WO2020005149A2

WO2020005149A2 - Method for digital flow measurement in pulsating flows

Info

Publication number: WO2020005149A2
Application number: PCT/SE2019/050631
Authority: WO
Inventors: Daniel OTTOSEN; Marianne Dalene; Gunnar Skarping
Original assignee: Provtagaren Ab
Priority date: 2018-06-29
Filing date: 2019-06-27
Publication date: 2020-01-02
Also published as: WO2020005149A3

Abstract

The invention relates to a method for flow measurement in pulsating flows, wherein said method comprises the steps of: a) providing a flow meter (1) comprising an inlet (2) and an outlet (3), a flow channel (4) extending between said inlet (2) and outlet (3), a first mass flow sensor (6) located inside said flow channel (4), a control unit in communication with the first flow channel, b) repeatedly b1) acquiring a signal sample of the output signal from the mass flow sensor (6), b2) translating the signal sample to a mass flow value using a previously made calibration, and c) providing an output signal comprising the last mass flow value.

Description

METHOD FOR DIGITAL FLOW MEASUREMENT IN PULSATING FLOWS

Technical Field

The present invention relates generally to a method for method for flow measurement in pulsating flows. More particularly, the present invention relates to a method for flow measurement in pulsating flows as defined in the introductory parts of the appended independent claims.

Background Art

There is a clear demand for the monitoring of air-borne compounds that can have health effects on exposed individuals. A great interest exists for compounds that have occupational exposure limit values, set by governmen tal bodies, to ensure that the levels of such compounds are satisfactory low.

In many cases, it is not known what the air contaminants consist of and for this reason, it is of interest to learn more details about the nature of these “unknown” compounds and to reveal the identity of the most predominate ones. Another field of interest is to study and check the effect of measures with a view to reducing these levels in air, e.g. to check the“true” ventilation efficiency or other measures to control the air levels. Devices for this purpose can also be used for the monitoring of the quality of compressed air and air in respiratory protective devices. Other fields of application for such devices are e.g. the control of different volatile compounds present in food. Such com pounds can be used as markers for degradation of certain food components or to monitor raw materials to ensure a satisfactory quality. Such devices may also be used to ensure that other compounds have not contaminated to food. In hospitals, such devices can be used to check the air levels of e.g. narcosis gases and to ensure that the personnel, patients or others are not exposed to toxic levels. Chemical warfare agents are compounds that need to be check ed for in order to reveal the presence thereof and to ensure that individuals are not exposed.

In environmental analysis there is a need to monitor the quality of air in cities, public places and in nature. One purpose is to obtain background data for statistical studies and to check if the levels are below the levels set by national and international bodies. They can also be used to check if the emission of industrial pollutants results in exposure in nature or in populated areas. The achieved data can have an impact on decisions and interpretation of a certain situation. There is therefore a demand of a satisfactory high qua lity of the data.

There are many examples of air pollutants that occur in both gas and particle phase. Of special interest are the size fractions that have the ability to reach the lower respiratory tract. There are reasons to believe that the toxicology is different depending on not only the chemistry as such but also on the distribution on different target organs in the body of humans. There is a need to know more about the exposure to the respirable particle fraction present in air.

Numerous devices exist for the monitoring of air-borne compounds and there is a great variety of technology used. In principle, the devices can be grouped in selective and non-selective devices. Non-selective devices give a response for several compounds and do not differentiate between two or several compounds and may also result in false positive results. Such devices are today still used, possibly due to the low cost. In many applications, false positive results can give rise to a high cost for the user, if costly measures are performed from invalid data.

Selective devices give a certain response for a selected compound or a group of compounds. Other present compounds do not interfere with the result. The frequency of false positive results will be much less as compared to non-selective monitoring. The quality of the data obtained is essential. Typical factors that describe the quality of the data are: repeatability, reproducibility, linearity (calibration graph characteristics with intercept and background), detection limit and quantification limit. In addition, knowledge regarding the interference from other compounds is necessary. It needs to be mentioned that a certain compound can influence the result even if the compound does not itself give rise to a response.

An important parameter in this area is the gas flow containing the compound to detect, i.e. the analyte, in the apparatus used for the detection. During the sampling of compounds in air it is of importance to be able to control and log the flow and volume of the acquired amount of air through the sampling device as there is a direct correlation between the contents in a sample and the air volume collected. Taking several samples simultaneously is also of importance for three reasons, more precisely for increasing the accuracy of a certain sample, for detecting erroneous samples, and for acquiring different compounds simultaneously. When handling sampling results, it is also important to be able to track how the sample was collected, the time, the flow, the temperature, the pressure, and the humidity.

Existing solutions to maintain a stable flow during sampling do not prove to maintain a stable flow over time and require field calibration. The flow speed needs to be calibrated before and after sampling to ensure that the sampling speed is correct and have not changed over time. A logging functionality is also often missing.

Some existing solutions where a differential pressure sensor indicates if a change in the flow system back pressure has occurred, adjusts the pump control signal to compensate for this. However, this solution has proven to give drift errors over time, and a calibration with an external flow meter is required in order to set a certain flow rate of its pump.

Another existing solution has a logging function, an ability to transfer logged data to a PC, and an ability to control the flow via a display and buttons. Tests on such pumps did not concur with its specifications, as the pumps did not manage to keep a stable flow due to the fact that a sampler inducing a certain backpressure was attached to it.

A problem with existing pump systems is that the flow sensors incorpo rated in them may fluctuate with the temperature of the flow sensor electronics. Most flow sensors, using different techniques for the actual measurement of gas flow, have an output voltage signal corresponding to the measured flow. The output signal is however easily affected by the

temperature of the electronic components in the flow sensor.

A further problem with the pumps for sampling purposes of the prior art is that the calibration of the pump mass flow sensor and thereby its

measurement results is/are degraded relatively fast due to wear and damages to the sensors of the pump and to the pump engine. The pumps are often used in rough conditions at industrial work places and often outdoors.

Pumps used for sampling purposes have varying levels of flow fluctuation due to the principle of the pump function for each pump.

Membrane pumps by nature have large fluctuations, sometimes a fluctuation from zero to full flow if the pump only has one membrane. Rotation pumps also have relatively large fluctuation.

Flow meters are inherently poor in measuring fluctuating flows. Many flow meters are labelled to not function correctly in pulsating flows. Since all pumps produce fluctuation to some extent, almost always to a large extent, and since the flow is a key feature in a correct fluid sampling procedure, a large need for a better flow measurement method is needed in the industry to enable more accurate air sampling fluid measurements.

In that context there is further a demand for automatically detection of defects in sampling equipment e.g. in a pump assembly and a sampling device, or other devices used in sampling.

Summary of the Invention

It is an object of the present invention to improve the current state of the art, to solve the above problems, and to provide an improved method for flow measurement in pulsating flows. According to a first aspect, these and other objects, and/or advantages that will be apparent from the following description of embodiments, are achieved, in full or at least in part, by a method for flow measurement in pulsating flows. The method comprises the steps of

a) providing a flow meter comprising: an inlet and an outlet, flow channel extending

between said inlet and outlet, a first mass flow sensor located inside said flow channel, a control unit in communication with the first flow channel, b) repeatedly b1 ) acquiring a signal sample of the output signal from the mass flow sensor, b2) translating the signal sample to a mass flow value using a previously made calibration,

c) providing an output signal comprising the last mass flow value. Normal mass flow sensors that are placed in a flow channel are built up by an up-streams first temperature sensor, a heating element down streams of the first temperature sensor, a second temperature sensor down streams of the heating element. This kind of mass flow sensor is both cheap and extremely sensitive. However, mass flow meters containing that kind of sensor often measure incorrect values if the flow is not laminar. They are therefore normally labeled to only function for laminar flows. The reason that these mass flow meters produce incorrect values is found in the

implementation of the flow meter. The mass flow sensor is normally controlled by microelectronics that sample the output signal from the mass flow meter at a certain frequency. The output signal from the mass flow meter is simply the temperature difference between the first temperature sensor and the second temperature sensor. To avoid fluctuations in the signal, as the mass flow sensors are very sensitive, a floating average of number of the last sampled values are presented as an output value. The output value is then translated to a mass flow by using a previously made calibration. As the signal strength is not linear to the flow rate, a polynomial function has to be used to represent the calibration. This will produce a correct measurement as long as the flow is laminar.

However, an average of the signal from the mass flow sensor when measuring a fluctuating flow will not be correctly measured due to the non linear relationship between the mass flow sensor signal and the flow rate.

This problem is solved by the method of the first aspect. The method of the first aspect will output a correct momentary mass flow measurement. If the repetition rate is high fluctuations may be resolved by continuous and correct measurements.

By measuring and quantifying the characteristics of the pulsations in the flow this may be reported to e.g. a control unit of a device. When the method is used in conjunction with a sampling device for which the method is used to measure the through flow, the data of the flow characteristics may be used to exactly measure how much gas has been drawn through the sampling device. The step b) may further comprise the step b3) of calculating a mean mass flow value from the last of a predetermined number of mass flow values. The step c) may then provide an output signal comprising the last calculated mean mass flow value. In that way a correct mean value will be the output from the mass flow meter.

The repetition of step b) is made in the range of every 1 ps to every 1 s. A correct high resolution of the single values used to calculate a mean value is important to measure a correct mean value of a fluctuating flow.

The flow meter may further comprise a first pressure sensor (7) located near said first mass flow sensor (6) adapted to measure a first pressure inside said flow channel (4), and a second pressure sensor (8) located outside said flow channel (4), said second pressure sensor (8) being adapted to measure a second pressure being the ambient atmospheric pressure. The method may further comprise the steps of measuring a first pressure with the first pressure sensor, measuring a second pressure with the second pressure sensor, calculating a first volume flow based on the mass flow value, the first pressure, the second pressure and the ideal gas law, and providing the first volume flow as a second output signal.

The method may further comprise the steps of: calculating a second volume flow from the pressure difference between the first pressure sensor and the second pressure sensor and a pre-prepared calibration to volume, comparing the calculated first volume flow and the second volume flow, and if the flow rate is above a predetermined threshold value, providing the second volume flow as the second output signal. Since a delta P volume flow sensor is more sensitive at high flows, while a differential thermal mass flow sensor is more sensitive at low flow rates, the flow meter may be optimized by using the sensor that is most sensitive at each instance in time.

The method may further comprise the step of when both the first volume flow and the second volume flow indicate that no flow is present, measuring a zero mass flow value with the flow meter, adjusting the calibration of the mass flow sensor to match the zero mass flow volume. If the flow meter is connected to a pump and the flow meter has information from the pump that it is not running, the zero mass flow value may be measured without estimation by its own flow sensors.

Measuring the zero mass flow recurrently, e.g. before each use of the mass flow meter, gradually coating of the sensors will be compensated for, thereby avoiding long term deviations. Such coating may e.g. be due to particles from the fluid flow adhering onto the sensor as dust will eventually reach every surface.

The method may further comprise the step of, if the zero mass flow signal indicates that the mass flow sensor is covered by coating, briefly increasing the temperature of heating elements of the flow meter to burn off the coating from the sensor. Other sensors indicating problems with coatings could also be used to trigger a burn-off using the heating element of the mass flow sensor. One or multiple heaters with the only purpose of purging sensors from coatings may also be used. Other means for purging the sensors when needed can natural y also be used.

The translation in step b2) may be performed by a calculation by a control unit based on the signal sample and a pre-calibrated polynomial. The translation step b2 may however also be translated using analog electronic components analog to how linear output sound is achieved in audio speakers.

The flow meter may further comprise an inlet filter, and an outlet filter, and the method may further comprise the step of providing an error signal if the calculated pressure difference is below a pre-determined value, thereby indicating a broken inlet or outlet filter of the flow meter. A sudden deviation of more than about 5% from a previous measured value in a measurement session, e.g. measured one second to 10 minutes ago, indicates malfunction. If the error signal indicates an error of more than 30% deviation from values set at the factory calibration, but deviates less than 5% from recent

measurements, the error is probably due to wear and tear of the flow meter. Both of these error types are preferably displayed for the operator of the flow meter. By detecting broken filters before measurements are initiated, the flow meter and other equipment attached for the measurement are saved from wear and extra service. Thereby costs can be saved. The method may further comprise the steps of fluidly connecting a sampling device to the inlet of the flow meter, providing a flow through the assembly by providing a sub-pressure source to said outlet of the flow meter, measuring the backpressure induced by the sampling device, calculating its restriction and evaluating the condition of the sampling device, and logging said restriction and said evaluated condition to a memory. The logged values may then be read at a later stage to provide further information regarding the measurement made using the sampling device.

The flow meter may further comprise an ambient temperature sensor, wherein the method further comprises the step of: measuring the ambient temperature with the ambient temperature sensor, measuring the temperature in the flow channel using a reference temperature measurement provided by the mass flow sensor, calculating the temperature difference between the measured ambient temperature and the measured reference temperature in the flow channel by use of the control unit, providing an error signal if the calculated temperature difference is above a predetermined threshold. The ambient temperature sensor may be used to increase the accuracy of the correction of the flow measurement value when e.g. converting the mass flow to a volume flow. The above method makes it possible to detect if either the ambient temperature sensor or the reference temperature sensor of the mass flow meter is broken. The method may further comprise the step of providing the option of manually setting the ambient temperature to a certain value. If the above method indicate that the ambient temperature sensor is broken, the temperature value to be used in calculations may be set manually by the operator of the flow meter, e.g. by looking at a normal thermometer close by or checking the weather forecast.

The method may further comprise the steps of: detachably connecting a second mass flow sensor to said outlet or inlet, measuring a first mass flow with the first mass flow sensor and a second mass flow with the second mass flow sensor, calculating the difference between the values of said first and second mass flows, and providing an output signal representing said calculated difference. If such a reference measurement produces an error signal, the first mass flow sensor of the flow meter has to be re-calibrated. This can, however, be achieved automatically by adjusting the output signal of the first mass flow sensor to compensate for said calculated difference.

The external mass flow sensor may be connected via a USB port in the flow meter and thereby be controlled by the control unit of the flow meter. The external mass flow sensor may optionally also be built into the flow meter, and only be used for occasionally checking the calibration of the first mass flow sensor. The method may further comprise the step of adjusting the output signal of the first mass flow sensor to compensate for the calculated difference.

According to some embodiments the method may further comprise measuring and/or quantifying pulse characteristics as e.g. minimum values, maximum values, and/or a discrete histogram of a specific flow range. This is useful since oscillations in the flow affect how much the sampler is exposed to the flow.The flow meter in the method may further comprise: a memory, wherein all method steps are instructions in a computer program stored on said memory, said computer program being executed by said control unit, and wherein all calculations steps are performed by use of said control unit.

Other objectives, features and advantages of the present invention will appear from the following detailed disclosure, from the attached claims, as well as from the drawings. It is noted that the invention relates to all possible combinations of features.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to“a/an/the [element, device, component, means, step, etc.]” are to be interpreted openly as referring to at least one instance of said element, device, component, means, step, etc., unless explicitly stated otherwise.

As used herein, the term“comprising” and variations of that term are not intended to exclude other additives, components, integers or steps. Brief Description of the Drawings

The above objects, as well as additional objects, features and advantages of the present invention, will be more fully appreciated by reference to the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, when taken in conjunction with the accompanying drawings, wherein:

Fig. 1 is a schematic illustration of an embodiment of the flow meter of the present invention.

Fig. 2 is a block diagram showing the method of the invention.

Fig. 3 is a schematic graph showing characteristic sensor signals Detailed Description of Preferred Embodiments of the Invention

Fig. 1 is a schematic illustration of an embodiment of the flow meter 1 of the method present invention. The flow meter 1 has a flow channel 4 having an inlet 2 and an outlet 3. Gas, e.g. in the form of breathing air or modified breathing air, is drawn through the flow channel 4 by a pump or suction means (not shown). The mass flow of gas flowing through the flow channel 4 is measured by a mass flow sensor 6. Adjacent to the mass flow sensor 6 is a first pressure sensor 7, measuring the pressure in the flow channel 4. A second pressure sensor 8 is located on the outside of said flow channel 4 and said flow meter 1. The second pressure sensor 8 thus measures the ambient pressure. The flow meter is controlled by a CPU 9, wherein the CPU 9 uses a memory 10 to store control algorithms and data. The flow meter 1 is further equipped with an ambient temperature sensor 12, measuring the ambient temperature. The flow meter further has a display 13 for presenting information and options to an operator of the flow meter. The display is preferably a touch display to provide interaction with the flow meter. In case of a non-touch display, buttons (not shown) are present near the display. The flow channel 4 of the flow meter 1 further has an inlet filter 14 and an outlet filter 15. Fig. 2 shows a block diagram of the method for flow measurement in pulsating flows, with the steps of: a) providing a flow meter 1 comprising an inlet 2 and an outlet 3, a flow channel 4 extending between said inlet 2 and outlet 3, a first mass flow sensor 6 located inside said flow channel 4, and a control unit in communication with the first flow channel. The next step b is then iterated with the following sub steps: b1 ) acquiring a signal sample of the output signal from the mass flow sensor 6, b2) translating the signal sample to a mass flow value using a previously made calibration,

d) providing an output signal comprising the last mass flow value. Step b) may further comprise a step b3) calculating a mean mass flow value from the last of a predetermined number of mass flow values. Step c) is then providing an output signal comprising the last calculated mean mass flow value. The repetition of step b) is made in the range of every 1 ps to every 1 s.

Fig. 3 shows the typical relation between a gas mass flow rate and the signals two sensors measuring that mass flow rate exerts, these two sensors being a delta-pressure mass flow sensor whose flow-to-signal characteristics is according to 1 ) and a thermal mass flow sensor whose flow-to-signal characteristics is according to 2). Since the two sensors are having different flow to signal characteristics, they have their optimal range for flow

measurements, being range 1’) for the thermal mass flow sensor and range 2’) for the delta-pressure mass flow sensor. The f-axis thus indicate flow; and the s-axis indicate signal from flow sensor. 1 ) is the characteristic signal from a thermal mass flow sensor; and 2) is the characteristic signal from a delta- pressure mass flow sensor. 1’) is the range where a signal from 1 ) gives better accuracy; and 2’) is the range where a signal from 2) gives better accuracy.

According to one embodiment a method for flow measurement in pulsating flows is provided wherein the flow meter of Fig 1 , used in the method of Fig. 2 further comprise a first pressure sensor 7 located near said first mass flow sensor 6 adapted to measure a first pressure inside said flow channel 4, and a second pressure sensor 8 located outside said flow channel 4, said second pressure sensor 8 being adapted to measure a second pressure being the ambient atmospheric pressure, the method further comprising the steps of measuring a first pressure with the first pressure sensor, measuring a second pressure with the second pressure sensor, calculating a first volume flow based on the mass flow value, the first pressure, the second pressure and the ideal gas law providing the first volume flow as a second output signal. This method may further comprise calculating a second volume flow from the pressure difference between the first pressure sensor and the second pressure sensor and a pre-prepared calibration to volume, comparing the calculated first volume flow and the second volume flow, and if the flow rate is above a predetermined threshold value, providing the second volume flow as the second output signal.

According to a further embodiment a method for flow measurement in pulsating flows a method is provided with the step of: when both the first volume flow and the second volume flow indicate that no flow is present, measuring a zero mass flow value with the flow meter, adjusting the calibration of the mass flow sensor to match the zero mass flow volume. This method may further comprise the step of if the zero mass flow signal indicates that the mass flow sensor is covered by coating, briefly increasing the temperature of heating elements of the mass flow sensor 6 to burn of the coating.

The method of Fig 2 my further be modified so that the translation in step b2) is performed by a calculation by a control unit based on the signal sample and a pre-calibrated polynomial. The translation step b2 can be translated using analog electronic components.

The skilled person realizes that a number of modifications of the embodiments described herein are possible without departing from the scope of the invention, which is defined in the appended claims.

Claims

1. A method for flow measurement in pulsating flows, wherein said method comprises the steps of:

a) providing a flow meter (1 ) comprising

- an inlet (2) and an outlet (3),

- a flow channel (4) extending between said inlet (2) and outlet (3),

- a first mass flow sensor (6) located inside said flow channel (4),

- a control unit in communication with the first flow channel,

b) repeatedly

b1 ) acquiring a signal sample of the output signal from the mass flow sensor (6),

b2) translating the signal sample to a mass flow value using a previously made calibration,

c) providing an output signal comprising the last mass flow value.

2. The method according to claim 1 , wherein the step b) further comprises the step b3) calculating a mean mass flow value from the last of a predetermined number of mass flow values, and wherein the step c) is providing an output signal comprising the last calculated mean mass flow value.

3. The method according to any one of the preceding claims, wherein repetition of step b) is made in the range of every 1 ps to every 1 s.

4. The method according to any one of the preceding claims, wherein the flow meter further comprises

- a first pressure sensor (7) located near said first mass flow sensor (6) adapted to measure a first pressure inside said flow channel (4), and

- a second pressure sensor (8) located outside said flow channel (4), said second pressure sensor (8) being adapted to measure a second pressure being the ambient atmospheric pressure,

the method further comprising the steps of measuring a first pressure with the first pressure sensor,

measuring a second pressure with the second pressure sensor, calculating a first volume flow based on the mass flow value, the first pressure, the second pressure and the ideal gas law

providing the first volume flow as a second output signal.

5. The method according to claim 4, further comprising the steps of: calculating a second volume flow from the pressure difference between the first pressure sensor and the second pressure sensor and a pre-prepared calibration to volume,

comparing the calculated first volume flow and the second volume flow, and

if the flow rate is above a predetermined threshold value, providing the second volume flow as the second output signal.

6. The method according to any one of claims 3-5, further comprising the step of

when both the first volume flow and the second volume flow indicate that no flow is present, measuring a zero mass flow value with the flow meter, adjusting the calibration of the mass flow sensor to match the zero mass flow volume.

7. The method according to claim 6, further comprising the step of if the zero mass flow signal indicates that the mass flow sensor is covered by coating, briefly increasing the temperature of heating elements of the flow meter to burn off the coating.

8. The method according to any one of the preceding claims, wherein the translation in step b2) is performed by a calculation by a control unit based on the signal sample and a pre-calibrated polynomial.

9. The method according to any one of the preceding claims, wherein the translation step b2 is translated using analog electronic components.

10. The method according to any one of the preceding claims, wherein said flow meter (1 ) further comprises an inlet filter (14), and an outlet filter (15), and wherein the method further comprises the step of

providing an error signal if the calculated pressure difference is below a pre-determined value, thereby indicating a broken inlet (14) or outlet filter (15) of the flow meter (1 ).

11. The method according to any one of the preceding claims, further comprising the steps of

fluidly connecting a sampling device to the inlet (2) of the flow meter, providing a flow through the assembly by providing a sub-pressure source to said outlet of the flow meter,

measuring the backpressure induced by the sampling device, calculating its restriction and evaluating the condition of the sampling device, and

logging said restriction and said evaluated condition to a memory.

12. The method according to any one of the preceding claims, wherein the flow meter (1 ) further comprises an ambient temperature sensor (12), wherein the method further comprises the step of:

measuring the ambient temperature with the ambient temperature sensor (12),

measuring the temperature in the flow channel (4) using a reference temperature measurement provided by the mass flow sensor,

calculating the temperature difference between the measured ambient temperature and the measured reference temperature in the flow channel (4) by use of the control unit (9),

providing an error signal if the calculated temperature difference is above a predetermined threshold.

13. The method according to any one of the preceding claims, wherein it further comprises the steps of:

detachably connecting a second mass flow sensor to said outlet (3) or inlet (2),

measuring a first mass flow with the first mass flow sensor (6) and a second mass flow with the second mass flow sensor,

calculating the difference between the values of said first and second mass flows, and

providing an output signal representing said calculated difference.

14. The method according to claim 12, further comprising the step of adjusting the output signal of the first mass flow meter to compensate for said calculated difference.

15. Method according to any one of the preceding claims, wherein said flow meter further comprises:

- a memory (10), wherein all method steps are instructions in a computer program stored on said memory (10), said computer program being executed by said control unit (9),

and wherein all calculations steps are performed by use of said control unit (9).