Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B5/00—Measuring for diagnostic purposes; Identification of persons
  • A61B5/08—Detecting, measuring or recording devices for evaluating the respiratory organs
  • A61B5/087—Measuring breath flow
  • A61B5/0878—Measuring breath flow using temperature sensing means
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
  • G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
  • G01F1/05—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by using mechanical effects
  • G01F1/34—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by using mechanical effects by measuring pressure or differential pressure
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
  • G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
  • G01F1/05—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by using mechanical effects
  • G01F1/34—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by using mechanical effects by measuring pressure or differential pressure
  • G01F1/36—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by using mechanical effects by measuring pressure or differential pressure the pressure or differential pressure being created by the use of flow constriction
  • G01F1/40—Details of construction of the flow constriction devices
  • G01F1/42—Orifices or nozzles
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
  • G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
  • G01F1/68—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by using thermal effects
  • G01F1/684—Structural arrangements; Mounting of elements, e.g. in relation to fluid flow

Definitions

• the present invention relates generally to gas flowmeters containing one or more differential pressure transducers measuring pressure drop across a flow-resistive element placed inside the gas passageway. More particularly, the present invention relates to improvement in the accuracy of flowmeters used in spirometry, by special design of the gas passageway, by increasing its immunity to vibrations and by providing special signal processing and a linearization method applicable to a flowmeter or other like transducer signals.
• a gas flowmeter contains a gas flow receiver (GFR) (also called gas passageway), a tube through which gas flow passes, and a differential pressure transducer thus connected to the GFR.
• GFR gas flow receiver
• the transducer measures differential pressure generated by a flow-resistive element placed inside the tube.
• the particular design of the GFR used in spirometry is the result of a trade-off between the imperative to increase differential pressure generated by the flow-resistive element to obtain higher sensitivity of the flowmeter, and the requirement not to exceed a maximum acceptable back-pressure specified by spirometry standards.
• the American Thoracic Society Standards for Spirometry (1994) require the back-pressure to be not greater than 150 Pa-s/l in the whole operating range. Therefore a GFR with highest ratio (differential pressure)/(back pressure) is preferable for this particular application.
• Another problem to be solved in the described flowmeter is related to reproducibility of the transformation characteristic of the GFR, which defines flow-to-differential-pressure conversion.
• simpler shapes of the GFR are easier to manufacture, with better reproducibility of the dimensions of the inner features and surface of the tube, which eventually guarantee reproducibility of the conversion.
• Another result of the tube shape simplification is the reduction of manufacturing costs, which is of great importance, especially for disposable tubes.
• GFRs with flow- resistive elements made in the form of plane diaphragms have essentially non-linear flow-to-differential-pressure conversion characteristics.
• the differential pressure which is generated by this type of GFR is close to a square function of the flow, which results in the following problem.
• thermoanemometer-type transducers connected in parallel to the GFR for flow measurements has been reported earlier [U. Bonne, K.Fhtsch, "Mikroanemometer fur die fürflu ⁇ mecanic für Gasen,” Technisches Messen, 1994, v.61 , n.7, pp.285-294; T.R.Ohnstein, R.G.Johnson, R.E.Higashi, D.W.Burns, J.O.Holmen, E.A.Satren, G.M.Johnson, R.E.Bicking, S.D.Johnson, "Environmentally Rugged, Wide Dynamic Range Microstructure Airflow Sensor," Proceedings on Solid- State Sensors and Actuators Conference (1990), pp.158-160].
• thermoanemometer-type transducer has a linear operating range of about four orders of magnitude and total operating range of more then six orders of magnitude [Frolov G.A., Gendin A.V., Grudin O.M., Katsan 1.1., Krivoblotskiy S.N., Lupina B.I, "Micromechanical thermal sensors for gas parameters measurements," Proceedings of 1996 ASME International Engineering Congress and Exposition, DSC-Vol.59, Micro-electromechanical systems, pp. 61-65].
• This particular transducer covers the whole operating range that makes it attractive for usage in flowmeters.
• Another advantage of thermoanemometer-type sensors is that their resolution can be improved to a level as fine as several mPa without degrading their dynamic properties.
• thermoanemometer-type transducer contains a functional element sensitive to gas flow passing through a specially designed gas flow assembly [US Patent 4548078]. This gas flow is proportional to pressure drop across the transducer, which makes it also possible to use it also for differential pressure measurements.
• the physical principle of gas flow measurement is based on flow-induced disturbance of a symmetrical temperature distribution in the gas around a heater. This disturbance, caused by the shift of the heated volume of gas in the direction of flow, is usually detected by a pair of temperature-sensitive elements.
• the flow-sensitive element contains one or two heaters, which warm the gas in a certain region of the flow channel.
• the flow sensor may use only two self-heated temperature-sensing elements, which warm the gas and measure the temperature difference simultaneously [H-E. de Bree, P. Leussink, T. Korthorst, H. Jansen, T S J. Lamme nk, M.
• Thermoanemometer-type flow or differential pressure transducers are sensitive to acceleration acting in the direction parallel to the gas flow in the transducer flow channel. This effect, caused by the shift of the heated volume of gas having lower density than the surrounding colder gas, results in a temperature difference sensed by the two temperature-sensitive elements. Acceleration applied in directions perpendicular to the gas flow also causes shift of the heated volume of gas, but this shift is in a direction which does not significantly change the temperature difference measured at the two temperature-sensitive elements. Therefore the transducer has low sensitivity to acceleration perpendicular to gas flow. In general, in a single sensor, the acceleration-induced output signal is indistinguishable from a flow-induced signal.
• Sensitivity to acceleration of the considered differential pressure transducers adversely affects the accuracy of the transducers and hence the gas flowmeter exposed to mechanical disturbances such as vibrations, rotation and displacement.
• Another problem is also caused by the necessity to detect and process differential pressure signals approximately 1000 times lower than those generated by near-linear GFRs (for example, Fleisch- or Lilly-type tubes in spirometry).
• near-linear GFRs for example, Fleisch- or Lilly-type tubes in spirometry.
• some vibrations or shocks of the device parts such as the pneumatic hoses connecting the GFR with differential pressure sensor, may occur. Being negligibly small with respect to differential pressure signals generated by linear GFRs, these parasitic signals may nevertheless be significant compared to useful signals generated by nonlinear GFRs.
• a flowmeter containing two nonlinear functional elements such as the GFR and thermoanemometer-type sensor, has a complex and essentially nonlinear conversion characteristic from flow to output voltage, which must be linearized.
• its calibration curve F(V) which specifies the correspondence between the flow F and the measured voltage V, must be defined.
• the present invention addresses also linearization of an essentially nonlinear flowmeter, containing a GFR with flow-resistive element generating differential pressure close to the square of the flow.
• Contemporary spirometers typically contain analog-to-digital converters (ADC), digitizing analog signals from the transducer for subsequent processing.
• ADC analog-to-digital converters
• ADC with resolution higher than 16-18 bits should be used. Meanwhile, comparatively cheap, simple and widespread 12-bit ADCs are preferable. Therefore the problem of resolving of low flows (in the flowmeter with nonlinear GFR), arises not only from the restricted sensitivity of differential pressure transducers, but also from the limited resolution of the preferred electronic circuitry.
• thermoanemometer-type differential pressure and flow transducers • Improve immunity of the flowmeter to vibrations or shocks of the device and its parts, including the thermoanemometer-type differential pressure and flow transducers.
• the flow-resistive element is made in the form of an obstacle to gas flow having non-symmetrical shape when viewed along the long axis of the GFR tube.
• the flow-resistive element is designed to simultaneously obtain low overall back-pressure, and high local differential pressure measured between two points inside the GFR.
• the local differential pressure is created by placing an obstacle in the GFR, directly between the two points inside the GFR at which the differential pressure is measured, typically, in the case of bi-directional flow measurement, as close as possible to the midpoint of the line connecting these two points.
• a gas flow receiver comprising a flow tube having a sidewall and guiding flow therethrough while inducing minimal resistance, an upstream sensing tube having an upstream orifice communicating with the flow tube via the sidewall, and a downstream sensing tube having a downstream orifice communicating with the flow tube via the sidewall.
• the receiver also has a non-symmetrical-flow-inducing diaphragm mounted in the flow tube between the upstream and the downstream orifices, and causing non-symmetrical flow in the flow tube with an accentuated higher pressure near the upstream orifice than would be sensed in a corresponding cross- section of the flow tube and an accentuated lower pressure near the downstream orifice than would be sensed in a corresponding cross-section of the flow tube, the orifices being positioned with respect to the diaphragm so as to sense accentuated pressure substantially without sensing pressure oscillations due to any turbulence induced by the diaphragm.
• the diaphragm is mounted to the sidewall between the orifices.
• the diaphragm may be shaped so as to exhibit high drag and generate maximum accentuated pressure for its size.
• the flow tube may have a smaller cross-section between the orifices and may be similarly tapered on both sides of the small cross-section.
• thermoanemometer-type differential pressure transducer To improve immunity to vibrations of the thermoanemometer-type differential pressure transducer, two or more thermoanemometer-type flow- sensitive elements are connected and used such that the parasitic acceleration-induced components of the signals can be separated from the flow-induced components, and cancelled, thus allowing identification of the flow-induced signals.
• thermoanemometer- type flow-sensitive elements are connected in a specific way, such that the gas flows through each of the elements in opposite directions.
• the output signals of the two flow-sensitive elements are processed electronically so that the acceleration-induced components of the signals are cancelled, while flow-induced components of the signals are doubled.
• Another combination may also include at least one flow-sensitive element through which no gas flows. Being subjected to the applied acceleration, this reference element generates an output signal which is used to cancel the acceleration-induced component of the element through which the gas flows.
• a gas flow transducer apparatus with immunity to vibration or acceleration which comprises a plurality of gas flow transducer elements each sensitive to vibration or acceleration in at least one direction and generating an output signal proportional to gas flow and to a perturbation component resulting from the vibration or acceleration.
• the transducer elements are arranged on a common support and a plurality of gas flow passages leading gas flow from an inlet to an outlet through at least one of the transducer elements.
• the elements are arranged on the common support and connected to the passages such that at least one of the perturbation component and the gas flow is measured differently by the transducer elements.
• Circuitry is provided that receives the output signal of each of the transducer elements and outputs a vibration or acceleration immune output signal corresponding to the gas flow with the perturbation component substantially cancelled.
• the gas flow passages may cause the gas flow to be equal through the transducer elements, and the gas flow may be split between the transducer elements, or it may pass serially through the transducer elements.
• two transducer elements are provided that are sensitive to vibration or acceleration along only one axis and are arranged parallel to one another, the gas flow passages being arranged such that the gas flow is in opposite directions through the transducer elements.
• Gas flow may be blocked in at least one of the transducer elements, wherein the at least one of the transducer elements measures only the perturbation component.
• the at least one of the transducer elements communicates with the gas flow such that the at least one of the transducer elements is subjected to a same gas composition and temperature as other ones of the transducer elements.
• the transducer elements preferably comprise thermoanemometer- type transducers.
• the output analog signal of the differential pressure transducer should have high frequency components at frequencies f>1/At, where At is the ADC sampling rate, where the amplitude of these high-frequency components must exceed one quantization unit of the ADC;
• the average output signal voltage is calculated by arithmetic averaging of several samples from the output of the ADC, during a time ⁇ - N-At, where ⁇ />2, resulting in resolution better than the quantization unit of the ADC;
• a method of estimating a value of an analog signal using an analog-to-digital converter (ADC) with a level of precision greater than a minimum quantization value of the ADC comprises: adding a secondary signal to the analog signal, the secondary signal having a zero DC component, a substantially even and symmetric amplitude distribution and a peak-to-peak amplitude greater than the minimum quantization value; recording and storing a digital output value of the ADC; averaging the digital output value recorded over a sampling period to obtain an estimated higher precision digital value with a precision greater than a precision of the digital output value.
• the secondary signal is provided by noise generated in amplifier circuitry used to amplify the analog signal
• the analog signal may be a gas flow transducer signal in addition to other types of signal.
• the sampling period varies as a function of an amplitude of the analog signal, wherein the sampling period is longer for lower amplitude values and is shorter for higher amplitude values. 4.
• the output analog signal of the differential pressure transducer conditioned with high-frequency components as specified in 3.1 )-3.2), can be processed as follows:
• the whole operating range of the flowmeter is divided into at least 2 (preferably more) non-overlapping sub-ranges.
• the number of subranges can be theoretically up to the number of quantization units in the ADC.
• the averaging times can then be different for each sub- range;
• a method of filtering a signal comprising the steps of measuring an amplitude of the signal, determining an averaging period ⁇ as a function of the amplitude, wherein ⁇ is longer for lower values of the amplitude and ⁇ is shorter for higher values of the amplitude, and averaging the amplitude over the period to provide a filtered output signal.
• the function may be a step function.
• the filtered output signal may be the instantaneous value of the amplitude.
• the step of measuring comprises converting an analog gas flow transducer signal to a digital signal providing the amplitude.
• N is equal or greater than 3; parameters #,>1 (preferable values of a, are close to 2); and A are coefficients determined experimentally for a particular flowmeter, to give the best linearization results.
• N is equal or greater than 3; parameters #,>1 (preferable values of a, are close to 2); and A are coefficients determined experimentally for a particular flowmeter, to give the best linearization results.
• the whole operating range of the flowmeter is divided into several (at least two) sub-ranges, and a calibration function is found separately for each of the sub-ranges by the same method.
• a method of processing a transducer output signal that is non-linear with respect to a physical parameter being measured to obtain a calibrated output signal representing the physical parameter on a given scale comprises: subjecting the transducer to a number of calibrated physical parameter conditions; recording a value of the output under each of the conditions; obtaining an analytical solution for a non-linear function relating the output value to the physical parameter, the solution being expressed as:
• V is the transducer output signal
• N is greater than or equal to 3
• parameters A are coefficients determined from the recorded values
• a are real numbers; determining the calibrated output signal for the transducer output signal using the analytical solution.
• ct ⁇ are greater than 1 , and may be non-integers.
• the step of determining may comprise: calculating a value of the physical parameter for each possible value of the transducer output signal using the analytical solution; and building a table of the physical parameter values indexed by digital output values; converting the transducer output signal into a digital output value; and obtaining a value of the calibrated output signal from the table using the digital output value.
• the analytical solution may be exact for each of the recorded values, and the analytical function may be divided into subranges.
• the transducer output signal may be derived from a gas flow transducer signal having a square transfer function, and the gas flow transducer is preferably a thermoanemometer-type transducer apparatus.
• Fig. 1 is a schematic of the flowmeter containing the Gas Flow Receiver, differential pressure transducer, analog electronic module and ADC module;
• Fig. 2 is the Gas Flow Receiver with star-like diaphragm (prior art);
• Fig. 3 is the Gas Flow Receiver with the invented non-symmetrical flow-resistive element;
• Fig. 4 shows the measured back-pressure versus flow for both types of the GFRs;
• Fig. 5 shows the measured ratio ⁇ of differential pressures generated by non-symmetrical and symmetrical flow resistive elements versus flow
• Fig. 6 shows deviations of experimentally-measured syringe volume from its actual value obtained during "expiration” and “inspiration”, at different averaged flow rates for the GFR with symmetrical star-like obstacle
• Fig. 7 shows deviations of experimentally-measured syringe volume from its actual value obtained during "expiration” and “inspiration,” at different averaged flow rates for the GFR with invented non-symmetrical flow-resistive element
• Fig. 8 is a schematic side view of the GFRs
• Fig. 9 is a schematic front view of the flow resistive element within the GFR tube (viewed along the long axis of the tube);
• Figs. 10 and 11 show the configuration of the invented differential pressure and flow transducer containing two thermoanemometer-type flow- sensitive elements.
• the channel for flowing gas connects two elements in series (Fig. 10), or in parallel (Fig. 11 );
• Fig. 12 depicts a schematic of a single flow-sensitive element with acceleration applied to it
• Fig. 13 shows an example of a more complex configuration involving three flow-sensitive elements connected in a triangle;
• Fig. 14 shows the configuration of two identically-aligned flow- sensitive elements with flow passing through only one of them;
• Fig. 15 shows (a) unfiltered, and (b) filtered output signals of the flowmeter at low flows;
• Fig. 16 is a block-diagram of the electronic module of the flowmeter;
• Figs. 17 and 18 are flow charts describing the invented signal processing;
• Fig. 19 shows the reaction of the flowmeter to a flow impulse
• Fig. 20 shows a calibration curve of the flowmeter. Description of the Preferred Embodiment
• a gas flow receiver has a non- symmetrical-flow-inducing diaphragm mounted in a flow tube and causes non-symmetrical flow in the flow tube with an accentuated higher pressure near an upstream orifice than would be sensed in a corresponding cross- section of the flow tube and an accentuated lower pressure near a downstream orifice.
• a gas flowmeter using thermoanemometer-type transducers receiving gas flow from the upstream orifice is made immune to vibration or acceleration, for example, by arranging a pair of the transducers parallel to one another with the gas flow passing serially through them, but in opposite directions. The resulting transducer signals processed to cancel the effect of the vibration or acceleration.
• the transducer output is amplified by a noisy amplifier which injects a secondary signal prior to digital conversion using an ADC.
• the digital signal is averaged over a sampling period to obtain a sample having a level of precision greater than a minimum quantization value of the ADC.
• the sampling period is varied as a function of the transducer's analog signal amplitude, such that the sampling period is longer for lower amplitude values and is shorter for higher amplitude values.
• the sampling period variation provides signal filtering.
• a calibrated output signal representing flow is obtained by recording samples under a number of calibrated flows and obtaining an analytical solution for the non-linear function, using the analytical solution to obtain calibrated values for all sample values, and, in operation, looking up a calibrated value corresponding to a sample value.
• the present application describes a variety of inventive features applicable to a gas flowmeter. These features improve the characteristics of the flowmeter. Maximum effectiveness of the improvement can be reached when the inventions discussed below are implemented jointly and are therefore presented together herein.
• Fig. 1 shows a schematic view of the flowmeter containing GFR 1, differential pressure transducer 2, analog electronic module 3 and ADC module 4.
• the flow-resistive element 5 of the invented gas flow receiver (GFR) 1 is designed to simultaneously obtain low overall back-pressure, and high local differential pressure measured between two points inside the GFR 1 for bi-directional flow measurement, for example in spirometry.
• the local differential pressure is created by placing an obstacle 5 in the GFR 1 , directly between the two points inside the GFR 1 at which the differential pressure is measured, as close as possible to the midpoint of the line connecting these two points. In some circumstances, it may be desirable for the obstacle or diaphragm to be positioned offset from the midpoint.
• each of the star-like symmetrical diaphragm 7 and the invented non-symmetrical diaphragm 5 were placed in the middle of identical tubes 120mm long with input inner diameter of 21 mm and inner diameter of 19mm at the center of the tube (Figs. 2 and 3).
• the shapes of the flow-resistive elements have been chosen so as to generate back pressure lower then 150Pa s/l (required by ATS standards for spirometry).
• the six beams of the star-like diaphragm 7 have width of 1 mm each.
• the central spot of the diaphragm has a diameter of 4mm.
• the non-symmetrical diaphragm 5 has the shape of a circular segment with height of 4mm.
• the star-like diaphragm 7 has thickness of 1 mm while the segment-type diaphragm 5 has a thickness of 0.1-0.2mm.
• Fig. 4 shows the dependence of back pressure on air flow, for the two GFRs.
• the invented GFR generates lower back pressure and higher differential pressure than the GFR with symmetrical diaphragm 7.
• each tube (after calibration) was connected to the syringe, and gas was "inspired” from the ambient through the tube into the syringe by piston strokes at different flow rates - "inspiration”.
• the inspired volume was measured by integrating flow measurements over time, and compared with the actual volume of the syringe.
• each tube was rotated by 180 degrees, such that its flow direction was reversed, and connected to the syringe with its opposite end, and the same experiments were conducted except with gas being pumped out of the syringe, by "expiration" piston strokes.
• the direction of gas flow in the tubes was the same, but the connections, and thus, the gas flow velocity distributions in the flux, were different.
• the GFR input is connected directly to an infinite volume of ambient gas
• the gas flux entering the GFR is shaped by a 40mm-long connecting tube with inner diameter of 30mm.
• the invented GFR has a simpler shape than the GFR with symmetrical star-like diaphragm or crest-type flow resistive element. This can simplify its manufacturing and improve reproducibility of the conversion characteristic.
• FIGs. 8 and 9 show several possible designs of the GFR, which do not exhaust all possible shapes of the GFRs. The particular choice of GFR should be made to provide optimal adaptation to the given application and manufacturing techniques.
• different cross-sections of flow- resistive element 8, 9, 10 are shown inside the GFR 1.
• the GFR 1 may also contain tubing 11 attached at a certain angle.
• Front views of plane diaphragms 12, 13, 14 used as flow-resistive elements are shown in Fig. 9.
• the flow measured is bi-directional.
• the positioning of the non- symmetrical diaphragm may preferably be in a position different from the midpoint between the sensing tube orifices.
• Figs. 10 and 11 show two possible configurations of the invented transducer, where two flow-sensitive elements 15 are connected in series (Fig. 10), and in parallel (Fig. 11 ).
• the gas flowing through the channels passes through the flow-sensitive elements in opposite directions. Therefore, the heated volumes of gas 16 near the heaters 17 in both flow-sensitive elements 15 (shown as shaded circles), are also shifted in opposite directions causing inverted output signal components.
• heated volumes of gas 16 are shifted in the same direction for both flow-sensitive elements 15 causing increments in output signals which are the same for both sensors.
• the output signals V sen sori and V sens0 r2 of the two flow-sensitive elements 15 are then processed by electronic circuitry 3, such that one signal is subtracted from the other.
• Vsensori toiv + V acceleration
• ⁇ sensor2 ⁇ vflow + acceleration * sensor1 ⁇ V sensor 2 — £- • /ow i
• V7/ o v and 'acceleration are the sensor output voltage components caused by gas flow and applied acceleration, respectively.
• Figs. 10 and 11 The choice of schemes presented in Figs. 10 and 11 depends on the particular application.
• the transducer with two flow-sensitive elements 15 connected in series has flow impedance two times higher than a single element, while the second transducer (Fig. 11) has flow impedance two times lower.
• a prototype of the transducer was assembled on the basis of two AWM2200 mass flow sensors (Honeywell) connected in series by plastic hoses. The performance of the assembled prototype was compared with the performance of a single AWM2200 sensor. The electronic circuitries of the transducers provided the same sensitivity to differential pressure. Then both devices were rotated in the Earth's gravity. The single AWM2200 sensor has sensitivity to acceleration of 14mV/g while the prototype of the invented transducer is completely immune to the same acceleration within the resolution of the electronic circuitry (less than 1 mV). Both tested transducers have the same sensitivity to gas flow. The invented configuration of the transducer 2, immune to acceleration, can be realized by a variety of methods.
• flow-sensitive elements may be commercially available sensors as described above or specially designed functional sensing elements.
• Fig. 13 where three flow-sensitive elements are arranged in the same plane and connected in the shape of a triangle.
• the output signals of each flow-sensitive element can be written as:
• Vsensort " Vfiow ⁇ * acceleration OOS(p) where ⁇ s the effective angle of the acceleration, as shown in Fig. 13.
• This system of three equations with three unknown parameters, Vfiow, Vg c DCeration and ⁇ can be solved to cancel the influence of the acceleration-induced component of the signal.
• Fig. 14 depicts another possible combination of two flow-sensitive elements.
• the gas passage 18, which is opened to the main gas passageway at one end allows the reference sensor to experience the same gas composition, pressure and temperature, without experiencing the 5 gas flow. When conditions of gas composition, pressure and temperature are constant, sensor 2 may be isolated without using the passage 18.
• the following three equations summarize the sensors output signals and the obtaining of the flow signal.
• the output signals of the sensors are:
• acceleration suppression is effective 0 independently of the particular embodiment shown in Figs. 10, 11 , 13, 14 and independent of the method of possible compensation of gas temperature, gas composition and ambient pressure.
• the invented method to improve flowmeter resolution at low flows
• the minimum detectable flow defined by the resolution of the ADC 4 (flow corresponding to +1 mV or -1 mV, the quantization unit of the ADC) was found to be approximately 50ml/s (without the invented signal processing).
• the reasonable useful frequency bandwidth of the electronic module for the detection of variable flows need not be greater than 100-150Hz.
• the frequency bandwidth of the electronic module was intentionally increased up to 10kHz.
• the increased high- frequency noise component of the analog output signal determined by approximately white noise of the operational amplifiers of the circuitry, had amplitude of approximately +3mV, equivalent to three quantization units of the ADC 4. This added noise is shown in Fig. 15a.
• a schematic block- diagram of the device is shown in Fig. 16. In the described experimentally- checked embodiment, additional noise was generated inside the analog circuitry module.
• Flow charts describing signal processing in the microprocessor- based module are shown in Figs. 17 and 18.
• the operating flow range of the flowmeter was divided into four sub-ranges: 0 ⁇ flow ⁇ 0.5 l/s;
• Fig. 15b The effectiveness of presented signal processing is shown in Fig. 15b.
• the suppression factor for vibration- generated signals is approximately 7-8.
• Another possible filtering sequence is shown in Fig. 18.
• the difference of this signal processing from previous one is the following:
• the high frequency response of the flowmeter is required mainly at high flows (for example, spirometry), which would be degraded by a long averaging time ⁇ .
• the usage of several sub-ranges with low averaging times at high flows maintains satisfactory speed of response at medium and high flows.
• the filtered signal (b) has the same shape as the unfiltered signal (a). Its fall time is estimated to be less than 10ms. The effect of filtering at low flows can be recognized by the effective suppression of the oscillating acoustic signal generated by the collision of the piston with the syringe bottom.
• the parameters of this filtering method i.e. number of flow sub- ranges, averaging times and amplitude of the analog signal noise component, can be chosen to optimize the operation of the flowmeter for a particular application.
• the parameters of the investigated physical process resulting in this choice are: required frequency response and flow operating range of the flowmeter, and intensity and frequency spectrum of the parasitic signals to be suppressed.
• V is the output voltage of the flowmeter; N is greater than or equal to 3; parameters A are coefficients determined experimentally; a, are real numbers (not necessary integer), typically greater than 1.
• a prototype flowmeter was used.
• the flow meter was based on the GFR 1 described above, containing the 120mm long tube with input inner diameter of 21 mm and inner diameter of 19mm at the center of the tube.
• a planar non- symmetrical diaphragm 5 having the shape of a circular segment with height of 4mm (Fig. 3), was used as a flow-resistive element. This diaphragm with thickness of 0.2mm was located at the center of the tube, and generated differential pressure close to the square of the flow (square transfer function).
• Mass flow sensor AWM2200 Honeywell was connected to the GFR 1 with two plastic hoses 6 to measure flow-induced differential pressure.
• the flowmeter also contained a 12-bit ADC AD7890-10 operating in the range +10V.
• the operating flow range of the flowmeter was divided into two sub-ranges, 0 - 2 l/s and 2 - 15 l/s.
• Two functions defined separately for the two sub-ranges are graphed in Fig. 20.
• the calibration curve was stored in form of the table in a computer file.
• the choice of the parameters N and a may differ from those used in the example presented herein, depending on the applications.
• the parameters can be chosen so as to give better fitting of the calculated calibration curve with actual flow response of the flowmeter. At low flows, the dominant contribution is given by the first element of the
• the parameter i defines conversion from flow to output voltage at low flows, which mainly depends on the construction of the GFR 1.
• the GFR 1 with diaphragm-type flow-resistive elements 5 generates differential pressure which varies near to the square of flow (square transfer function).
• some deviations of the parameter ai as well as parameters a, (1 ⁇ / ⁇ /V) from the value of 2 are also included in the invented linearization method.
• the number of sub-ranges may also vary from one to some number greater than two. This choice depends on the particular application.

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• 2000
  • 2000-09-01 AU AU2000269752A patent/AU2000269752A1/en not_active Abandoned
  • 2000-09-01 JP JP2001522038A patent/JP2003529051A/ja active Pending
  • 2000-09-01 CA CA002422398A patent/CA2422398A1/en not_active Abandoned
  • 2000-09-01 KR KR10-2003-7003190A patent/KR20030043951A/ko not_active Application Discontinuation
  • 2000-09-01 WO PCT/CA2000/001026 patent/WO2001018496A2/en active Search and Examination
  • 2000-09-01 EP EP00958063A patent/EP1315948A2/en not_active Withdrawn

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