Abstract:
Two resistance elements ( 3, 6 ) are used for eliminating the influence of wall temperature on the gas pressure in a vessel, determined by a Pirani manometer. The first resistance element ( 3 ) is present in a first branch of a Wheatstone bridge ( 1 ), and the voltage is tapped by means of a voltage divider ( 7 ). The second resistance element ( 6 ) is present with a series resistance ( 5 ) in the second branch and is adjusted to a lower temperature. The changes in the voltages tapped at the branches are essentially identical for identical temperature changes at the resistance elements ( 3, 6 ), so that the Wheatstone bridge ( 1 ) remains balanced. The adjustment is improved by a constant current source ( 11 ). Another embodiment uses only one resistance element, whose temperature is reduced periodically during the balancing of the Wheatstone bridge and, after thermal equilibrium has been established, is determined by determining its resistance by means of a low constant current and is used for the computational compensation of the effect of the wall temperature. In a further embodiment, transient effects are produced by periodically switching a resistance back and forth and the frequencies are measured, from which the gas pressure is then determined.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The invention relates to a method and apparatus for measuring gas pressure in a vessel. 
     It has long been known that the pressure in a vessel can be measured by the Pirani method, by measuring the electric power obtained at a resistance element, for example a slide wire, which power gives the heat transfer from the resistance element to the vessel wall. Using the known relationship between pressure and thermal conductivity of the gas, it is finally possible to determine the required gas pressure in the vessel from the heat transfer between resistance element and vessel wall. 
     However, it is also known that there are in this method of measurement various interfering factors which can be eliminated or compensated with difficulty or only with considerable effort. Not only is the thermal conduction by the gas dependent on the temperature of the vessel wall, but the heat transfer between resistance element and vessel wall also contains components which are caused by radiant exchange and thermal conduction in the region of the connections of the resistance element. The two components are also highly dependent on the temperature of the vessel wall, which can be measured only with considerable effort to the extent required for a sufficiently accurate measurement. 
     Attempts have already been made to compensate the temperature influences by installing a temperature-dependent resistance in a Wheatstone bridge containing the resistance element. However, it is very difficult to achieve satisfactory compensation over a relatively large pressure range in this way. 
     DE-A-43 08 434 also discloses that, in such a solution, it is possible to measure the resistance value of temperature dependent resistance and to use it for an additional, for example computational temperature compensation. Here too, however, the compensation of the temperature influence is not absolutely optimal in spite of the considerable effort. 
     SUMMARY OF THE INVENTION 
     It is accordingly the object of the invention to provide a method of measurement in which interfering factors are eliminated in a radical and simultaneously simple manner and in particular the influence of the wall temperature on the value determined for the gas pressure is compensated over a large pressure range. 
     The invention provides a method in which the gas pressure is determined in a simple manner in a form essentially unimpaired by interference, from readily obtainable electrical variables. Also provided are apparatuses by means of which the method according to the invention can be carried out in a particularly advantageous manner. The proposed embodiments make it possible, depending on the requirements regarding the accuracy of measurement, to carry out a very accurate determination of the gas pressure by means of separate measurement and digital processing of the electrical variables. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Below, the invention is explained in more detail with reference to Figures, which merely show embodiments. 
     FIG. 1 shows a first embodiment of the circuit diagram of an apparatus according to the invention for carrying out the method according to the invention, 
     FIG. 2 shows a second embodiment of the circuit diagram of an apparatus according to the invention for carrying out the method according to the invention, 
     FIG. 3 shows a third embodiment of the circuit diagram of an apparatus according to the invention for carrying out the method according to the invention and 
     FIG. 4 shows curves for constant pressure and for constant temperature in a plane of variables determined by means of the apparatus according to FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The measurement of the pressure is based on the formula for the power output as a result of heat transfer from the resistance element to the vessel wall, as a function of the gas pressure p in the vessel, the temperature T of the resistance element and the wall temperature T W : 
     
       
           N ( p )=α(ε T   4 −ε W   T   W   4 )+β( T−T   W )/ T   W   ×p +γ( T−T   W ),  (1) 
       
     
     which formula is known, for example, from H. R. Hidber and G. Süiss: “Pirani manometer with linearized response”, Rev. Sci. Instrum. 47/8 (1976), 912-914. 
     Here, the first term relates to the heat transfer by radiation and the last term to that by thermal conduction in the region of the connections of the resistance element, while the middle term describes the heat transfer caused by the pressure-dependent thermal conduction by the gas, for a range of pressures below 1 mbar, which is of particular interest here. With the use of a slightly more complicated formula for the dependence of this term on the pressure, which also takes into account the saturation at higher pressures, the method according to the invention can however also be applied, without major changes, to a substantially larger pressure range. 
     (1) contains, as a factor difficult to eliminate, the wall temperature T W , which can substantially influence the results of the measurement. According to the basic concept of the invention, the effect of the wall temperature is either suppressed at the outset by designing the circuit in such a way that changes due to the influence of the ambient temperature in the resistance of resistance elements do not substantially influence the electrical state of the apparatus or are computationally compensated by determining the temperature of a resistance element, at low electrical load, over its resistance when the latter is in thermal equilibrium with the environment, or by switching back and forth between two different temperatures and heuristically compensating the dependence of the measured results obtained on the ambient temperature. 
     The apparatus according to FIG. 1 comprises a Wheatstone bridge  1  having a first branch which contains in series a first bridge resistance  2  and, in the vessel to be monitored, a first resistance element  3 , preferably a slide wire. In a second branch parallel thereto, are a second bridge resistance  4  and a series resistance  5  with a second resistance  6 , likewise a slide wire, in series, arranged in the same vessel, preferably close to the first resistance element  3 . 
     Between the first bridge resistance  2  and the first resistance element  3  is a tap which is connected to an inverting input of an operational amplifier  10  via a voltage divider  7  consisting of high-impedance resistances  8 ,  9 , while a second tap between the second bridge resistance  4  and the series resistance  5  is connected to its noninverting input. The output of the operational amplifier  10  feeds the Wheatstone bridge  1  and also delivers the output voltage U a . 
     A constant current source  11  which supplies a constant current I 0  is connected to the output of the voltage divider parallel to the first resistance element  3 , between the resistances  8  and  9 . The resistance R P1  of the first resistance element  3  and the resistance R P2  of the second resistance element  6  each have a positive temperature coefficient. 
     The Wheatstone bridge  1  is kept in a balanced state by the operational amplifier  10 . If the values of the first bridge resistance  2  and of the second bridge resistance  4  are denoted by R G1 , R G2 , the value of the series resistance  5  is denoted by R V  and the dividing factor of the voltage divider  7  is denoted by k and the constant current I 0  is neglected at first, we have: 
     
       
           k×R   P1 /( R   P1   +R   G1 )=( R   V   +R   P2 )/( R   G2   +R   V   +R   P2 )  (2) 
       
     
     The resistances can now be dimensioned in such a way that, for a specific pressure, which is preferably in the upper part of the measuring range, and for an average wall temperature, the derivative of the left term in (2) with respect to the temperature T 1  of the first resistance element  3  corresponds to that of the right term with respect to the temperature T 2  of the second resistance element  6 . 
     Since the temperatures T 1 , T 2  of the first resistance element  3  and of the second resistance element  6  change in the same way when the wall temperature T W  changes, the equality of the terms is therefore essentially retained. As a result of supplying the constant current I 0 , the left term is slightly shifted so that, with the same temperature changes, the two terms correspond at two points and are very close together in between. There is therefore substantially no adjustment of the Wheatstone bridge  1  with identical changes of the temperatures T 1 , T 2  of the resistance elements  3 ,  6  in said range, i.e. the output voltage U a  experiences virtually no change when the wall temperature T W  changes. 
     The dimensioning can be chosen, for example, as follows: R G1 =100Ω, R G2 =475Ω, R V =128Ω. The resistance elements  3 ,  6  each have a cold resistance of 87Ω. The resistances  8 ,  9  of the voltage divider  7  are high-impedance, as mentioned above, so that they have virtually no effect on the current through the Wheatstone bridge  1 . They may be chosen, for example, to be 82.2 kΩ and 100 kΩ. Finally, the constant current I 0  may be, for example, 1 μA. 
     The two branches of the Wheatstone bridge  1  are in any case adjusted so that a relatively high current flows through the branch containing the first resistance element  3  and a substantially lower current flows through the branch containing the second resistance element  6 . The resistance elements  3  and  6  therefore also have different temperatures T 1 , T 2 . The temperature changes of the resistances R P1 , R P2  of the resistance elements which are caused by changes in the pressure and in the resultant heat loss are therefore likewise different and lead to a rebalancing of the Wheatstone bridge  1  by the operational amplifier  10 , which is also reflected in a change in the output voltage U a . 
     The evaluation of the output signal is carried out essentially according to a recorded calibration curve since the purely computational evaluation would be possible only with considerable effort, owing to the relatively complicated design of the apparatus. It is also possible to measure the resistance at different temperatures at one and the same resistance element by changing the temperature, for example by periodically switching a resistance of a Wheatstone bridge containing the resistance element. 
     This takes place specifically in a slightly different way in the apparatus according to FIG.  2 . It likewise comprises a Wheatstone bridge  1  having a first branch which contains, in series, a first bridge resistance  2  and a resistance element  3  arranged in the vessel and preferably in the form of a slide wire and a second branch comprising a series circuit of two base resistances  12 ,  13 . The Wheatstone bridge  1  is in turn balanced by an operational amplifier  10 , this being effected by means of a switch  14 , for example a transistor. A constant current source  15  supplies a constant current I 0  to the Wheatstone bridge  1 . The bridge resistance  2  may have, for example, a value of 100Ω and the base resistances  12 ,  13  a value of 10 kΩ and 14 kΩ, respectively, while the cold resistance of the resistance element  3  is 87Ω. The constant current I 0  may be 100 μA. 
     When switch  14  is closed, the Wheatstone bridge  1  is balanced by the operational amplifier  10 . After thermal stabilization of the circuit, the resistance R p  of the resistance element  3  is obtained in a known manner from the output voltage U a  and constant resistance values, and the temperature T 1  of the resistance element  3  is also obtained from the known relationship between temperature and resistance R p , and in addition the voltage drop across said resistance element  3  and hence the pressure-dependent power N(p) output by it as a result of radiation and thermal conduction, whereby of course, according to (1), the wall temperature T W  which is not accurately known influences this variable. 
     By opening the switch  14 , the balancing of the Wheatstone bridge  1  is discontinued. After thermal stabilization—it may be possible to determine the final value beforehand from the curve of the transient effect—a component of the constant current I 0 , which is preferably rated so that it does not significantly heat the resistance element  3 , flows through said resistance element  3 . From the voltage U 0  drop across the resistance element  3  and the known values of the further resistances in the Wheatstone bridge  1 , it is now possible in turn to determine the resistance of the resistance element  3  and to determine its temperature T 2 , which now corresponds to the wall temperature T W . After substitution of this value in (1), the equation can be solved for p. 
     The apparatus according to FIG. 3 in turn comprises a Wheatstone bridge  1  having a bridge resistance  2  of, for example, 100Ω and a resistance element  3  connected in series to said bridge resistance  2  and arranged in the vessel, preferably a slide wire having a cold resistance of 87Ω, in a first branch and two base resistances  12 ,  13  of, for example, 10 kΩ and 14 kΩ in a second branch, which bridge is balanced by an operational amplifier  10 . A parallel resistance  13 ′ of, for example, 120 kΩ can be connected in parallel to the second base resistance  13  by means of a switch  16 . 
     An amplifier  17 , which is led back to the noninverting input of said operational amplifier  10  via a coupling resistance  18  of, for example, 1 MΩ, and, parallel thereto, an amplitude controller  19  are connected to the output of the operational amplifier  10 . In addition, the output voltage U a  is fed to a frequency meter  20 , which transmits the result of its measurement to a microprocessor  21 . The microprocessor  21  also actuates the switch  16 . 
     If the switch  16  is, for example, opened, a transient effect is triggered, in which the resistance element  3  assumes a temperature T 1 . As a result of the feedback of the Wheatstone bridge  1  via the amplifier  17 , which feedback is controlled by the amplitude controller  19 , the oscillation about the rest position corresponding to complete balance is stabilized, the amplifier  17  being controlled in such a way that the amplitude of the oscillation assumes a fixed, relatively small value. The frequency ν 1  of this oscillation, which depends on the gas pressure p and also on the wall temperature T W  through the thermal conductivity and the heat capacity of the gas, is measured by the frequency meter  20  and transmitted to the microprocessor  21 . 
     This closes the switch  16 , for example after the apparatus and hence the measured frequency ν 1  have completely stabilized, and thus results in a change in the dividing ratio and causes rebalancing of the Wheatstone bridge  1  by adjustment of the resistance element  3  to a lower resistance and hence a lower temperature T 2 . About this rest position, too, an oscillation is generated whose frequency ν 2 , after stabilization of the apparatus, is measured by the frequency meter  20  and in turn depends on the gas pressure p and also on the wall temperature T W  through the thermal conductivity and the heat capacity of the gas. At a wall temperature T W  of about 25° C., frequencies of the order of magnitude of 50 Hz at 10 −3  mbar and 2 kHz at 1 bar result in the case of the above-mentioned dimensions. 
     In the plane defined by ν 1  and ν 2  (cf. FIG.  4 ), each point corresponds to a pressure-temperature pair (p, T W ). The ν 1  and ν 2  values corresponding to specific pressures and wall temperatures can be determined by calibration measurements and are stored in tabulated form in a memory. Conversely, the microprocessor  21  can determine the pressure p and the wall temperature T W  from a frequency pair (ν 1 , ν 2 ) by interpolation from the closest stored values. FIG. 4 shows lines of constant wall temperature T W  and lines of constant gas pressure p. Here too, the gas pressure p is therefore determined on the basis of electrical variables which occur in succession at one and the same resistance element  3 .