DE2456751A1 - Variable convertor for gas vol measurement - has no moving parts and suitable for very slight temp and pressure variations - Google Patents

Abstract

The period of transverse vibration of a vibrating wire is taken in linear approximation as a measure of the variable of state. The wire is subjected to constant tension and is relaxed through a diaphragm in proportion to the difference between the working pressure and the temperature-dependent pressure of a reference gas in a gas thermometer. Pref. the diaphragm is a metal bellows which undergoes elastic deformation in the axial direction to load the vibrating wire with constant tension. Pref. the nodal point of the wire vibrations is determined by perforated blocks with the seating of the wire in the blocks controlled by slightly off-centre fixing of the wire beyond the range of vibration.

Description

Volume corrector for small ranges of variability of the state variables The subject of this patent application is a mechanical-electronic device with which Help measure the volume of gases, especially natural gases and their mixtures, using a suitable approximation of the real gas equation to the normal pressure pO and standard temperature To are converted and in a counter can be accumulated. An operating volume counter of the usual design, which with is equipped with an electronic pulse pick-up is assumed to be given The operating pressure p and the operating temperature T are recorded by a common sensor with frequency output, which can be compared with a constant Oscillator frequency enables direct digital signal processing.

The volume corrector for small ranges of variability of the state variables has a much simpler structure than the current state of corrector technology corresponding devices and does not contain any mechanically moving parts. He comes for Use cases in question where operating pressure and temperature fluctuates only slightly are exposed.

Consumption measurements are made according to the mass of the gas supplied or billed according to the volume based on the normal state. The gas mass can Determine the standard volume using the volume corrector using the operating density sensor.

Operating density transmitters are not suitable because the signal is too small for measurements in low pressure networks. These networks form the area of application of the volume corrector, which, designed for a certain gas composition, the volume ratio of the Normal condition to the operating condition, the so-called condition number Vo Z = V ran. Together with an operating volume counter of the usual design, the respective Standard volume can be determined and continuously accumulated in a counter.

The state number can be measured with the help of a Reference gas or using a suitable approximation of the real gas equation from the operating pressure and temperature.

With the reference gas method, the volume of a defined closed Amount of gas which is in pressure and temperature equilibrium with the gas to be converted the same composition, the primary measurand. The reciprocal of this voluge, a measure for the desired state number is used in reference gas devices with a mechanical Calculating gear formed.

The conversion using the real gas equation is carried out using the p # To Z = po # T # k (p, T) The analytical representation is linked to the form of the ideal Gas laws. The compressibility factor k (p, T), describes the deviations from the behavior of ideal gases, approximately either regarded as constant or through defined non-linearities in the acquisition of the state variables p and rr are taken into account. If one sets these sizes with the help of suitable Sensor into mechanical deflections, then their quotient results as an output variable a mechanical calculating gear.

In both conversion processes, the controls exactly or approximately determined state number a continuously adjustable gear, which the revolutions of the operating volume counter proportional to Z is transferred to the standard volume counter.

Volume corrector with reference gases as well as devices for simulation the real gas equation require a high level of precision engineering. The multitude Mechanically moving parts are subject to wear and tear that change over the long term the measurement properties can result.

The installation of a volume corrector between the pressure regulator and operating volume counter cause small relative fluctuations in the operating pressure and thus also a narrow range of values for the state number. With a fixed assignment of the pressure regulator to the volume corrector in an integrated assembly, for example there are also possible uses for technically simple correctors limited range of variability of the state variables.

According to the invention recorded in the volume corrector for small ranges of variability of the state variables is connected to a pretensioned vibrating wire as a frequency generator Diaphragm the difference between the operating pressure and the operating temperature proportional Pressure in a closed reference gas volume. Within small difference ranges is the sufficiently high constant frequency that can be measured digitally with an oscillator The oscillation of the wire is proportional to the approximation used here the quotient of the operating pressure and temperature that occurs in the state number.

Draw transducers based on detunable mechanical oscillators are characterized by a high inherent long-term stability.

Since deflections of the diaphragm are negligible within the measuring range stay small, there are also no mechanical hysteresis phenomena.

Fig. 1 shows an example of the principle arrangement. The one as a diaphragm effective metal bellows (i) separates the gas to be converted from the reference gas, which is in the gas thermometer container (2) and in temperature equilibrium with the gas to be converted. The vibrating wire (3) leads from the fixed bracket (4) in (2) through the perforated stone (5) in the base (6) to the insulating bushing (7) in the closure cap (8) of the metal bellows P Transverse mechanical vibrations of (3) in the field of the permanent magnet (9) induce an alternating voltage in the oscillating wire In a manner known per se, it is possible to use feedback via the two-pole oscillator circuit (10) in the correct phase to the induction signals, a current can be impressed on them Vibrations undamped.

In the pulse shaper stage (11), j each generates positive voltages leading zero crossing of the signals from (10) a square pulse. After the flip-flop (12) from a signal of the operating volume meter fed in via input (13): ers has been set, the gate (14) acting as a gate leaves the square-wave pulses out (11) reach the signal switch (15.). From it becomes through the first incoming impulse the start / stop blip-flop (16) is set and by the k-th pulse over the gate (17) deleted again. At the Applying the supply voltage generated the switch-on normalization unit (18) this reset signal. The start / stop flip-flop is thus set for k oscillation periods of the corrector and leaves during this Time the pulses of the oscillator (20) controlled by a quartz crystal that reach the input of the counter (21) as the effective gate (19) '. Its Based on the previously entered initial value, the content is increased with each oscillator cycle increased by one. The negative initial value shown as a complement is chosen so that. after n oscillator pulses the content of (21) reaches zero at this point in time is characterized in that the highest value counting stage in (21) from one to zero jumps. This activates the circuit groups "Preset counter" (22) and the delay stage (23) controlled. The circuit group (22) causes the renewed takeover of the Initial value from the read-only memory (24) in the counter, which thus for a Another counting cycle is prepared. The delay stage (23) stretches when it is repeated twice Start of counting cycles within the period of k oscillation periods the temporal Distance between the signals from the highest value counting stage of (21) so far that they over the pulse shaper and amplifier stage (25) processed by the step counter (26) can be. After deleting the start / stop flip-flop and canceling the counting process the respective content of (21) remains as a carryforward for the next revaluation process in this counter.

A comparison with the standard volume registered in (26) is provided the step counter (27), which via the pulse shaper and amplifier stage (28) and the input (13) is controlled by the operating volume counter and the operating volume indicates.

The read-only memory (24) is expediently mechanically separated or programmed to leave pathways. This programming corresponds to a digital calibration of the corrector and is via an electronic control automatable in their process. Mechanical adjustment devices for influencing the period of oscillation can thus be dispensed with. The counter can be used for calibration purposes Pick up the supplied pulse train at (29).

The state number is the quotient of the number of pulses and n.

The electronic assemblies drawn framed in FIG possibly form the assembly together with the components for the mains supply a circuit board.

The vibrating wire (3) is made of a material with density # and have the cross section q. Due to (4) and (5) the node distance L is more transversal Given vibrations.

In the case of a tensile load due to the bias PO of the pressureless Netall bellows, which is advantageously used as a spring element, the period of oscillation of transverse fundamental oscillations is If F is the effective bellows cross-section of (1), then the vibrating wire is relieved by the operating pressure p with the force p # F and loaded by the pressure p * of the reference gas with P * # F = Pr # T / To # F.

Here p is the reference gas pressure at the standard temperature To.

This results in the period of oscillation under operating conditions For abbreviation, Po / F = Pv was set. If an average operating pressure pm and an average operating temperature Tm are defined for the metrological application, then t can be developed according to the deviations Ap = p - p ,, AT = T - Tm. It surrenders In comparison, the representation is the correspondingly developed state number with the compressibility factor considered to be approximately constant If the normalization of t is initially disregarded, then the linear terms match if is set. It follows from this As part of a linear approximation, the period of oscillation can be used as a measure for the state number if 1) the reference gas filling is dimensioned so that the metal bellows is free of differential pressure at p = p and g = Tm and 2) the mechanical preload of the metal bellows is closed. Po = PmB is chosen.

However, this design is only optimal for very small Ap and AII.

For the metrological recording of larger intervals of the state variables the corrector parameters are easy to modify.

Fig. 2 shows an example of the mechanical structure. Of the Metal bellows (1) consists of a material with good spring properties, e.g. Bronze or beryllium copper. The lower shelf of (1) is in a recess of the Base (6), the upper in a profile of the cap (4) soldered or glued.

The cap, drawn from sheet metal, contains one in its center insulating vacuum-tight bushing (7) for the vibrating wire (3). The vibrating wire is soldered slightly off-center in (7), so that it defines the node position determining perforated brick (5) rests. Bending moments are allowed for this bracket in the nodal area of the wire are not taken into account. Detail C shows enlarged the details of the bracket.

The cylindrical interior of (1) is largely replaced by a cylindrical Approach of the base (6) filled out. As a result, the reference gas proportion remains inadequate defined temperature in the remaining volume negligible * with complete relaxation of the vibrating wire when the permissible operating pressure is exceeded this approach the cap. The pressure equalization between the metal bellows and the gas thermometer container (2) is the hole bridging the perforated stone (5) (30) sischer The fixed mounting of the vibrating wire provides the rotating part (4) with the perforated stone (31). Here, too, a slightly asymmetrical soldered connection is guaranteed between (3) and (4) a defined node position in (31). Details shows detail D in an enlarged view. An edge rounding of the holes in (5) and (31) prevents notch effects on the oscillating wire. The rotating part (4) is pressed onto (6) by the Sleeve (32) added. The hole (33) in (32) ensures pressure equalization to (2). Via the spacer (34), the two is magnetized in opposite directions permanent magnet (9) of (4) composed of mirror-image parts in one recess the sleeve (32) held. The gas thermometer container (2) is in an annular groove cylindrical collar from (6) - soldered. With this arrangement, the heat flow remains small to the socket during the soldering process. The nozzle at the end of (2) is after the filling of the reference gas squeezed and soldered or welded so that a hermetic seal is guaranteed.

Mechanical protection of the metal bellows is provided by the base crimped closure cap (35). The pressure equalization to the outside takes place via the Bore (36). The electrical bushing (37) provides the connection to the plug-in Oscillator circuit. The bores (38) ensure suitable cams an anti-twist device on this electronic assembly.

Fig. 3 shows an example of the installation of the volume corrector into a gas pipe. As a recording for the corrector (39) serves the Cast body (40;), which for this purpose ends with a recess extending obliquely to the axis is provided in a lateral approach and via the hole (41st) the tap of the Operating pressure permitted. The round cord seal (42) prevents flow processes along the recess. The sealable threaded ring (43) presses over the metallic Foot ring of the 'plastic-cast oscillator circuit (10) the end flange the cap on the seal (44). The gas to be converted flows through the cast body in the direction of the arrow.

The following numerical example shows the design of a U'mwerter described for a low pressure network.

The operating temperature range is O - 3000. With the operating pressure range of 0.95 - 1.05 ata, atmospheric pressure fluctuations, controller fluctuations and the influences of different installation heights above sea level are taken into account for a given discharge pressure of the burner. Tab. 1 shows for Löningen-Menslage natural gas the course of the state number Z in this range of dr state variables. 0.95 ata 1.00 ata 1.05 ata to 0.94995250 0.99995000 1.04994750 1000 0.91612822 0.96434550 1.01256270 200C 0.88461175 0.93117027 0.97772878 3000 0.85526014- 0.90027384 0.94528753 Tab. 1 If you do not perform a volume conversion and use Z = 0.94265655 as a basis for calibrating the operating volume counter, then systematic measurement errors in the range of + 10.22% must be accepted.

If one leaves the metrological interpretation of the volume corrector for small ranges of variability of the state variables for the key values of Tab. 1 equal absolute errors of the approximation result, then with pv = = 0.49832073 ata pr = 0.95592697 ata the approximate representation for the state number Z * = 0.95059870 # [1 - 2.0067397 P + 0.0070228687 T] @@@ (p in ata, T in 0K). Here the period of oscillation to was replaced by a dimensionless adjustment parameter. The pressures pv and pr deviate by a few per thousand from those from the linear approximation the following values.

Tab. 2 shows the approximation values Z * for the state variables in Tab. 1 0.95 ata 1.00 ata 1.05 ata 0 ° C 0.94499549 0.99564689 1.05542630 1000 0.91381651 0.95937617 1.01250850 200C 0.88553314 0.92680089 0.97443395 300C 0.85972316 0.89733336 0.94035487 Tab. 2 Tab. 3 the relative errors Z - ZZ of the approximate representation in percent. In the one for this example 0.95 ata 1.00 ata 1.05ata 0 ° C -0.52 -0.43 +0.52 10 ° C -0.25 -0.52 -0.01 20 ° C +0.10 -0.47 -0.34 30 ° C +0.52 -0.33 -0.52 gave. 3 they are in the interval # 0.52%.

The required parameters can be identified with a list that is available Bronze bellows, whose inner diameter is 9 mm and whose outer diameter is 14.5- mm. With 8 waves, compressed by 0.88 mm, it loads one 0.2 mm strong steel wire with 548.15 p. The node distance L = 92.23 mm results in the case of no differential pressure Bronze bellows to = 1.25 msec or fo = 800 Hz.

If you use k = 5 as the time base for the signal switch (15) and as Frequency of the oscillator (20) 1 MHz, then you get Z * from the in the counter (21) fed in pulse train when the read-only memory (24) for the reduction 1: 5000 is programmed. Operating volume meters with pulse pick-up usually deliver each for decadic multiples of a cubic meter (.. 0.1 m3, 1 m3, 10 m3, ..) a signal. If you feed this in at (13), the step counter (26) shows the sum of these operating volume increments multiplied by the respective Z *.

If you change the reduction by one unit for digital calibration, i.e. from 1: 5000 to 1 | 4999 or 1: 5001, then this has a later change in the display at # 0.01 ffi as a reference gas, dry nitrogen comes with a small Adding helium in question. This admixture facilitates the leak test during manufacture of the corrector. The filling pressure at 0 ° C is pr # 0.956 ata.

If the corrector is designed precisely, that of the vibrating one Wire-entrained reference gas mass must be taken into account, which leads to a slight constant effective magnification of the product Qq. Depending on the in the corrector The materials used cause varying operating temperatures across the different linear expansion coefficient and the temperature-dependent modulus of elasticity of the metal bellows Additional influences on the period of oscillation caused by modification the parameters pv and Pr can be corrected.

A corrector with comparable systematic error margins leaves can also be designed for measurements in the medium pressure range by setting the interval 0.95 - 1.05 ata is replaced by approximately 0.95 pm - - 1-05pm. This restricted pressure area must be observed safely behind a modern pressure regulator.

The higher reference pressure can lead to a mechanical overload of the vibrating wire. Such an overload leaves avoid if, as shown in Fig. 4, the rotating part (4) on the The snap ring (45) engaging the helical spring (46) is pressed into its conical seat will.

If the tension of the swing wire (3) now exceeds one specifiable Value, then (4) stands out and the end piece (8) of the metal bellows is after a small deflection of the cap (35).

Metal bellows with smaller diameters are used for medium pressure correctors into consideration. It proves to be advantageous that the axial spring constant this component grows with decreasing diameter. For correctors at the limit Additional metal or quartz coil springs can be used to generate the high pressure area the pretensioning of the vibrating wire

Claims (8)

Claims 1. State set valerator for small ranges of variability of the state variables, characterized in that the period of oscillation is transversal mechanical fundamental vibrations of the vibrating wire (3), which at constant pretension Via a suitable diaphragm proportional to the difference between the operating pressure and the pressure proportional to the operating temperature in the gas thermometer container (2) enclosed and in temperature equilibrium with the gas to be converted Reference gas is relieved, in a linear approximation as a measure of the state number is used. 2. Volume corrector for small ranges of variability of the state variables according to claim 1, characterized in that a metal bellows (1) is used as a diaphragm finds the oscillating wire after elastic deformation in the axial direction (3) loaded with a constant preload. 3. State quantity numerator for small ranges of variability of the state variables according to claim 1 and 2, characterized in that the nodal position of the transversal mechanical fundamental vibrations of (3) determined by the perforated bricks (5) and (31) is, with a defined support of the vibrating wire in the perforated stones a slightly eccentric wire attachment outside the vibration range is guaranteed is. 4. Volume corrector for small ranges of variability of the state variables according to claims 1 to 3, characterized in that suitably running zero crossings the alternating voltage tapped at the two-pole oscillator circuit (10) the pulse shaping stage (11), the gate (14) and the signal switch (15) the flip-flop (16) which control the signals of the oscillator (20) during a period of time of k oscillation periods of the oscillating wire for the purpose of implementing this period of time can get into a digital number via the gate (19) in the counter (21). 5. c State volume conversion for small ranges of variability of the State variables according to Claims 1 to 4, characterized in that the content of the Counter (21), based on the previously entered complementary shown negative initial value, increased by one unit with each signal from the oscillator (20) until the highest counting level of the counter switches from one to zero, - one renewed takeover of the initial value from the read-only memory (24) and that Step counter (26) for standard volume display via the pulse shaper and amplifier stage (25) controls. 6. Volume corrector for small ranges of variability of the state variables according to claim 1 to 5, characterized in that a variable programming of the read-only memory (24) by separating or leaving conductor paths is made and a calibration of the automated in its technical process Corrector enables. 7. Volume corrector for small ranges of variability of the state variables according to claim 1 to 6, characterized in that the delay stage (23) at two triggering of the step counter (26) within the time span of k Oscillation duration is the time interval between the switching signals from the highest value The counting stage of (21) expands so that it can be processed by the step counter and a standard cubic meter display can be compared with one of the operating cubic meter display Provide resolution. 8. Volume corrector for small ranges of variability of the state variables according to claim 1 to 7, characterized in that in correctors for higher Reference pressures when the pipeline network is ventilated, the rotating part (4) for the oscillating wire holder against an overload of (3) against the action of the suitably preloaded helical spring (46) lifts from its seat until the end piece (8) of the metal bellows (1) from the closure cap (35) is caught. L e r s e i t e