Fluid level gauge

Abstract: A fluid level monitor (14) for monitoring the level of fluid (13) in a vessel (10) comprises an inverted U-shaped propagation member (16) which allows the propagation of stress waves therethrough. A first limb (15) of the propagation member (16) is immersed in the fluid in the vessel (10) and a second limb (17) is positioned outside the vessel (10). A transmitter transducer (18) and a receiver transducer (20) are acoustically coupled to the second limb (17) of the propagation member (16). The transducer (18) transmits stress wave pulses into the propagation member (16), the stress wave pulses produce a diffuse stress wave field in the propagation member which is damped by any fluid contacting the first limb (15) of the propagation member (16). The transducer (20) detects the stress waves propagating in the propagation member (16) and produces an electrical signal corresponding to the level of the diffuse stress wave field. An analyser (24) measures the decay rate of the diffuse stress wave field which is indicative of the level of fluid in the vessel (10) contacting the propagation member (16).

Description: The present invention relates to fluid level monitors for monitoring the level of fluid in a vessel. A known method of monitoring fluid levels in a vessel is to perform a time of flight measurement. A transducer emits a pulse of ultrasound from either above or below a fluid level and the reflection of the ultrasound from the fluid surface is detected by the transducer and the time taken for the ultrasound to travel from the transducer to the fluid surface and back is measured. Using the knowledge of the velocity of the ultrasound in the fluid, or acoustic conductor, it is possible to then calculate the fluid level as described in EP0247908A and J61-120020A. A prior art fluid level monitor for monitoring the level of fluid in a vessel comprises a propagation member which allows the propagation of stress waves therethrough, at least one acoustic emission transducer is acoustically coupled to the propagation member, the at least one acoustic emission transducer is arranged to transmit one or more stress wave pulses into the propagation member, the at least one acoustic emission transducer detecting the stress waves propagating in the propagation member and producing an electrical signal, analyser means to analyse the electrical signal to indicate the level of fluid in the vessel contacting the propagation member. This known method of monitoring fluid levels in a vessel is to position a pair of transducers one vertically above the other on the vessel wall. One of the transducers emits a pulse of ultrasound from either above or below the fluid level, and the other transducer detects the ultrasound travelling vertically in the vessel wall. The ultrasound propagating in the vessel wall is damped by any fluid contacting the vessel wall. The amplitude of the detected ultrasound is a measure of the level of the fluid in the vessel. This method however is not completely satisfactory because the amplitude of the detected ultrasound is dependent also upon the transducer sensitivity as well as the fluid level. This method would require careful calibration to compensate for the transducer sensitivities. The invention is characterised in that at least one transducer is an acoustic emission transducer arranged to produce or detect stress waves the stress wave pulse propagates in the propagation member for a period of time to produce a diffuse stress wave field in the propagation member, the diffuse stress wave field is damped by any fluid in contact with the propagation member so that after each of said at least one stress wave pulses the diffuse stress wave field in the propagation member dies out with a decay rate, the at least one transducer detects the diffuse stress wave field in the propagation member and produces an electrical signal corresponding to the level of the diffuse stress wave field, the analyser means has means to measure the decay rate of the diffuse stress wave field, the decay rate of the diffuse stress wave field is dependent upon the area of contact between the fluid and the propagation member and thus is indicative of the level of fluid in the vessel contacting the propagation member. The propagation member may be formed integral with the wall of the vessel. At least a first portion of the propagation member may extend into and be positioned within the vessel. A second portion of the propagation member may be positioned outside of the vessel, the at least one transducer is positioned on the second portion of the propagation member. A first transducer may be arranged to transmit stress wave pulses into the propagation member, a second transducer is arranged to detect the stress waves propagating in the propagation member. The means to measure the decay rate may measure the slope of the envelope of the diffuse stress wave field. The means to measure the decay rate may measure the decay rate for a transient stress wave pulse. The means to measure decay rate may measure the decay rate for repeated stress wave pulses. The at least one transducer may be arranged to produce or detect stress waves having a frequency in the region of 150kHz. The present invention will be more fully described by way of example with reference to the accompanying drawings in which:- Figure 1 is longitudinal cross-sectional view of a vessel having a fluid level monitor according to the present invention. Figure 2 is a graph of stress wave level against time for a typical transient stress wave pulse. Figure 3 is a graph of stress wave level against time showing the envelope of the typical stress wave pulse. Figure 4 is a graph of stress wave level against time showing three different envelopes of typical stress wave pulses. Figure 5 is a graph of stress wave level against time showing different envelopes of a single typical stress wave pulse for different transducer sensitivities. Figure 6 is a graph of diffuse stress wave field level decay rate against fluid level. A vessel 10 with a fluid level monitor 14 is shown in figure 1. The vessel 10 contains a fluid 12, and the fluid 12 has a fluid level 13. The fluid level monitor 14 comprises a propagation member 16 which allows the propagation of stress waves, elastic waves, ultrasound or acoustic emission therethrough. The propagation member 16 is formed from any suitable material with this property. The propagation member 16 as shown is of inverted U-shape and a first limb 15, or portion, of the propagation member 16 extends into and is positioned in the vessel 10 and a second limb 17, or portion, of the propagation member 16 is positioned outside the vessel 10. A first transducer 18 and a second transducer 20 are acoustically coupled to the second limb 17 of the propagation member 16. The first transducer 18 is a transmitter and is electrically connected to a pulse generator 22. The second transducer 20 is a receiver and is electrically connected in series to an amplifier 24 and an analyser 26. The analyser 26 is electrically connected to a display 28, and a recorder 30. In operation the pulse generator 22 sends an electrical pulse, or pulses, to the first transducer 18, which transmits stress waves into the propagation member 16. The stress waves propagate in the propagation member 16 and reverberate around the propagation member 16 for a considerable time period to produce a diffuse stress wave field. During this reverberation, the stress waves are reflected around the propagation member many times, for example there may be several thousand reflections. The diffuse stress wave field is damped as the stress waves propagate through the propagation member 16, by the propagation member itself but more significantly by any fluid contacting the propagation member. The amount of damping of the diffuse stress wave field is dependent on the area of the propagation member contacted with fluid. The second transducer 20 detects the level of the diffuse stress wave field in the propagation member, and converts the stress waves into an electrical signal. The electrical signal is amplified by the amplifier 24 and is then analysed by the analyser 26 to measure the damping of the diffuse stress wave field, the amount of damping of the stress wave field is indicative of the area of the propagation member 16 contacted by the fluid, which is dependent on the level of fluid in the vessel contacting the propagation member 16. The analyser is arranged to send an electrical output signal to a display 28 and a recorder 30. The display 28 for example may be a Visual Display Unit or a gauge. Preferably the transducer 18 emits a single transient stress wave pulse which reverberates in the propagation member 16 and is detected by the transducer 20. The detected reverberating stress wave level 30 from a single transient stress wave pulse is shown in figure 2, and figure 3 indicates an envelope 32 which the analyser 26 applies to the reverberating stress wave level 30 about the peaks and troughs of the reverberations. The analyser 26 measures the decay rate of the diffuse stress wave field ie. the decay rate of the reverberating stress wave level from the single transient stress wave pulse. Figure 4 shows the envelopes 34, 36 and 38 detected by the transducer 20 from a single transient stress wave pulse for three different fluid levels in the vessel, it can be clearly seen that the envelope 36 has a smaller decay rate ie. slope than envelope 34 and that the envelope 38 has the smallest decay rate. Envelope 34 corresponds to the greatest level of fluid and envelope 38 corresponds to the smallest level of fluid. Figure 5 shows envelopes 40, 42, 44 and 46 detected by transducers 20 with varying sensitivities but for the same level of fluid. It can be clearly seen that although the maximum stress wave level decreases with sensitivity the decay rate of the reverberate stress wave level remains constant. The measurement of the decay rate of the diffuse stress wave field is a more accurate and repeatable technique for measuring the fluid level, and is unaffected by transducer sensitivity or changes of transducer sensitivity. Figure 6 shows a graph of the decay rate of the diffuse stress wave field level against fluid level, and this shows a steady increase of decay rate with fluid level over the whole range. A monatonically increasing linear relationship appears to exist over the whole range. It may be possible to compensate for the non-linearities by appropriate shaping of the propagation member, or by using suitable corrections in the analyser. It may be possible to use more than a single transient stress wave pulse for example a repeated stress wave pulse emission. In some applications it may be necessary to surround the propagation member with a sleeve which does not allow stress waves to propagate therethrough so that the propagation member is isolated from the fluid. The second portion of the propagation member may form a waveguide such that the transducers are located at a position remote from the vessel, particularly if the vessel is in hostile environments, to protect the transducers. Although in the embodiment a pair of transducers have been used, it is possible to use a single transducer which acts as a transmitter/receiver. However such an arrangement requires more complex electronics. In the example of figure 6 which gives actual measurements of stress wave level decay rate against fluid level, stress waves with a frequency content in the region of 150kHz were generated and detected using resonant piezo-electric transducers. The propagation member may form a portion of the vessel wall, however it is preferred that the propagation member is separate from the vessel because the propagation member would be isolated from external noise sources, and it would allow the propagation member to be shaped as discussed above.

Claims: A fluid level monitor for monitoring the level of fluid in a vessel comprising a propagation member (16) which allows the propagation of stress waves therethrough, at least one transducer (18,20) acoustically coupled to the propagation member (16), the at least one transducer (18,20) being arranged to transmit one or more pulses into the propagation member (16), the at least one transducer (18,20) detecting the pulses propagating in the propagation member (16) and producing an electrical signal, analyser means (26) to analyse the electrical signal to indicate the level of fluid in the vessel contacting the propagation member (16) characterised in that the at least one transducer (18,20) is an acoustic emission transducer arranged to produce or detect stress waves the stress wave pulse propagates in the propagation member (16) for a period of time to produce a diffuse stress wave field in the propagation member (16), the diffuse stress wave field is damped by any fluid in contact with the propagation member (16) so that after each of said at least one stress wave pulses the diffuse stress wave field in the propagation member (16) dies out with a decay rate, the at least one transducer (18,20) detects the diffuse stress wave field in the propagation member (16) and produces an electrical signal corresponding to the level of the diffuse stress wave field, the analyser means (26) has means to measure the decay rate of the diffuse stress wave field, the decay rate of the diffuse stress wave field is dependent upon the area of contact between the fluid and the propagation member (16) and this is indicative of the level of fluid in the vessel contacting the propagation member (16). A fluid level monitor as claimed in claim 1 in which the propagation member (16) is formed integral with the wall of the vessel (10). A fluid level monitor as claimed in claim 1 in which at least a first portion (15) of the propagation member (16) extends into and is positioned within the vessel (10). A fluid level monitor as claimed in claim 3 in which a second portion (17) of the propagation member (16) is positioned outside of the vessel (10), the at least one transducer (18,20) is positioned on the second portion (17) of the propagation member (16). A fluid level monitor as claimed in any of claims 1 to 4 in which a first transducer (18) is arranged to transmit stress wave pulses into the propagation member (16), a second transducer (20) is arranged to detect the stress waves propagating in the propagation member (16). A fluid level monitor as claimed in any preceding claim in which the means to measure the decay rate measures the slope of the envelope of the diffuse stress wave field. A fluid level monitor as claimed in any preceding claim in which the means to measure the decay rate measures the decay rate for a transient stress wave pulse. A fluid level monitor as claimed in any preceding claim in which the means to measure the decay rate measures the decay rate for repeated stress wave pulses. A fluid level monitor as claimed in any preceding claim in which the at least one transducer (18,20) is arranged to produce or detect stress waves having a frequency in the region of 150kHz.

Inventors: WEBSTER JOHN RICHARD, WEBSTER, JOHN RICHARD
Applicants: ROLLS ROYCE PLC, ROLLS-ROYCE PLC
Application year: 1989
Main classification: G01F23
IPC codes: G01F23