to 100% hydrogen) with adequate dynamic signal range and without saturation of the associated amplifier. With prior art apparatus a substantial inaccuracy was caused by the fact that the measurements were made at temperatures which were not necessarily constant from measurement to measurement, and the thermal conductivities of all gases change with temperature; with the methods and apparatus of the invention the measurements are effectively ratios at the same temperature and these differences therefore disappear to make the measurements virtually temperature insensitive over the operating range. The desired performance characteristic for industrial test equipment used to determine the hydrogen content of aluminium is the ability to measure, under field conditions, over the ambient temperature range of 10°C-60°C, such contents to 1% relative to 1% concentration of the hydrogen in the nitrogen carrier gas at 60°C. Such precision has previously been only achievable under laboratory conditions, but has been achieved under field conditions with the methods and apparatus of the invention. Thermistors which are solid state temperature sensitive devices, usually consisting of a small bead of ceramic material, are particularly advantageously used as the temperature sensitive element of katharometers. In general their resistance correlates accurately and uniformly with their corresponding operating temperature and they are readily commercially available with a wide range of temperature/resistance characteristics such that it is possible to accomodate the necessary broad operating temperature range of 10°C-60°C. They are also readily available with a temperature/resistance characteristic that is negative over the desired operating range, (i.e. the resistance increases with increase in temperature), as contrasted with the positive characteristic of the hot filament katharometer elements used hitherto, and this simplifies the design of the accompanying electronic circuit. They are moreover more physically rugged than a heated filament. Thermistors of bead form have a specific thermal advantage in a katharometer following from their structure, as is illustrated by Figures 4A and 4B, which show respectively a typical mounting for a thermistor and a heated filament in the katharometer body. The physically small bead thermistor body 10 is mounted between the two relatively thick terminal rods and 44b, usually of diameter about 1.25mm (0.05in), by its two relatively thin terminal wires 50 of about 0.025mm (0.001in) diameter which are soldered to the rods. The heat generated in the thermistor by the current has therefore a relatively high resistance leakage path through the wires 50, as compared to the leakage path to the gas passing through the katharometer. Referring to equation (1) it will be seen that the value of K L is therefore small compared to K m and K i , while since K m is much greater than K L and smaller than K i for the thermistor the resultant variation in total thermal conductivity is large, with correspondingly large variation in the total voltage. The heated filament has no such small heat flow throttling wires, but is soldered directly to the heavy terminal rods 44a and 44b, from which the heat generated is transferred easily to the exterior. Consequently the leakage value K L is high and in practice will be many times the value of K m . The filament is usually of metal and consequently K i is also very high with the result that most of the heat produced tends to be drained to the exterior through the terminal rods without being removed by the circulating gas. The changes in K m from changes in the gas cause only small variations in the total thermal conductivity, resulting in only small variations in the output voltage. This can only be compensated by increasing the gain of the amplifier, and the variation obtained may be below the input offset voltage required for satisfactory operation of the amplifier to give the sensitivity required. Since the value of T t (thermistor temperature) in equation (1) is to be constant the value of its resistance is also correspondingly constant, and in order to obtain the greatest output voltage V₀ the thermistor resistance should therefore be as high as possible. A limitation exists in that the temperature/resistance characteristic of most commercially available thermistors reverses above a certain threshold temperature, with the reversal beginning earlier as the operating temperature increases. The thermistor is therefore chosen with the threshold temperature at 60°C to give maximum resistance value at this temperature, it being accepted that the resistance at the other lower operating temperatures will be lower. Figure 6 shows a series of curves at different ambient temperatures for a value B plotted against the resistance values R t of the thermistor, where B is given by the following expression derived from equation (1) B = R t (T t -T b ) The thermistor is chosen with the ambient temperature at 60°C to give maximum value of B at this temperature, it being accepted that the value at the other lower ambient temperatures will be higher. It will be seen that in this specific example the threshold value of R Th at 60°C is 960 ohms with a value of B = 231; at the lower ambient temperature of 15°C the value of B is 373.8 and these are the maximum and minimum values of this portion of equation (1); for a given value of R t (ambient), which can be obtained with the method and apparatus of the invention, this becomes a constant. It follows that the final expression K L + K i · K m K i + K m in equation (1), which sets out the total thermal conductivity as seen by the circuit, becomes the only variable, and with K L held to a neglible value by the use of a thermistor the resultant plot of the value of this expression against hydrogen content becomes highly linear, as shown by the graph in Figure 6, in which the ordinate represents specific numerical values of this final expression. The curve is taken below zero to be able to represent the values obtained with gases other than hydrogen, e.g. argon as used for calibration purposes will give about the same value as -13% H₂. At low hydrogen concentrations the value of K L is about the same order of magnitude as K m and the slope is greater; in the middle range the graph is virtually linear, while at high values the value of K m is of the same order of magnitude as K i which is constant and therefore the linearity is maintained. It will be noted moreover that the graph maintains a substantial positive slope over its entire range up to 100% hydrogen, so that there is no limitation from this aspect on the measurement of high hydrogen concentrations. The sensitivity of the circuit is given by the slope of the graph in Figure 6 and it can be shown that for this particular apparatus the variation of V₀ to give the required 1% precision in measurement is about 227 microvolts; such a value is within the capabilities of many low cost operational amplifiers. Such low cost amplifiers are also able to meet the modest power requirements required by a katharometer utilizing a thermistor of 30 volts total and about 10 mA output current. It will be seen therefore that the katharometers of the invention, particularly those using thermistor elements, below, have the desirable characteristics that they are stable and easy to operate and calibrate, provide the desired 1% precision of operation, can measure up to 100% hydrogen content of the test gas without the need to switch ranges, are operable in the desired ambient temperature range of 10°C to 60°C. The thermistor-using apparatus can operate with a requirement for less than 100 milliwatts of electric power that can readily be supplied by batteries in a portable apparatus. In the circuit of Figure 7 the same or a similar element is given the same reference number, wherever that is possible. The thermistor katharometer element 10 is connected in series with a resistor 18, and is connected across the input of a divider element 52 whose inputs are the potentials, V₁ and V₂ at the terminals of the thermistor element, and whose output is V₂/V₁, this output being fed to an operational amplifier 54. The value of resistor 18 is set to give a desired value (e.g. 2) to this ratio at the predetermined operating temperature and resistance of the element, and the power output of amplifier 54 is sufficient to maintain the ratio stable at that value. As the thermistor 10 cools and its resistance decreases the value of the ratio decreases, whereupon the output of amplifier 54 increases to heat the element and restore the resistance to its original equilibrium value. The value of V₀ is measured at the output of the amplifier 54. A third embodiment is illustrated by Figure 8 in which the accuracy and sensitivity of the circuit is improved by eliminating any ambient temperature effects on the amplifier and the reference resistor 18 of the two previously described circuits, permitting the use of a resistor with a temperature coefficient that is reasonably low (e.g. 50 ppm/°C) instead of requiring it to be zero, or accepting the otherwise resultant lower sensitivity. The amplifier 32 supplies operating current to the thermistor 10 and the reference resistor 18 in series, so that they both receive the same current and the voltages across them are therefore equal to their respective resistances; the curcuit is then arranged to control the value R Th of the thermistor to make it equal to the value of resistor 18 at equilibrium, when the voltages are the same, or to bring them to some convenient predetermined ratio. The series connection of the resistor and thermistor makes the comparison of their voltages difficult, since they cannot be directly referenced to a common reference point such as the ground point 56. This difficulty is overcome in the circuits of this aspect of the invention by transferring the voltage value across resistor 18 to a holding capacitor 58, and transferring the voltage value across the thermistor 10 to a holding capacitor 60. The two holding capacitors are connected between the same ground reference point 56 and the respective inputs to the amplifier 32, which can therefore more accurately produce an output such that the two voltages are equal, or are equal to the predetermined ratio. Such a circuit can control the ratio of the values of the voltages with a high degree of precision (e.g. about 2 ppm). Precision capacitors are not required and the operation of the amplifier is independent of ambient temperature. The sensitivity is determined primarily by the input sensitivity of the amplifier, and it is found that the overall precision is better and more stable than an equivalent bridge circuit. The ratio between the two resistances R and R Th must be kept small, but this is not a disadvantage since the value 1 is quite satisfactory, and it is relatively easy to select a resistance sufficiently close in value to the operating range of the thermistor. In this circuit the voltage transfers are produced by a commercially available dual switching module 62 providing two separate switch blocks 64a and 64b, each including a respective pair of "in" switches 66a and 66b and a pair of "out" switches 68a and 68b. The block 64a controls the voltage transfer from resistor 18 to capacitor 58 via a transfer capacitor 70a, while the block 64b controls the transfer from thermistor 10 to the capacitor 60 via a transfer capacitor 70b. When each two "in" switches are closed the respective "out" switches are open, and vice versa, the switches being operated to break before make in both directions under the control of an internal oscillator that synchronises the opening and closing sequences. A suitable range of operating frequency for the switch blocks is from 100Hz to 1KHz, the frequency being controlled by an externally adjustable capacitor 72. Each switch block operates as a differential voltage translator, although theoretically in the circuit as illustrated the block 64b transferring from the thermistor 10 is not essential, since it is already connected to the reference point 56. It is needed in practice in this particular circuit so that there is a capacitor at each input to the amplifier; the bias current at each input produces equal voltage variation in both capacitors and a smoother output is obtained. Each transfer capacitor is first connected across its respective resistive element and will charge or discharge until its voltage is equal to that across the element; it is then disconnected and connected to its respective holding capacitor, to which it charges or discharges until their voltages are equal. The switching sequence is repeated and eventually the voltages across each set of resistive element, transfer capacitor and holding capacitor will be equal with the required high degree of accuracy.

Claims: Apparatus for measurement of thermal conductivity of a test gas comprising a katharometer body with a single enclosure in its interior and an inlet and outlet, a single thermistor katharometer element mounted in said enclosure, electrical leads for said thermistor passing through the walls of said enclosure but electrically isolated from said enclosure and said katharometer body, means for supplying test gas to the interior of the enclosure, means for supplying electrical power to said thermistor via said electrical leads in order to heat said thermistor to a predetermined temperature, control means for maintaining said thermistor at the predetermined temperature for different thermal conductivities of said test gas, means for measuring the amount of power required to maintain said thermistor at its pre-determined temperature, and respective isothermal heat sinks (46a, 46b) connected to said electrical leads (44a, 44b) where they leave to enclosure, such that the said means for measuring the amount of power will measure a difference in said power equivalent to a change in said thermal conductivity of said test gas equal to a change of 1% in the hydrogen content of a 1% hydrogen in nitrogen gas mixture for ambient temperature changes between 10°C and 60°C. Apparatus as claimed in claim 1, characterised by the provision of thermometer means for independent measurement of the temperature (T b ) of the katharometer enclosure (12) to permit the employment of a single measurement to obtain the value of Km for the test gas. Apparatus as claimed in claim 1 or 2, characterized in that the control means (90) comprises a resistance (18) connected in series with the thermistor (10); means for supplying electrical power to the thermistor (10) and the resistance (18) in series to produce voltages across each of them corresponding to their respective resistances; means (64a, 64b) for transferring at least one of the voltages thus produced as a first transfer voltage to a common reference point (56) of the circuit with the other voltage; and means (32) for comparing the two voltages having the common reference point and controlling the supply of electrical power to the thermistor (10) and resistance (8) in series in accordance with the comparison of the two voltages. Apparatus as claimed in claim 3, characterised in that the control means (90) also includes means for transferring the other of the two voltages thus produced as a second transfer voltage to the common reference point; and the said comparing means (32) compares the first and second transfer voltages and controls the supply of electrical power in accordance with the comparison. Apparatus as claimed in claim 3 or 4, characterised in that the transferring means (64a, 64b) for the resistance voltage or the thermistor voltage comprises a transfer capacitor (70a, 70b), a holding capacitor (58 or 60) connected to the common reference point, and switch means (64a, 64b) periodically connecting the transfer capacitor (70a or 70b) in parallel with the resistance (18) or the thermistor (10) to be charged or discharged thereby, and thereafter connecting the transfer capacitor (70a or 70b) in parallel with the holding capacitor (58 or 60) for the latter to be charged or discharged thereby. Apparatus as claimed in claim 3, characterised in that the transferring means (64a, 64b) comprises for each of the resistance voltage and the thermistor voltage a respective transfer capacitor (70a, 70b), a respective holding capacitor (58,60) both connected to the common reference point, and switch means (64a, 64b) periodically connecting each transfer capacitor (70a, 70b) in parallel respectively with its resistance (18) and its thermistor (10) to be charged or discharged thereby, and thereafter connecting each transfer capacitor (70a, 70b) in parallel with its respective holding capacitor (58, 60) for the latter to be charged or discharged thereby. A method of measuring the gas thermal conductivity of a test gas employing a katharometer comprising a single katharometer enclosure having an inlet thereto for the test gas and an outlet therefrom for the test gas, the single katharometer element within the enclosure being a thermistor having a required temperature/resistance characteristic and mounted therein by a pair of leads passing through but electrically isolated from the walls of the enclosure and connected directly thereafter to respective isothermal heat sinks (44a, 44b), the method comprising supplying electrical power from a source thereof to the thermistor to heat it to a predetermined temperature value and a corresponding resistance value, passing the test gas into the katharometer enclosure over the thermistor to thereby change its temperature from the predetermined value and thereby change its resistance from the corresponding value, utilising the change of resistance of the thermistor to change supply of electrical power thereto to restore its temperature to the predetermined value and its resistance to the corresponding value, and measuring the amount of power supplied to the thermistor in the presence of the test gas with the temperature thereof restored to the predetermined value to determine the test gas thermal conductivity; whereby the katharometer enables for changes in ambient temperature between 10°C and 60°C measurement of the change in said power resulting from a change in the thermal conductivity of said test gas caused by a change of 1% in the hydrogen content of a 1% hydrogen in nitrogen gas mixture. A method as claimed in claim 7 characterised in that the temperature (T b ) of the katharometer enclosure (12) is obtained by independent measurement and a single measurement is employed to obtain the value of Km for the test gas. A method as claimed in claim 7 or 8, characterised by passing a gas mixture comprising a carrier gas having a test gas entrained therein through the katharometer (12) enclosure for a first period of time and thereafter making a first measurement; thereafter purging the katharometer with carrier gas to remove the gas mixture therefrom and making a second measurement within a short period of time after the first measurement, the two measurements being made sufficiently closely spaced in time to minimize any resultant changes in temperature of the katharometer and for T b to be correspondingly substantially constant; and thereafter comparing the first and second measurements to obtain the value of Km for the test gas with elimination of the variable T b . A method as claimed in claim 9, characterised in that the first period of time is 10-15 minutes and the second period of time is 10-30 seconds. A method as claimed in claim 10, characterised in that the second period of time is 15-20 seconds. A method as claimed in any one of claims 7 to 11, characterised in that: in the katharometer electric circuit a resistor (18) is connected in series with the thermistor (10) to a reference point (56) in the circuit; the resistor (18) and the thermistor (10) are supplied with current from the same source to establish voltages across them corresponding to their respective resistances, one of which voltages is measurable from said reference point (56); the voltage across at least the other one of the resistor (18) and thermistor (10) not connected directly to the reference point (56) is transferred to another circuit element (58 or 60) connected to the reference point (56); and the voltages having the reference point (56) in common are compared and the current supplied to the resistor (18) and the thermistor (10) is controlled in response to the comparison to maintain their resistances in a predetermined ratio. A method as claimed in claim 12, characterised in that the voltage across either the resistor (18) or the thermistor (10) is first transferred to a transfer capacitor (70a or 70b), and thereafter is transferred from the transfer capacitor (70a or 70b) to a holding capacitor (58 or 60) connected to the reference point (56). A method as claimed in claim 12, characterised in that the respective voltages across the resistor (18) and the thermistor element (10) are first transferred to respective transfer capacitors (70a, 70b), and thereafter are transferred to respective holding capacitors (58, 60) both connected to the reference point (56). A method as claimed in any one of the claims 7 to 14, characterised in that the control means and the power measuring means are calibrated using measurements of K i and K L of three known gases, where: K i is the equivalent gas thermal conductivity corresponding to the thermal resistivity of the thermistor (10); and K L is an equivalent gas thermal conductivity corresponding to the heat loss of the thermistor electrical leads (44a, 44b).

Inventors: HACHEY RAYNALD, LAMARRE DANIEL, MARCOTTE JACQUES, HACHEY, RAYNALD, LAMARRE, DANIEL, MARCOTTE, JACQUES
Applicants: ALCAN INT LTD, ALCAN INTERNATIONAL LIMITED
Application year: 1990
Main classification: G01N27
IPC codes: G01N25, G01N27