GB2176007A

GB2176007A - Dynamic calibration method and system for pressure measurement circuits

Info

Publication number: GB2176007A
Application number: GB08514455A
Authority: GB
Inventors: Willy Kamiel Caekebeke; Craecker Roger Jean De
Original assignee: Labofina SA
Current assignee: Labofina SA
Priority date: 1985-06-07
Filing date: 1985-06-07
Publication date: 1986-12-10
Also published as: AT394451B; ATA149886A; GB2176007B; GB8514455D0; CH670889A5; DE3618504A1

Abstract

This method for the dynamic calibration of pressure measurement circuits comprising a piezo-electric quartz pressure transducer, a charge amplifier and a digitizing device consists in using a source 1 of inert gas under a determined high pressure which is slightly higher than the calibration pressure. The exact pressure of this gas is determined with high accuracy by using a dead weight tester 3. This source of inert gas is also employed to pressurize oil and the high oil pressure is transmitted to the pressure transducer, which is alternatively subjected to this high oil pressure and to atmospheric pressure by a device 10. <IMAGE>

Description

SPECIFICATION Dynamic calibration method and system for pressure measurement circuits This invention relates to a method and a system for the dynamic calibration of measurement circuits which are used to determine with accuracy dynamic pressures which prevail during some chemical or physical processes, such as for example pressures inside the cylinders of combustion engines.

The measurement circuits comprise a piezoelectric quartz pressure transducer, a charge amplifier nd a digitizing device for generating values of digital numbers corresponding to pressure signals.

The circuits have tobe calibrated not only before use, but also fromtime to time during their use, where precise measurements are required.

Up to now, two types of calibration methods have been employed.

The first type consists in carrying out a static calibration of the whole circuit. A dead-weight pressure balance is used and the time constant switch of the amplifier in the circuit is in the "long" position. The pressure is raised in the balance several times from atmospheric pressure to the highest pressure value which is encountered in the measurement application. The voltage jump at the output of the amplifier is noted and is used to calibrate the circuit.

Such a static method is however several drawbacks. In the "long" position of the time constant switch, the measurement circuit is very sensitive to zero drift cause by dirt or moisture in the circuit input. Removing this drift is very difficult, so that accurate calibration becomes impossible. Furthermore, the observed repeatability is poor and the difference between two measurements may reach 5%, a value that is not acceptable where precise measurements are required. Moreover, the mean value of the statically observed sensitivity (V/bar) can be 3 to 5% less than the mean value of the dynamic sensitivity, this difference being due to electrical effects.

The second type of calibration method is a dynamic method, which is however limited to the calibration of the charge amplifier. A charge calibrator provides the charge amplifier with a rectangular charge wave with selectable amplitude and reasonable precision (+ 1%). This method can provide an approximate value of the dynamic amplification factor of the sole charge amplifier but does not calibrate the whole measurement circuit.

There is thus a need for a dynamic calibration method for the whole measurement circuit.

An object of the present invention is to provide such a method for the accurate calibration of the whole circuit, i.e. the pressure transducer, the charge amplifier and the digitizing device. A further object of the invention is to provide a calibration system which is characterised by an exceptional repeatabil ity and a high sensitivity.

Accordingly, the method of the present invention for the dynamic calibration of a pressure measure ment circuit including a piezo-electric pressure transducer, a charge amplifier and a digitizing device consists in using a source of inert gas under a particular high pressure which is slightly higher than the calibration pressure, accurately measuring the gas pressure, employing the gas to pressurize oil and applying alternatively the resultant oil pressure and atmospheric pressure to the pressure transducer.

The present invention also provides a dynamic calibration system for use in that method and which comprises: - a source of inert gas under high pressure which is linked, on one hand, to a dead weight pressure tester, and, on the other hand, to a closed oil container wherein a tube is immersed in oil, the oil being pressurized by the gas, and a calibration assembly comprising:: - a stator, in which the piezo-electric pressure transducer is mounted and which has two opposed holes one for high oil pressure input and the second for atmospheric pressure input, and - a rotor wherein a bore connects said pressure transducer alternatively with the high oil pressure input and the atmosphere pressure input, said rotor being motor driven at variable speed, the high oil pressure being transmitted to the pressure transducer through the tube which is immersed in the oil and which is connected to the high pressure input of said stator.

The pressure of the inert gas may be controlled by valves and is brought to a value slightly higher than the calibration pressure. The dead weight tester can measure the selected pressure with accuracy and typically the average error does not exceed 0.1%.

Any usual dead weight tester exhibiting high precision may be employed for the accurate measurement of the gas pressure.

The gas pressure prevails also in the closed oil container which is provided with a tube or pipe which is immersed in the oil and which is connected to the calibration assembly. The oil is therefore pressurized by the gas pressure and the high oil pressure is transmitted along the tube to the calibration assembly, more particularly to the high pressure input in the stator and hence to the piezo-electric pressure transducer. The exactly calibrated transducer, with the charge amplifier and the digitizing device can now be used to determine the calibration pressure which occurs for example in a cylinder of a combustion engine.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a general view of the dynamic calibration system together with the dead weight tester and the calibration assembly in accordance with the present invention; and Figure 2 is an enlarged and sectional view of the calibration assembly which is shown in Figure 1.

In he embodiment illustrated in Figure 1, a source of inert gasunder pressure is a bottle 1 of nitrogen, which is linked through a pipe 2 to a dead weight tester 3 and to a smaller closed container or bottle 4 containing oil.

The pipe 2 is provided with a primary pressure regulator 5, a pressure compensating needle valve 6 and a valve 7. The bottle 4 is equipped with excess pressure regulating valves 8.

Atube-9 is immersed into the oil which is contained in bottle 4. The oil is pressurized by the nitrogen pressure and the high oil pressure is transmitting along the tube 9 to the calibration assembly which is described hereinbelow.

The dead weight tester 3, which is a known device, comprises an oil container 11 and a pump-12. A piston 13, with calibrated-weights 14, has a lift of about 20 mm. Each calibration is made when the weights are spinning and floating freely.

A gauge 15 enables a user to know approximately the pressure in the whole system.

Figure 2 is a sectional and enlarged view of the calibration assembly 10. The main parts of the calibration assembly 10 are a rotor 16, a flange 17, a stator 18, and a transducer holder 19 for holding a pressure transducer 20.

The rotor 16 is mounted for rotation in a hole in the flange 17 which is annular. The stator 18 is tubular and is mounted between the flange 17 and the transducer holder 19 so as to surround a portion of the rotor 16. The pressure transducer 20 is mounted along the axis of the rotor 16.

The stator 18 has two threaded holes 21 and 22 which are radially opposed, one hole being con nected to the high pressure or calibration pressure, as the case may be, and the other hole being connected to atmospheric pressure. These two con nections are interchangeable. Their threads are NPT (National Pipe Thread), which assure a perfectly tight-fit and resistance to high pressures. The wall thickness of the stator 18 must be. sufficient to enable the mounting ofa high pressure connector with each of the holes 21,22. The material of the stator 18 has self-lubricating properties.

The portion of the rotor 16 which is enclosed by flange 17, stator 18 and transducer holder 19 has a bore therethrough consisting of an axial first bore 23 which exits on the axial end face of the rotor 16 opposite the pressure transducer 20 and a radial second bore 24 which exits at an axial position which corresponds to the axial position of the two holes 21, 22.

When the rotor 16 is rotated, it can act as a three way valve because the holes 21 and 22 are con nected successively to the transducer 20 through the bores 23 and 24. Thus, in use the transducer 20 is connected successively to the precisely measured calibration pressure and atmospheric pressure. In order to prolong the time of connection between eitherthe calibration pressure or atmospheric press ure with the transducer 20,the bore 24 is radial, but has an angle lower than 1800 so as to avoid a short circuit connection between both pressure inputs.

The clearance between the rotor and the stator is as small as possible in order to avoid oil leakage and a consequential pressure drop. The end play of the rotor is limited in order to avoid backwards and forward motion and a pumping effect of the rotor.

The rotor is driven by a motor and a variable speed device (not shown).

The flange 17 closes the assembly and enables the passage of the rotor-axis therethrough without oilleakage. A space for two o-rings 25 is provided in the cylindrical wall of the hole in the flange 17 for this reason. The closeness between the flange 17 and the stator 18 is assured first by accurately machining the internal face of the flange 17 and secondly by locating a round seal 26 between the opposing faces oftheflange 17andthestator 18.

The transducer holder 19 acts as a flange at the opposite end of stator 18 from flange 17 as well as having a holderforthe piezo-electric pressure transducer 20. Each type of transducer is used with a respective particular type of transducer holder. In the embodiment shown in Figure 2, the holder 19 is designed for the AVL transducer type 8 OP 500 CA (a product of AVL - Austia). The transducer is mounted between an insert 27 and a gasket 28 in the holder 19 and is held in position by means of a screw 29. The close fit between the transducer holder 19 and the stator 18 and the centering between the components.

are assured in the same manner as for the flange 17.

A purge screw 30 (see Figure 1) is provided in the calibration system 10 so as to permit purging of the whole system. The oil resulting from the purging operation and the oil lost by little leakages in the calibration assembly are recovered in a can 31.

The piezo-electric quartz transducer 20 can be employed to determine with great accuracy the pressure which prevails for example in the cylinders of an internal combustion engine. The transducer, with the charge amplifier and digitizing device for giving the valuers of the digital numbers corresponding to this pressure signal, has to be calibrated with high precision.

To this end, the pressure transducer 20 is screwed into the transducer holder 19 of the calibration assembly 10.

The nitrogen bottle 1 is opened and, with the aid of the regulator 5, the nitrogen pressure is brought up to a value which is a little higher than the calibration pressure. The valves 6 and 7 between the bottle 1and the bottle 4 are opened, in order to increase the nitrogen pressure of the whole system. A user can read this pressure approximately on the tester gauge 15. The valves are then closed when the tester weights 14 in the dead-weight tester go up. The motorfor driving the rotor 16 is started and the speed of the rotor is brought up to the desired value.

The weights 14 are spun by hand. The correct pressure will be maintained in the whole system as long as the piston 13 is floating freely and the weights are spinning. In the case of incorrect calibration,the piston plate is so low that it rests on the bushing or so high that the internal stop of the piston is touching the underside of the bushing.

Further fine pressure adjustments can be made with the pump 12 or by releasing the excess pressure with the valves 8. Any pressure loss during a calibration canbe compensated by adjusting the needle valve 6.

The dead weight tester 3 measures the pressure with high accuracy, the average error being about 0.1% or even less.

The same nitrogen pressure is present in the bottle 4 containing oil. The volume of the bottle and the amount of oil are not critical. Good results were obtained byemlploying a 10 liter bottle containing about 3 liters of oil. The oil is pressurized by the nitrogen and high oil pressure is transmitted through tube 9 to the hole 21 of the stator 18 (the hole 22 being in this case connectedto atmospheric pressure). The rotor 16 connects the pressure transducer 20, through the bores 23 and 24, alternatively with the high oil pressure and the atmospheric pressure.

Due to the poor compressibility of oil and the small oil volume in the rotor after a pressure cycle, a certain amount of oil is lost by leaving the atmospheric input hole 22 of the stator. That small leakage flow prevents the direct connection of the dead weight tester 3 with the calibration assembly 10.

Indeed, as a result of the leakage, the piston 13 supporting the calibrated weights 14 would drop against the bushing and, from that moment, the oil pressure could no longer be correct. With the system of the present invention, that drawback is obviated, because the upper part of the bottle 4 contains nitrogen, which has a much better compressibility than oil and acts therefore as a buffer. After rotation of the rotor, the nitrogen is expanded and its pressure decreases, but with a percentage which does not exceed 0.1% or, in other terms, with a percentage which corresponds to the accuracy of the dead weight tester.

The nitrogen buffer also exhibits a second advantage; it dampens the waves caused in the oil by the discontinuous flow through the calibration assembly. Without the nitrogen being present as a buffer, the waves would be transmitted to the dead weight tester 3 in a discontinuous manner and the calibrated weights 14 would move with accelerations and decelerations. In that case, the oil pressure would not be constant, but would contain a component due to the weights' inertia.

After calibration, the pressure transducer 20 is unscrewed from the transducer holder 19 for further use.

The hydraulic-pneumatic system of the present invention provides the piezo-electric pressure transducer with a rectangular pressure wave of controll- able frequency, varying between the high calibration pressure - with a precision of 0.1% - and atmospheric pressure. This system enables a user to calibrate dynamically the whole measurement circuit, including transducer, amplifier and digitizing device, i.e. to determine under the real working conditions, the value of the digital number corresponding to each dynamic pressure change in the system to be measured.

Claims

1. Amethodforthedynamiccalibration of pressure measurement chains including a piezoselectic pressure transducer, a charge amplifier and a digitizing device, said method consisting in: - using a source of inert gas under a determined high pressure slightly higher than the calibration pressure, - measuring with accuracy this determined gas pressure, - employing this gas to pressurize oil, and - applying alternatively the so obtained oil pressure and the atmospheric pressure to the pressure transducer.

2. A method according to claim 1, wherein the inert gas is sent, on one hand, to a dead weight tester for accurate determination of its pressure, and, on the other hand, to a closed container provided with a tube immersed in oil, said oil being pressurized by this gas.

3. A dynamic calibration system for use in a method according to any one of the preceding claims, said system comprising essentially; - a source of inert gas under a determined high pressure which is linked, on one hand, to a dead weight tester, for accurate determination of this pressure, and, on the other hand, to a closed oil container wherein a tube is immersed, this oil being pressurized by this gas.

- a calibration assembly comprising a stator in which axis the piezo-electric pressure transducer is mounted and which has two threaded holes, radially opposed, one for high oil pressure input and the other for atmospheric pressure input; a rotor wherein a slot connects said pressure transducer alternatively with the high oil pressure input and the atmospheric pressure input, said rotor being motor driven at variable speed, the high oil pressure being transmitted to the stator through the tube which is immersed into the oil container and which is connected to the high pressure input of said stator.

4. A dyncamic calibration system according to claim 3, wherein the calibration assembly is closed by two flanges, one of these flanges being a holder for the pressure transducer to be calibrated.

5. A method of calibrating a fluid pressure measuring device, including a piezo-electric pressure transducer for detecting changes in fluid pressure, so that the device can subsequently be used to measure fluid pressures up to a given value, the method comprising the steps of: (a) providing a source of an inert gas at a pressure which is greater than the said given value; (b) accurately measuring the pressure of the inert gas; (c) providing a liquid and pressurizing the liquid with the inert gas to the pressure of the inert gas; and (d) applying alternatively the pressurized liquid and atmospheric pressure to the pressure transducer thereby to obtain pressure readings with the device corresponding to the accurately measured pressure of the inert gas and atmospheric pressure.

6. An apparatus for calibrating a fluid pressure measuring device, the device including a piezoelectric pressure transducer for detecting changes in fluid pressure, so that the device can subsequently be used to measure fluid pressures up to a given value, the apparatus comprising means for providing an inert gas at a pressure which is greater than the said given value, means for accurately measuring the pressure of the inert gas, a reservoir for a liquid, means for pressurizing the liquid with the inert gas to the pressure of the inert gas, and means for applying alternatively the pressurized liquid and atmospheric pressure to the pressure transducer thereby to obtain pressure readings with the device corresponding to the accurately measured pressure of the inert gas and atmospheric pressure.

7. A method of calibrating a fluid pressure measuring device substantially as hereinbefore described with reference to the accompanying drawings.

8. An apparatus for calibrating a fluid pressure measuring device substantially as hereinbefore described with reference to the accompanying drawings.