WO1992001940A1

WO1992001940A1 - Method and device for flow rate measurement

Info

Publication number: WO1992001940A1
Application number: PCT/SE1991/000494
Authority: WO
Inventors: Bengt NORÉN
Original assignee: Noren Bengt
Priority date: 1990-07-17
Filing date: 1991-07-15
Publication date: 1992-02-06
Also published as: SE9002446D0; SE9002446L; SE466820B; EP0540595A1; JPH05508923A

Abstract

A process and an apparatus for flow rate measurement are based on the utilization of a measuring body exposed to the medium, the flow of which is to be measured, a physical quantity of said measuring body, for instance the resistance, changing with the temperature. The quantity (R1, R2) of the measuring body is measured in a measurement sequence at least two times during the cooling or heating of the measuring body, the time interval between the measurements being known or the same. Furthermore, the quantity (R0) of the measuring body is measured before heating or after cooling, i.e. when the measuring body has substantially the same temperature as the medium flow. Finally, the flow rate is calculated on the basis of the measurement values (R0, R1, R2) and the time interval (between R1 and R2). At least three quantity measurements are instead carried out with a known or the same time interval according to an alternative during heating or cooling and the flow rate is calculated on the basis thereof.

Description

Method and device for flow rate measurement

FIELD OF THE INVENTION AND PRIOR ART

This invention relates to a process and an apparatus for flow rate measurement. Media to be subjected to a flow measurement may be gases as well as liquids. A specific example is wind measurement. A measuring body exposed to a medium, the flow of which is to be measured, is included in the process and the apparatus, a physical quantity of said body, in particular an electrical quantity, such as the resistance, changing with the temperature.

A common type of hot body flow meters is based on the principal that a measuring body, the resistance of which changes with the temperature, is a part of a bridge with feed-back coupling. The bridge is kept balanced by changing the heating current through the measuring body. A not heated resistor with the same tempe¬ rature coefficient as the measuring body is arranged in the "cold branch" of the bridge so as to compensate for temperature changes in the medium flow or stream to be measured. This compensating resistor is placed in the medium flow close to the measuring body, which may be a wire. Apart from the energy exchange between the hot measuring body and the environment, the value read will be dependent on the nominal resistance of tae wire and the symmetry between the wire and the compensating resistor with respect to a temperature coefficient. Lack of symmetry gives rise to measurement errors. Further disadvan¬ tages of this known technique are constituted by the fact that a sensor comprising the measuring body ■ and the reference transmitter cannot be replaced without a re-calibration of the bridge. An additional disadvantage is that the electronics used has to be specially constructed for the purpose and may accor¬ dingly not be used to other things.

Another known flow meter with a heatable measuring body is based on that in a measurement sequence the measuring body is heated and after that allowed to cool under influence of the medium flow. The heating and the cooling are carried out successively, so that the temperature of the measuring body will vary between two temperature levels, which both are substantially higher than the temperature of the medium, the flow of which is to be measured. The upper temperature level is in a typical example 146°C and the lower one 96°C, and it is then meant that the medium, the flow of which is to be mea¬ sured, is air at room temperature. The duration of the heating of the measuring body from the lower to the higher temperature level and the duration of the cooling from the higher tempera¬ ture level to the lower one are utilized as measurement values being the basis for the flow rate calculation. This known technique leads to a rather complicated embodiment so as to make the heating take place with a constant power and between temperature levels being accurately defined. Variations of the heating power cause considerable measurement errors and the same thing is valid for inaccuracies of the temperature levels. An additional measuring body arranged in the medium flow, which is unheated and has to show the current temperature of the air flow, is also required in this known embodiment. Thus, unba¬ lances between the heated measuring body and the unheated measuring body also here give rise to measurement errors.

SUMMARY OF THE INVENTION

The object of the present invention is to further develop the process and the apparatus according to the preambles of claims 1 and 8, respectively, said preambles defining the second example given above of known technique, so that the process and the apparatus are simplified but in spite thereof possibilities to a higher measuring accuracy are created by the reduction of the dependence on potential sources of errors.

According to the invention this object is obtained by the characteristics defined more in detail in the characterizing parts of claims 1 and 8. Accordingly, the present invention is based on the fact that the heat exchange between the measuring body and the medium flow follows an exponential progress related to the flow rate. Thus, the central idea of the inven¬ tion is diametrically opposed to the prior art described in the introduction to carry out measurements of the physical quantity of the measuring body varying with the temperature with such known or constant time base that data is obtained, which give an unambiguous description of the exponential progress. The flow rate may thereby be determined or calculated on the basis of the measurement values and time data while using mathema¬ tical/ physical principles known per se. The physical quantity measured is preferably, although not necessarily, an electrical quantity, such as the resistance.

In the utilization of the process and the apparatus according to the invention an elimination or reduction of several sources of errors associated with the prior art is obtained. Any not heated measuring body with the same temperature coefficient as the heated measuring body is for example not required. The dependence upon the heating current and time is according to an embodiment of the invention (claim 3) completely eliminated. The dependence on variations of the heating current is accor¬ ding to another embodiment (claim 2; measuring during heating) at least substantially reduced, since no long time stability of the current is required as in accordance with the prior art. Accordingly , the process and the apparatus according to the invention are apart from the elimination of the need of a separate unheated reference sensor or measuring body characterized in that the measuring takes place during a progress or sequence, which is actively initiated (as far as the change of the temperature of the measuring body is con¬ cerned) but not controlled so as to obtain any balance or any final value aimed at; nor is any feed-back coupling required in order to obtain the balance or the final value thanks to the absence of such a control. This is the key to the obtainment of the simplicity and accuracy of the object of the invention.

According to a first embodiment of the process a quantity of the measuring body R (see Fig 1), which here is exemplified as the resistance, when the measuring body has substantially the same temperature as the medium flow. It is then suitable to measure R_Q as the first step in the measurement sequence. A heating current is after that applied (at A) through the measuring body, which raises the temperature of the body and thereby its resistance. The heating current is after the heating required switched off (at B) and at least two measure¬ ments R and R of the resistance of the measuring body is after that carried out at short intervals. It is then important (but easy to achieve) that the time interval between the measurements is known or the same for a plurality of measure¬ ment sequences. The same time intervals are always used in the simplest case, but different time intervals may also be used provided that they are known for each measurement sequence. In the case when the same time interval between the measurements is always used, it will in fact not be necessary that the time interval is known, but a calibration is instead carried out in a way known within the measurement field, so that the time interval continuously used and being always the same leads to the obtainment of the measurement values aimed at.

After switching off the heating of the measuring body at B, the measuring body will cool and finally reach the temperature of the medium flow, i.e. R . It appears from this that R_Q could be measured after completed cooling instead, but this results in an unnecessary time consumption and furthermore in that the flow conditions prevailing may change considerably. In a measurement system including several sensors with measuring bodies, the next sensor may be switched on immediately after the measurement of R- if R_ is measured first.

The resistances may be directly translated into energy levels when the temperature dependence of the wire is constant within the temperature range utilized. The relative energy loss from the measuring body during cooling will then be:

Experiments carried out show that the relative energy loss of the measuring body after a heating calculated in this way is only dependent on dtl2, the aerodynamical properties of the measuring body as well as the current flow rate. The relative energy loss turned out to be almost independent of:

- the heating current

- the heating time

- the delay after heating

- the length of the measuring body

- the nominal temperature coefficient of the measuring body

(independent means that the dependence is not stronger than that it may be made negligible without increasing the costs of the measuring equipment) .

These results are explained by the fact that the heat exchange between the measuring body and the medium flow follows an exponential progress as mentioned above. The exponential progress is in the case with a measuring body in the form of a wire characteristic for a certain wire diameter. An exponential progress or development is unambiguously described when the end point and two points with a known time interval are known.

Thus, all information about the exponential progress for the temperature adjustment (heat exchange) between the measuring body and the medium flow is according to this embodiment of the present invention obtained by R_, R-, R, and the time interval between R and R . Accordingly, the flow rate may be calculated by means of these measurement values and the time interval.

An unambiguous description of the exponential progress of the temperature adjustment between the measuring body and the medium flow dependent on the flow rate will according to a second embodiment of the invention be obtained by measuring the physical quantity of the measuring body, for instance the resistance, during the cooling of the measuring body at least three times, namely R- , R_ and R_, the time interval between R₁ and R as well as R and R being known or always the same. Thus, it is possible to calculate the flow rate according to principles well know per se thanks to the knowledge of these measurement values R., R₂ and R₃ and the time intervals. A measurement of R-, i.e. the value of the physical quantity when the measuring body has substantially the same temperature as the medium flow, is not required any longer but is defined by the exponential progress when at least three measurement values are obtained during the cooling. This means that it is accor¬ ding to this embodiment of the invention not necessary to let the measuring body cool down to substantially the temperature of the medium flow any longer, but the measuring body may instead be brought to comparatively quickly oscillate between two different temperature levels, and at least three measure¬ ments are carried out in quick time during the cooling between these levels.

In accordance with a third embodiment of the invention it is also possible to carry out the flow rated measurement by measuring the physical quantity, especially the resistance, three times during the heating of the measuring body, see R ,

R5._ and R,6. ,' the time interval between R4. and R5_c and between R5_c and R being known or always the same. The measurement values

R4. ,' R5_c and R6, and the time intervals therebetween deliver also in this case an unambiguous description of the exponential progress of the heat exchange between the measuring body under heating and the medium flow, so that the flow rate may be calculated on the basis of the measurement values and the time intervals. Since the quantity measurements are carried out without any maintained balance or without observing defined temperature levels, variations of the heating conditions (the heating current) over a longer period of time will not in¬ fluence the flow rate value obtained in a disturbing manner. It is valid also for this embodiment with measurement during the heating that the physical quantity (R ) of the measuring body has neither to be measured when the measuring body has sub¬ stantially the same temperature as the medium flow nor when the measuring body has any end value with respect to the tempera¬ ture, but the measuring body may even here by alternating heating and cooling be brought to oscillate rapidly between two different temperature levels both located above the temperature of the medium flow. Thus, in accordance with the invention these temperature levels have not to be known or determined.

In measuring while heating it is suitable to carry out the heating with a substantially constant power or otherwise with a known characteristic.

In accordance with a fourth embodiment of the invention it is also possible to carry out the flow rate measurement in parti¬ cular in media with comparatively high temperatures by, in a measu: - ent sequence, first of all actively cool the measuring body to a temperature lying below the medium temperature and after that allow the measuring body to be heated under the influence of the medium flow in the direction towards and to the temperature thereof. Measurements of a physical quantity of the measuring body changing with the temperature may be carried out during the heating caused by the medium flow for obtaining measurement values (analogous to R_Q, R. , R_ and R , R_, R_ , respectively) and time data sufficient to ensure an unambiguous description of the exponential progress of the heat exchange in a way substantially similar to the one already described above for the cooling case, although reverse with respect to the temperature change. The physical quantity may also analogous to what has already been described above with reference to the third embodiment be made at least three times during the possible cooling process so as to obtain measurement values corresponding to those indicated by R , R and R in the heating case.

The process and the apparatus according to the present inven¬ tion deliver, when a gas is measured, as the hot body flow sensors in general the mass flow, i.e. that the relation between the measurement value and the flow rate in meter per second is changed according to the general gas law when the temperature and the air pressure of the medium are changed.

The expression "determining or calculating" used herein with respect to the flow rate on the basis of the measurement values and the time intervals between the measurements should be interpreted within their broadest meaning as it already appears from the above. A strict mathematical operation may on one hand be carried out by hand or in a computer when the time intervals are known. However, when the time intervals between measure¬ ments carried out are always the same, such a calibration may instead be carried out, that the measurement values obtained may be the basis of the derivation of values indicative of the flow rate without having any knowledge of the real time intervals between the measurements.

The exponential progress or curve related to the heat exchange between the measuring body and the media flow is of course influenced by variations of the temperature and the flow rate of the medium during a measurement sequence.

The flow rate may according to the invention not be followed entirely continuously. However, the embodiments defined more in detail in claims 2 and 9 allow oscillation leading to heating and cooling or conversely with a relatively high periodicity, which permits to follow the flow rates variations rather well. It is for the rest not interesting to follow the flow rate continuously in measurements of average flow rates.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a specific description of embodiments according to the invention cited as examples.

In the drawings:

Fig 1 is a diagram illustrating the relationship resistance/ time (or approximately energy/time or temperature/time) ,

Fig 2 is a principal circuit diagram for the apparatus accor¬ ding to the invention, and

Fig 3 is a schematic view illustrating another embodiment of the measuring body than the one in Fig 2.

DETAILED DESCRIPTION OF EXEMPLIFYING EMBODIMENTS

Fiσs 1-2

The embodiment described below concerns the case in which R₀, R. and R are measured.

A suitable computer 1, which preferably is in the form of a programmable data logger, is used as means for measuring the resistance values R_Q, R- and R. indicative of the flow rate and means for calculating the flow rate of the medium in question on the basis of these values. In experiments such a data logger marked by the company Campbell Scientific Inc., Logan, Utah, USA under the designation "CR 10 Measurement and control Module" was used with success. This was in the experiment provided with two AM 416 16-channels relay multiplexors 2. A control connection between the data logger 1 and the relay multiplexor is in Fig 2 designated by 3.

Measuring bodies 4, the resistance of which changes with the temperature, were connected to each relay multiplexor 2. Measuring bodies in the form of wires were used. These wires were more exactly of Pt and their diameter was 0,1 mm. A diameter interval suitable from the practical point of view would here be less than 0,5 mm, preferably 0,01-0,2 mm.

Only two sensors 5 provided with measuring wires 4 are illus¬ trated in Fig 2. Although it is not illustrated in Fig 2, several sensors were connected to the relay multiplexors in the experiment. The measuring wires 4 may for example be attached, such as by soldering, between the legs of a holding clamp. A set of switches 6 is controlled by the relay multiplexors 2 for each of the sensors 5. The distance between the sensors 5 and the respective relay multiplexor 2 was in the practice between 10 and 80 meters. Resistors 7 with a resistance value dependent on the cable length in question were used for normalizing the cable resistance to the resistance of the four longest cables of 80 meters.

A relay 8 including a switch 9 for closing and interrupting, respectively, the heating current through a conductor 10 and a current-limiting resistor 11 to the sensor 5 in question was connected to the data logger 1.

The data logger 1 has an output 12, which is excited, delivers a constant voltage, in resistance measurements and is through a reference resistor 13 applied to the measuring wire 4 of the respective sensor 5 through a switch of the relay multiplexor 2 in question.

Furthermore, the data logger 1 has connections 14, 15 for voltage measurement, which connections are coupled over the respective measuring wires 4 through conductors and perspective switches 6.

The relay multiplexors 2 make it possible to operate a big number of sensors 5 by one single data logger 1, said sensors being successively connected to the connections of the data logger for heating current and measurement.

When a certain sensor 5 shall carry out a measurement sequence it is connected to the data logger 1 by means of the relay multiplexor 2. The resistance R_Q of the measuring wire 4 is first measured without any preceding heating, i.e. when the measuring wire 4 has the temperature of the medium flow. The switch 9 is after that closed by means of the relay 8, so that a heating current flows through the measuring wire 4 and this is heated. The heating current is interrupted at the point B in Fig 1 (this point has to be well defined) and the data logger 1 operates after that automatically with always the same time interval the two resistance measurements R. , R_ in Fig 1 during the cooling of the measuring wire 4. The localization of the measuring points - and R_ on the time scale, i.e. during the exponential heat exchange sequence between the wire and the medium flow, is not critical; however, it is appropriate to make the two measurements R and R in a part of the exponen¬ tial curve being not too flattened.

Measurement sequences are after that carried out in an analo¬ gous way with the other sensors connected to the data logger 1 through the relay multiplexors 2. An experiment series of 22 sensors manufactured by handicraft and having measuring wires of 0,1 mm Pt-wire turned out to deliver measurement values with very small mutual discrepancies. The dispersion of the measuring values were within ± 1%, which is a remarkably good result.

Apart from the fact that the process and the apparatus accor¬ ding to the invention lead to an excellent measurement accu¬ racy, a particular advantage is that the sensors may be of a very simple and cheap type. The electronic components needed to adapt the sensors to the data logger used were restricted to the reference resistor 13 for the resistance measurement, the relay 8 and the current limiting resistor 11 for the heating current and a number of resistors 7 for normalizing the cable resistances.

Sensors 5 with the measuring wires or bodies 4 may also be used to measure the temperature besides the flow rate, which re¬ quires an individual calibration of the sensors with respect to the temperature. Such a combination of flow rate and tempera¬ ture measurement is of particular interest under meteorological circumstances and may be used in measurements of energy ex¬ change.

The equipment described by means of Fig 2 may through a simple programming be adapted for excluding the measuring of R and instead measuring of at least the three values R₁, R and R₃ during cooling.

Adaptation of measuring equipment suitable for the purpose so as to carry out the measurement of the measurement values . , R and R on heating lies in consideration of the instructions according to the invention given above within the field of competence of a man with ordinary skill in the art, so that no further description is made with respect thereto. Fig 3

An alternative embodiment of the measuring body 4 is illustra¬ ted in Fig 3. The measuring body comprises in this case a hollow member 16, preferably in the form of a tube closed at one end 17. At least the part of the tube located closest to the closed end 17 of the tube is intended to be located in the medium flow 18. The tube may extend into the medium flow through an opening in a wall 19 of a channel delimiting the medium flow. A sealing member 20 is suitably arranged between the tube 16 and the channel wall 19. The tube 16 may suitably extend substantially perpendicularly to the flow direction of the medium.

The medium in question will in the following description have a comparatively high temperature. For instance the channel wall 19 could in such a case be provided with cooling arrangements, for instance internal cooling channels.

The means for making the temperature of the measuring body 4 to deviate from the temperature of the medium flow is in this case arranged to cool the measuring body. The cooling means com¬ prises more exactly a suitable cooling fluid source 21, which delivers cooling fluid to a conduit 22, which extends into the measuring body 4 made as a tube 16 so as to deliver a cooling fluid inside the tube 16 and by that cool the inner side thereof. The conduit 22 may for instance be arranged to deliver cooling fluid towards the inner side of the closed end of the tube 16, so that this inner side is cooled down, whereupon cooling fluid is forced to turn and flow back inside the tube 16 away from the closed end 17 thereof so as to finally be drained off out of the tube. As illustrated in Fig 3, it is suitable that the conduit 22 enters into the tube 16 at a point outside the channel wall 19 and extends along the tube 16 thereinside to a point rather close to and opposite to the inner side of the closed end of the tube. The measuring means for measuring the physical quantity of the measuring body 4 changing with the temperature is here an infra-red-meter, which is schematically indicated at 24 and comprises an infra-red-sensor 25 arranged to detect infra¬ red-radiation from the measuring body 4, more exactly from the inner side of the closed end of the tube 16. The infra-red- meter 24 is as indicated in Fig 3 connected to the end of the tube 16 turned away from its closed end 17 and the infra-red- sensor is directed along the tube in order to be able to detect the infra-red-radiation from the inner side of the closed end 17. This location leads in combination with the cooling fluid to a comparatively high security against excessive temperatures of the infra-red-meter and the sensors thereof.

When the embodiment according to Fig 3 is used to carry out a flow rate measurement cooling of the tube 16 by means of the cooling means 21, 22 is carried out according to a first variant, so that the tube 16 is cooled down to a temperature below the medium temperature. After that the cooling is termi¬ nated, which means that the tube 16 will tend to adapt its temperature to the temperature of the medium flow. Measurements of the infra-red-radiation (corresponding to the temperature) on the inner side of the closed end of the tube are carried out by means of the infra-red-meters 24, 25 during this adaptation. At least two measurements may be carried out in the way de¬ scribed as principal by means of Fig 1, said measurements delivering in combination with a measurement when the measure¬ ment tube 16 has substantially the same temperature as the medium flow measurement values which in combination with required information concerning the time interval between the two measurements first mentioned lead to an unambiguous description of the exponential progress, according to which the temperature equalization between the measurement tube 16 and the medium flow takes place, so that the flow rate of the medium may be determined or calculated therethrough. Alter¬ natively, at least three measurements of the infra-red-radia¬ tion are carried out during the temperature adaptation of the measurement tube 16 to the medium flow, in which the tempera¬ ture equilibrium state has not to be measured.

At least three infra-red-radiation measurements with a known or the same time interval are carried out during the cooling duration in accordance with a second variant and an unambiguous description of the exponential progress of the heat exchange dependent on the medium flow is obtained on the basis thereof. It should be understood that the cooling should be carried out under constant conditions or otherwise conditions with a known characteristic.

The apparatus according to Fig 3 may if desired of course be modified so that the cooling means 21, 22 becomes a heating means instead, i.e. supplies fluid causing heating of the measuring body to a temperature above the temperature of the medium flow.

POSSIBLE MODIFICATIONS

The process and the apparatus according to the invention may of course be modified in several ways within the scope of the inventional idea. The measuring body may in the heating case be a part of a more complex sensor of varying type, which could have a separate heating element, for instance an electrical one, so as to heat the measuring body by conduction of heat thereto. The measuring body as well as the heating element could then be enclosed in a shell or body of the sensor. Such an embodiment could be particularly suitable to flow rate measurements in liquids and the heat exchange between the measuring body and the medium would accordingly take place indirectly through any form of an envelope. More regular thermistors could also be used as an alternative to the measu¬ ring body in the form of the wire described. Other types of measurin< bodies, for instance thermocouples, having other electrical quantities being dependent on the temperature may also be used. As it appears from Fig 3, other physical quanti- ties than electrical ones could also be measured in accordance with the inventional idea. It should be mentioned as an example that just the temperature is measured by means of a measuring body constituting a thermometer of a type being arbitrary or known per se, for instance a so called optical thermometer.

Finally it should be mentioned that in the case of a measuring body being heated in an electrical way the measuring current and the heating current could be the same. This could then be applied to the measuring body continuously but with such a varying intensity that the measuring body would oscillate between temperature levels spaced apart, which however do not have to be defined or known. As a consequence of the resistance change of the measuring body in dependence on the temperature, the measuring current (and also the heating current) will get a variance, i.e. be the physical quantity changing with the temperature aimed at according to the invention. Measurement values and time data sufficient for determination of the exponential progress of the heat exchange may be obtained by measurements of this quantity and the flow rate may also be determined therethrough. An embodiment according to these lines of aim could give a high measurement periodicity, so that also rapid changes of the flow rate can be followed.

Claims

1. A process for flow rate measurement, in which a measuring body (4) exposed to the medium, the flow of which is to be measured, is used and in a measurement sequence the temperature of the measuring body is brought to deviate from the tempera¬ ture of the medium flow and after that is allowed to be adapted towards or to the latter under influence of the medium flow so that the measuring body will be heated and cooled or conver¬ sely, and in which values indicative of the flow rate are measured and the flow rate is determined or calculated on the basis thereof, c h a r a c t e r i z e d in that in connection with the heating and/or the cooling of the measuring body measurements of a physical quantity of the measuring body changing with the temperature are carried out for obtaining measurement values and time data sufficient to give an unam¬ biguous description of the exponential progress, according to which the heat exchange between the measuring body and the medium flow takes place, and that the flow rate is determined or calculated on the basis of these measurement values and time data.

2. A process according to claim 1, c h a r a c t e r i z e d in that in a measurement sequence

a) the physical quantity (R₁, R₂, R₃ and R₄, ₅, R , respec¬ tively) of the measuring body is measured at least three times during the heating and/or the cooling of the measuring body, the time interval between the measurements being known or the same for a plurality of measurement sequences, and that

b) the flow rate is determined or calculated on the basis of the measurement values (R., R , R and R., R , R , respec¬ tively) and the time interval.

3. A process according to claim 1, c h a r a c t e r i z e d in that in a measurement sequence a) the physical quantity (R.,, ₂) of the- measuring body is measured at least two times during the temperature adaption of the measuring body towards the temperature of the medium flow, the time interval between the measurements being known or the same for a plurality of measurement sequences,

b) the quantity (R_Q) of the measuring body before or after the measurements in step a) is measured when the measuring body has substantially the same temperature as the medium flow, and that

c) the flow rate is determined or calculated on the basis of the quantity measurement values (R , R , R ) and the time interval.

4. A process according to any of the preceding claims, c h a r a c t e r i z e d in that the physical quantity measured is an electrical one, for instance a resistance.

5. A process according to any of the preceding claims, c h a r a c t e r i z e d in that a wire is used as measuring body.

6. A process according to claim 5, c h a r a c t e r i z e d in that a Pt-wire is used as wire.

7. A process according to claim 5 or 6, c h a r a c t e r i z e d in that the diameter of the wire is less than 0,5 mm, preferably 0,01-0,2 mm.

8. An apparatus for flow rate measurements, comprising a measuring body (4) arranged to be exposed to the medium, the flow of which is to be measured, means for making the tempera¬ ture of the measuring body to deviate from the temperature of the medium flow and after that allow the temperature of the measuring body to be adapted towards or to the temperature of the medium flow under influence thereof, so that the measuring body will be heated and cooled or conversely, means for measu- ring values indicative of the flow rate and means for determi¬ ning or calculating the flow rate on the basis of these values, c h a r a c t e r i z e d in that the measuring means is arranged to, in connection with the heating and/or cooling of the measuring body (4) , measure a physical quantity of the measuring body changing with the temperature for obtaining measurement values and time data sufficient to give an unam¬ biguous description of the exponential progress, according to which the heat exchange between the measuring body and the medium flow takes place, and that the determining or calcu¬ lating means is arranged to determine or calculate the flow rate on the basis of these measurement values and time data.

9. An apparatus according to claim 8, c h a r a c t e r i z e d in that the measuring means is arranged to measure the physical quantity (R , R , R and R , R , R , respectively) of the measuring body at least three times during the heating/and or cooling thereof at time intervals being known or the same for a plurality of measuring sequences, and that the determining or calculating means is arranged to determine or calculate a flow rate on the basis of the quantity measurement values (R., R₂, R and R , R , R_g, respectively) and the time interval.

10. An apparatus according to claim 8, c h a r a c t e r i z e d in that the measuring means is arranged to measure the physical quantity (R , R ) of the measuring body at least two times during its temperature adaption towards the temperature of the medium flow at a time interval being known or the same for a plurality of measurment sequences, that the measuring means also is arranged to measure the quantity (R_) of the measuring body when the measuring body has substantially the same temperature as the medium flow, and that the determining or calculating means is arranged to determine or calculate the flow rate on the basis of the quantity measurement values (R_Q, R., R ) and the time interval.

11. An apparatus according to any of the claims 8-10, c h a r a c t e r i z e d in that the measuring means is arranged to measure an electrical quantity, such as a resis¬ tance, of the measuring body.

12. An apparatus according to any of the claims 8-11, c h a r a c t e r i z e d in that the measuring body is a wire.

13. An apparatus according to claim 12, c h a r a c t e r i z e d in that the wire is of Pt.

14. An apparatus according to claim 12 or 13, c h a r a c t e r i z e d in that the diameter of the wire is less than 0,5 mm, preferably 0,01-0,2 mm.

15. An apparatus according to any of the claims 8-14, c h a r a c t e r i z e d in that the means for making the temperature of the measuring body to deviate from the tempera¬ ture of the medium flow is arranged to heat the measuring body.

16. An apparatus according to any of the claims 8-14, c h a r a c t e r i z e d in that the means for making the temperature of the measuring body to deviate from the tempera¬ ture of the medium flow is arranged to cool the measuring body.

17. An apparatus according to claim 15 or 16, c h a r a c t e r i z e d in that the measuring body comprises a hollow member arranged at least partially in the medium flow, that the apparatus comprises means for cooling or heating the hollow member, and that the measuring means is a infra-red- meter with an infra-red-sensor arranged to detect the infra- red-radiation from a portion of the inner surface of a wall of the hollow member.

18. An apparatus according to claim 17, c h a r a c t e r i z e d in that the infra-red-detector is arranged at such a part of the hollow member that is intended to be located outside a wall of a channel delimiting the medium flow.