Methods of measuring a flow rate of fluid flowing through various kinds of flow paths are largely divided into a method of measuring a volumetric flow rate and a method of measuring a mass flow rate. In case of fluid such as gas which is highly compressible and has a low specific gravity, flow control can be precisely performed by measuring the mass flow rate rather than the volumetric flow rate. Further, in a case where there are any chemical reactions, it is convenient to directly check and control the mass flow rate when measuring the flow rate in a chemical reaction apparatus, because the chemical reaction is performed based on a mass. The mass flow controller for measuring the mass flow rate and controlling a mass flow in response to measured values has been already widely employed in the whole industries. In order to measure an infinitesimal mass flow rate, a thermal measurement method of applying heat to fluid and measuring change in temperature of the fluid has been generally used.

Background Art FIG. 1 is a schematic view showing the constitution of a conventional thermal mass flow controller. The mass flow controller 200 comprises a sensor 220 for measuring a mass flow rate of fluid flowing through a flow path 210, a valve 230 for regulating the mass flow rate of the fluid flowing through the flow path 210 by adjusting an opening of the flow path 210, and a control unit 240 for adjusting the opening of the flow path 210 by actuating the valve 230 in response to the mass flow rate measured by the sensor 220.

The sensor 220 includes a sample fluid flowing tube 221 which is configured to be in fluid communication with the flow path 210 so that at least a portion of fluid flowing

through the flow path 210, a heating coil 222 which is wound on an outer surface of the sample fluid flowing tube 221 to function as a heating source for heating sample fluid flowing the sample fluid flowing tube 221 by converting electrical energy supplied from a power source 270 into thermal energy, a first temperature sensing coil 223 which is wound on the outer surface of the sample fluid flowing tube 221 upstream of the heating coil 222 to function as a temperature measuring device for measuring the temperature of upstream sample fluid, and a second temperature sensing coil 224 which is wound on the outer surface of the sample fluid flowing tube 221 downstream of the heating coil 222 to function as a temperature measuring device for measuring the temperature of downstream sample fluid. That is, an electrical signal corresponding to the temperature of the upstream sample fluid is obtained from the first temperature sensing coil 223, whereas an electrical signal corresponding to the temperature of the downstream sample fluid is obtained from the second temperature sensing coil 224.

The sample fluid flowing tube 221 of the sensor 220 is configured such that an upstream end thereof is connected with a hole penetrating through a sidewall of the flow path 210 and a downstream end thereof is connected with another hole penetrating the sidewall of the flow path 210 downstream of the upstream end. Thus, the sample fluid is introduced into the sample fluid flowing tube through the upstream end and then discharged from the tube through the downstream end. At this time, in order to obtain an accurate measured value, it should be ensured that the sample fluid is always drawn from the fluid flowing the flow path 210 at a constant mass flow ratio. To this end, a flow guide such as a laminar flow device 250 is provided in the interior of the flow path 210 so that a streamline of bypass fluid, which does not pass through the sample fluid flowing tube 221, can be changed.

Hereinafter, a principle of measuring mass flow rate using a difference between temperatures measured at upstream and downstream sides will be explained. FIG. 2 schematically shows a principle of measuring the mass flow rate using the temperature difference measured by the mass flow measurement sensor 220 constructed in the same manner as shown in FIG. 1.

As shown in FIG. 2, when the sample fluid has been heated by the heating coil 222,

electrical signals corresponding to the same temperature value are obtained from the first and second temperature sensing coils 223,224, respectively, if the sample fluid is not flowing. However, if the sample fluid is flowing, electrical signals corresponding to the temperature values different from each other by a temperature difference ST are obtained from the first and second temperature sensing coils 223,224, respectively. The reason that such a temperature difference DT is generated is that as the sample fluid flows downstream, a certain heat flux applied to the sample fluid by the heat source also flows downstream and accordingly the first temperature sensing coil 223 located at the upstream side is affected less than the second temperature sensing coil 222 located at the downstream side.

The temperature difference IT of the sample fluid is expressed as a function of a heat flux or heat flow rate Q provided from the heat source and a mass flow rate of the sample fluid heated by the heat flux and heat flow rate Q. Therefore, as expressed in Equation 1, the mass flow rate ni of the sample fluid flowing through the flow path 210 can be calculated from specific heat Cp of the sample fluid, the heat flux Q applied from heat source, and the temperature difference JT of the sample fluid obtained by the electrical signals from the first and second temperature sensing coils 223,224. m = Ql (Cp AT) (1) Generally, the heat flux applied from the heating source is kept constant, the control unit 240 causes a valve driving signal, which corresponds to the temperature difference between the upstream and downstream sample fluid and is obtained by the electrical signals from the first and second temperature sensing coils 223,224, to be transmitted to a valve actuator 260, and the valve actuator 260 allows the mass flow rate of the fluid flowing through the flow path 210 to be adjusted by actuating the valve 230 in response to the valve driving signal so as to adjust the opening of the flow path 210.

The mass flow measurement sensor may measure a mass flow rate using a power difference rather than the aforementioned temperature difference. A mass flow measurement sensor using the power difference is disclosed in Japanese Laid-Open Patent Publication No. (Hei) 1-318925 (December 25,1989) entitled"Mass Flow Controller"and filed in the name of Omi Tadahiro, et al. and in U. S. Patent No. 5,711, 342 (January 27,

1998) entitled"Mass Flow Controller, Operating Method and Electromagnetic Valve"and issued to Kazama Yoichiro, et al. The mass flow measurement sensor is constructed in such a manner that the first and second temperature sensing coils 223,224 of the sensor 220 receive electric power and generate heat so that they can function as heating sources for heating sample fluid without using any additional heating coils serving as the heating source. The mass flow measurement sensor is a type in which the mass flow rate of the sample fluid flowing through the flow path can be calculated by using the power difference obtained when the upstream and downstream temperatures of the sample fluid are caused to be always kept constant.

Such a thermal mass flow measurement sensor is constructed such that the sample fluid flowing tube through which the sample fluid flows is a single tube with a disk-type fluid path. According to the experiences, response time in sensing the flow rate of fluid flowing through the tube has been improved as the diameter of the sample fluid flowing tube is reduced, in case of the mass flow measurement sensor using the single sample fluid flowing tube.

However, if the diameter of the sample fluid flowing tube is caused to be reduced so as to increase the response time, other disadvantages are derived. That is, if the diameter of the sample fluid flowing tube and thus a sectional flow area thereof is reduced, a reduction ratio of the sectional flow area of the sample fluid flowing tube to a sectional flow area of a main tube through which a total amount of fluid to be controlled flows will be increased. At this time, the reduction ratio is a magnification factor to be multiplied to the mass flow rate measured by the sensor in order to obtain the total mass flow rate of the fluid. Thus, a measuring error of the sensor will be increased in proportion to the magnification factor, and thus, sensitive and accurate measurement cannot be expected.

In addition, if the sectional flow area of the sample fluid flowing tube becomes smaller by causing its diameter to be reduced in order to enhance the response time of the conventional mass flow measurement sensor, there is a disadvantage in that the mass flow measurement sensor is sensitive to fine particles contained in the sample fluid and change in pressure of the sample fluid.

FIG. 14 is a comparative graph plotting change in a temperature difference with

respect to change in mass flow rate measured by respective mass flow measurement sensors according to the prior art and the present invention.

As shown in FIG. 14, the conventional mass flow measurement sensor has a linear temperature difference characteristic up to a certain range of flow rate, but exhibits a nonlinear temperature difference characteristics after the range. Such a range within which the temperature difference is kept to be linear is called a linear range. Such a linear range is a range within which the mass flow rate can be precisely measured using the temperature difference. Thus, as the linear range is narrower, the range of measurement for the mass flow rate is also reduced. That is, it can be seen from FIG. 14 that the conventional mass flow measurement sensor has a narrow linear range of measurement.

Therefore, there is a disadvantage in that the range of measurement for the mass flow is narrow.

Accordingly, there is a need for the mass flow measurement sensor with a wide linear range of measurement so that the mass flow measurement sensor can be normally operated in a wide range of measurement.

Disclosure of Invention An object of the present invention is to provide a mass flow measurement sensor by which precise and sensitive measurement can be made, its response time is fast, and its linear range of measurement is sufficiently increased even in a case where a sectional flow area of a sample fluid flowing tube is large.

According to an aspect of the present invention, there is provided a mass flow measurement sensor for measuring a mass flow rate of sample fluid from a temperature difference or power difference which is obtained by heating the sample fluid and then measuring temperatures of the heated sample fluid at upstream and downstream sides or power required for causing the temperatures to be kept constant. The mass flow measurement sensor comprises a sample fluid flowing tube for defining a flow path through which the sample fluid flows, a heating source for heating the sample fluid flowing through the flow path, a first temperature sensing means for sensing the temperature of the upstream sample fluid affected by the heating source and generating an

electrical signal corresponding to the temperature, and a second temperature sensing means for sensing the temperature of the downstream sample fluid affected by the heating source and generating an electrical signal corresponding to the temperature.

The mass flow measurement sensor is characterized in that at least a portion of the flow path including an interval in which the heating source and the first and second temperature sensing means are located is divided into a flowing zone and a non-flowing zone. Preferably, the non-flowing zone is defined at the center of a cross-section of the flow path, and the flowing zone is defined to surround the non-flowing zone. Further, it is preferred that the non-flowing zone be defined by an interior rod which is inserted into the sample fluid flowing tube and spaced apart from an inner surface of the sample fluid flowing tube.

According another aspect of the present invention, there is provided a mass flow rate controller for causing a mass flow rate of fluid to converge on a control reference value in such a manner that an opening adjustment valve is allowed to be opened or closed so that change in the mass flow rate can be cancelled if the change in the mass flow rate occurs while the controller sets the control reference value for the mass flow rate of the fluid flowing through a main flow path and monitors the mass flow rate. The mass flow controller comprises a mass flow measurement sensor installed at the main flow path for measuring a change of the mass flow rate of sample fluid from a temperature difference which is obtained by heating the sample fluid and then measuring temperatures of the heated sample fluid at upstream and downstream sides, a bridge circuit for generating an electrical signal corresponding to the temperature difference in response to a signal outputted from the mass flow measurement sensor, a comparative control circuit for comparing a measured value from the mass flow measurement sensor with the reference control value and generating a control signal, and a valve actuator for actuating the opening adjustment valve in response to the control signal outputted from the comparative control circuit. At this time, the aforementioned thermal mass flow measurement sensor including the feature of the present invention may be utilized as the mass flow measurement sensor.

Brief Description of Drawings

FIG. 1 is a schematic view of a configuration of a mass flow controller in which a thermal mass flow measurement sensor is used, illustrating the thermal mass flow measurement sensor according to a prior art.

FIG. 2 schematically illustrates a principle of measuring a mass flow rate using a temperature difference in the mass flow measurement sensor constructed in the same manner as shown in FIG. 1.

FIG. 3 is a schematic view showing a configuration of a mass flow rate controller using a mass flow measurement sensor according to the present invention.

FIG. 4 is an enlarged sectional view illustrating the configuration of the mass flow measurement sensor according to a first preferred embodiment of the present invention.

FIG. 5 is a sectional view of a sample fluid flowing tube taken along line V-V of FIG. 4 illustrating a structure of the sample fluid flowing tube of the mass flow measurement sensor shown in FIG. 4.

FIG. 6 is an enlarged sectional view illustrating the configuration of the mass flow measurement sensor according to a second preferred embodiment of the present invention.

FIG. 7 is an enlarged sectional view illustrating the configuration of the mass flow measurement sensor according to a third preferred embodiment of the present invention.

FIG. 8 is an enlarged sectional view illustrating the configuration of the mass flow measurement sensor according to a fourth preferred embodiment of the present invention.

FIG. 9 is an enlarged sectional view illustrating the configuration of the mass flow measurement sensor according to a fifth preferred embodiment of the present invention.

FIG. 10 is an enlarged sectional view illustrating the configuration of the mass flow measurement sensor according to a sixth preferred embodiment of the present invention.

FIG. 11 is an enlarged sectional view illustrating the configuration of the mass flow measurement sensor according to a seventh preferred embodiment of the present invention.

FIG. 12 is an enlarged sectional view illustrating the configuration of the mass flow measurement sensor according to an eighth preferred embodiment of the present invention.

FIG. 13 is an enlarged sectional view illustrating the configuration of the mass flow measurement sensor according to a ninth preferred embodiment of the present invention.

FIG. 14 is a comparative graph plotting change in a temperature difference with respect to change in mass flow measured from the conventional mass flow measurement sensor shown in FIG. 1 and the mass flow measurement sensor according to the first preferred embodiment of the present invention shown in FIG. 4.

FIG. 15 is a graph plotting change in the temperature difference with respect to change in the mass flow rate measured ten mass flow measurement sensors which are configured according to the first preferred embodiment of the present invention shown in FIG. 2 but have different diameters of the sample fluid flowing tubes.

Best Mode for Carrying out the Invention Hereinafter, preferred embodiments of the present invention will be explained in detail with reference to accompanying drawings.

FIG. 3 shows an example of a configuration of a mass flow rate controller 100 in which a mass flow measurement sensor 120 can be employed according to the present invention. The mass flow rate controller 100 is a system for causing a mass flow rate to be kept within a predetermined range in such a manner that an opening adjustment valve is allowed to be opened or closed so that change in the mass flow rate can be cancelled if the change in the mass flow rate occurs while the system monitors the mass flow rate of fluid flowing through a flow path 110. In order to detect the change in the mass flow rate of the fluid, the mass flow measurement sensor of the present invention is installed in the flow path. The mass flow measurement sensor 120 is a thermal sensor for simultaneously outputting signals corresponding to upstream and downstream temperatures of the sample fluid, in the same manner as the conventional sensor mentioned above.

The mass flow rate controller 100 includes a bridge circuit 141 for generating electrical signals corresponding to a temperature difference between upstream and downstream sides in response to the signals outputted from the mass flow measurement sensor 120. The electrical signals outputted from the bridge circuit 141 are amplified

through an amplification circuit 142 and then transmitted to a comparative control circuit 143. That is, a measured value from the mass flow measurement sensor 120 and a reference value from a setting unit 144 are inputted into the comparative control circuit 143.

The comparative control circuit 143 outputs a control signal to a valve actuator 160 so as to actuate a valve 130 in a direction that the measured value is caused to converge into the reference value. The valve actuator 160 actuates the valve 130 in response to the input control signal so as to adjust the opening of the flow path 110.

Preferably, a display 145 for displaying the measured value from the mass flow measurement sensor 120 thereon is provided so that a user can view the measured value.

At this time, a displayed value on the display 145 may be in the form of the temperature difference or the mass flow rate converted from the temperature difference.

Further, in order to ensure that the mass flow rate of the fluid flowing through the flow path 110 can pass through the mass flow measurement sensor 120 at a predetermined ratio, a flow guide-150 for changing a streamline of fluid bypassing the mass flow measurement sensor 120 is provided at an inner side of the flow path 110. As described above in the prior art, the flow guide 150 may be a laminar flow device or other suitable means.

Although an example of the mass flow rate controller 100 in which the mass flow measurement sensor 120 using the temperature difference is employed has been described above, the mass flow measurement sensor using the power difference may be employed in the mass flow rate controller as disclosed in Japanese Laid-Open Patent Publication No.

(Hei) 1-318925 (December 25, 1989) and U. S. Patent No. 5,711, 342 (January 27, 1998).

Therefore, it is apparent that the mass flow measurement sensor 120 to be described later can measure the mass flow rate by using either the temperature difference or the power difference.

FIG. 4 specifically shows a configuration of the mass flow measurement sensor 120 according to the first embodiment of the present invention.

The mass flow measurement sensor 120 according to the embodiment includes a sample fluid flowing tube 121 serving as a flow path through which at least a portion of total mass flow to be measured that flows through the main flow path, in the same manner

as the aforementioned conventional mass flow measurement sensor. As to the heating source for heating the sample fluid flowing through the sample fluid flowing tube 121, there is provided a heating coil 122 for converting electrical energy into thermal energy.

As to a first temperature sensing means for measuring a temperature upstream of the sample fluid influenced by the heating source, there is provided a first temperature sensing coil 123 installed near an upper end of the sample fluid flowing tube 121 to generate electrical signals that varies depending on a change in temperature. As to a second temperature sensing means for measuring a temperature downstream of the sample fluid affected by the heating source, there is provided a second temperature sensing coil 124 installed near a lower end of the sample fluid flowing tube 121 to generate electrical signals that varies according to change in temperature.

At least a portion of the length of the sample fluid flowing tube 121 according to the embodiment is constructed to have a cross-section of which a central portion defines a non-flowing zone 127. This is a most important feature for achieving the object of the present invention. As shown in FIGS. 4 and 5, an interior rod 125 is disposed within the sample fluid flowing tube 121 in a state where it is spaced apart from an inner surface of the sample fluid flowing tube and longitudinally extends therein. The sample fluid flows through a space between the inner surface of the sample fluid flowing tube 121 and an outer surface of the interior rod 125. That is, a cross-section of the central interior rod 125 in FIG. 5 showing the cross-section of the sample fluid flowing tube 121 constitutes the non-flowing zone 127, and a flowing zone 128 is defined only around the non-flowing zone. There should be the non-flowing zone 127 in at least an interval of the length of the sample fluid flowing tube 121 in which the heating coil 122, the first temperature sensing coil 123 and the second temperature sensing coil 124 are wound. Further, it is preferred that the flowing cross-sectional area be uniform throughout the full length of the sample fluid flowing tube 121. In other words, it is preferred that the cross-sectional area of the flowing zone 128 surrounding the non-flowing zone 127 be identical with the flowing cross-sectional area defined by an interval of the sample fluid flowing tube 121 excluding the non-flowing zone 127.

Although FIG. 5 shows that both the sample fluid flowing tube 121 and the

interior rod 125 have disk-type cross-sections, the present invention is not limited to such a shape. Elliptical tube and/or interior rod, or rectangular tube and/or interior rod may be used so far as a flow path can be defined around the interior rod. Herein, the flow path defined by the inner surface of the tube and the outer surface of the interior rod will be referred to as"annular flow path"regardless of the shapes of the tube and interior rod.

Moreover, although FIG. 4 shows that the sample fluid flowing tube 121 is generally C-shaped and the interior rod 125 is disposed only at an intermediate portion of the tube, this is only for the sake of convenience of illustration and does not intend to limit the present invention. The interior rod 125 may be disposed to extend throughout the full length of the sample fluid flowing tube 121 or only over a portion of the full length thereof.

Furthermore, although FIG. 4 shows that the interior rod 125 is supported by sidewalls of the sample fluid flowing tube 121, this is also only for the sake of convenience of illustration and does not intend to limit the present invention. Only maintaining the interior rod 125 to be spaced apart from the inner surface of the sample fluid flowing tube 121 without hindering the sample fluid from flowing will suffice.

FIG. 4 shows a cladding member 126 surrounding the outer surface of the sample fluid flowing tube 121 in the vicinity of the upstream and downstream ends of the sample fluid flowing tube 121. The cladding member 126 functions to cause the sample fluid flowing tube 121 to be firmly fixed to the outer surface of the main flow path, and to cause the sample fluid flowing tube 121 and the sample fluid to be thermally isolated from the environment. The present invention is not limited by whether there is the cladding member 126. The cladding member 126 may be completely removed or formed to surround the entire sample fluid flowing tube 121.

In the embodiment, the heating coil 122 is wound around the outer surface of the intermediate portion of the sample fluid flowing tube 121, the first temperature sensing coil 123 is wound around the outer surface of a portion of the sample fluid flowing tube upstream of the heating coil 122, and the second temperature sensing coil 124 is wound around the outer surface of a portion of the sample fluid flowing tube downstream of the heating coil 122.

According to the mass flow measurement sensor using the sample fluid flowing

tube 121 with such an annular flow path, more precise and sensitive measurement over a conventional mass flow measurement sensor can be done, fast response time can be achieved, and the linear flow range can be sufficiently increased. The inventors performed various tests to verify these facts. Some of test results will be described below.

In the embodiment, the heating coil 122 has been described as being used as the heating source. However, various kinds of heaters such as a plane heater, an electrical hotbed wire, and a Nichrome wire capable of converting electrical energy into thermal energy may be properly used and other heaters capable of properly controlling the amount of heat supplied may also be used.

Further, although the embodiment employs the temperature sensing coils as the first and second temperature sensing means, other temperature sensing devices such as a Pt wire or a thermocouple may also be properly used.

Materials with superior durability and wear resistance such as stainless steel are useful as materials for the sample fluid flowing tube 121 of the mass flow measurement sensor 120 according to the embodiment.

Although the term"interior rod"is used herein for the sake of convenience, the interior rod accommodated in the sample fluid flowing tube 121 is actually a wire having a small diameter since the diameter of the sample fluid flowing tube 121 generally does not exceed a few millimeters.

The diameter of a conventional sample fluid flowing tube should be very small in consideration of the response time, the linear flow range and the like. However, even though the diameter of the sample fluid flowing tube 121 of the mass flow measurement sensor 120 according to the present invention is considerably large, satisfactory performance is obtained. Therefore, in a case where the total mass flow rate to be measured is small and the diameter of the main flow path is small, all the total mass flow rate can be used as the sample fluid to be measured by installing the mass flow measurement sensor 120 at an intermediate portion of the main flow path rather than a method of branching a portion of the total mass flow rate.

FIG. 6 shows a cross-section view for explaining a configuration of a mass flow measurement sensor 120 according to a second embodiment of the present invention. The

mass flow measurement sensor 120 according this embodiment is identical with the mass flow measurement sensor according to the first embodiment, except that at least one portion of the interior rod 125 is hollow and the heating coil 122 as the heating source is disposed in the hollow portion of the interior rod 125 between the first and second temperature sensing coils 123,124. Therefore, the other components will not be repeatedly described.

The entire amount of heat from the heating coil 122 disposed in the hollow portion of the interior rod 125 according to the embodiment can be transferred to the sample fluid flowing around the interior rod 125 without any influence by the environment. Thus, the amount of heat supplied can be precisely controlled.

FIG. 7 shows a cross-sectional view for explaining a configuration of a mass flow measurement sensor 120 according to a third embodiment of the present invention. The mass flow measurement sensor 120 according this embodiment is identical with the mass flow measurement sensor according to the second embodiment, except that a major portion of the interior rod 125 except for ends thereof is hollow and the heating coil 122 as the heating source is disposed in the hollow portion of the interior rod 125 so as to occupy positions overlapping with the first and second temperature sensing coils 123,124.

Therefore, the other components will not be repeatedly described.

In the mass flow measurement sensor 120 according this embodiment, the sample fluid begins to be heated while passing through a position where the first temperature sensing coil 123 is disposed, and then, passes through a position where the second temperature sensing coil 124 is disposed in a state where the temperature of the sample fluid is sufficiently increased.

FIG. 8 shows a cross-sectional view for explaining a configuration of a mass flow measurement sensor 120 according to a fourth embodiment of the present invention. The mass flow measurement sensor 120 according this embodiment is identical with the mass flow measurement sensor according to the third embodiment, except that the heating coil 122 is wound on the outer surface of the sample fluid flowing tube 121 and the first and second temperature sensing coils 123,124 are disposed in the hollow portion of the interior rod 125. Therefore, the other components will not be repeatedly described.

The first and second temperature sensing coils 123,124 disposed in the hollow portion of the interior rod 125 according to the embodiment can precisely measure the temperature of the sample fluid flowing around the interior rod 125 without any influence by the environment.

FIG. 9 shows a cross-sectional view for explaining a configuration of a mass flow measurement sensor 120 according to a fifth embodiment of the present invention. The mass flow measurement sensor 120 according this embodiment is identical with the mass flow measurement sensor according to the fourth embodiment, except that all of the heating coil 122 and the first and second temperature sensing coils 123,124 are disposed in the hollow portion of the interior rod 125. Therefore, the other components will not be repeatedly described.

The heating coil 122 and the first and second temperature sensing coils 123,124 disposed in the hollow portion of the interior rod 125 according to the embodiment allows the entire amount of heat from the heating coil to be transferred to the sample fluid flowing around the interior rod 125 and the temperature of the sample fluid flowing around the interior rod 125 to be precisely measured, without any influence by the environment, respectively.

FIG. 10 shows a cross-sectional view for explaining a configuration of a mass flow measurement sensor 120 according to a sixth embodiment of the present invention. The mass flow measurement sensor 120 according this embodiment is identical with the mass flow measurement sensor according to the first embodiment, except that an additional heating coil is not provided and the first and second temperature sensing coils 123,124 are constructed to perform both the heating and temperature sensing functions in this embodiment. Therefore, the other components will not be repeatedly described.

Since the materials, configurations and associated circuits of the first and second temperature sensing coils 123,124 for performing both the heating and temperature sensing functions can be easily conceived by those skilled in the art from the configuration of a conventional mass flow measurement sensor, the detailed description thereof will be omitted. However, the present invention is not limited by the materials, configurations and associated circuits of the first and second temperature sensing coils 123,124. It will

be apparent that the other configurations capable of serving as both the heating and temperature sensing means (Pt wire, Nichrome wire and the like) may be properly employed.

FIG. 11 shows a cross-sectional view for explaining a configuration of a mass flow measurement sensor 120 according to a seventh embodiment of the present invention.

The mass flow measurement sensor 120 according this embodiment is identical with the mass flow measurement sensor according to the sixth embodiment, except that at least a portion of the interior rod 125 is hollow and the first and second temperature sensing coils 123,124 further having the heating function are disposed in the hollow portion of the interior rod 125. Therefore, the other components will not be repeatedly described.

The first and second temperature sensing coils 123,124 disposed in the hollow portion of the interior rod 125 according to the embodiment can heat the sample fluid flowing around the interior rod 125 and precisely measure the temperature of the sample fluid without any influence by the environment.

FIG. 12 shows a cross-sectional view for explaining a configuration of a mass flow measurement sensor 120 according to an eighth embodiment of the present invention.

The mass flow measurement sensor 120 according this embodiment is identical with the mass flow measurement sensor according to the sixth embodiment, except that at least a portion of the interior rod 125 is hollow and the heating source such as the heating coil 122 is additionally disposed in the hollow portion of the interior rod 125. Therefore, the other components will not be repeatedly described.

The additional heating source such as the heating coil 122 disposed in the hollow portion of the interior rod 125 according to the embodiment facilitates the heating of the sample fluid flowing around the interior rod 125 to further improve response characteristics.

FIG. 13 shows a cross-sectional view for explaining a configuration of a mass flow measurement sensor 120 according to a ninth embodiment of the present invention. The mass flow measurement sensor 120 according this embodiment is identical with the mass flow measurement sensor according to the eighth embodiment, except that the first and second temperature sensing coils 123,124 further having the heating function are disposed

in the hollow portion of the interior rod 125 and the heating coil 122 serving as an additional heating source is wound on the outer surface of the sample fluid flowing tube 121. Therefore, the other components will not be repeatedly described.

The first and second temperature sensing coils 123,124 disposed in the hollow portion of the interior rod 125 according to the embodiment can precisely measure the temperature of the sample fluid flowing around the interior rod 125 without any influence by the environment. The heating coil 122 as the additional heating source wound on the outer surface of the sample fluid flowing tube 121 facilitates the heating of the sample fluid to further improve response characteristics.

Hereinafter, the results of tests that have been performed to examine the response time and the linear flow range of the mass flow measurement sensor according to the present invention will be described. FIG. 14 shows comparative graph for illustrating change in the temperature difference with respect to change in the mass flow rate measured in the conventional mass flow measurement sensor and the mass flow measurement sensor according to the first embodiment of the present invention, respectively. The mass flow measurement sensor of the present invention employed in this test is constructed to have the same sectional flow area and distance between the coils as the conventional mass flow measurement sensor.

As shown in FIG. 14, it can be seen that the linear flow range of the mass flow measurement sensor of the present invention is wider than that of the conventional mass flow measurement sensor. The wider linear flow range means a wider range of mass flow measurement. Thus, when the mass flow measurement sensor of the present invention is used, accurate mass flow measurement can be done over a wider range.

This is because the sample fluid flowing tube of the mass flow measurement sensor of the present invention is annular in cross-section whereas the tube of the conventional mass flow measurement sensor is a disk-type in cross-section. That is, in a case where the sample fluid flowing tube has the disk-type cross-section in the same manner as the conventional mass flow measurement sensor, fluid flowing characteristics and thermal characteristics are influenced by its diameter. However, in a case where the sample fluid flowing tube has the annular cross-section in the same manner as the mass

flow measurement sensor of the present invention, the fluid flowing characteristics and thermal characteristics are influenced by a gap between the inner and outer diameters thereof.

Furthermore, another test was performed using the mass flow measurement sensor according to the first embodiment of the present invention. In this test, measurement was made for ten mass flow measurement sensors having the sample fluid flowing tubes of which diameters were increased by 1 mm from 1 mm to 10 mm under conditions that the sensors have the configuration according to the first embodiment and the gap was kept constant were prepared. For the respective mass flow measurement sensors, temperature differences according to changes in the mass flow rate. The results are plotted in FIG. 15.

As shown in FIG. 15, as the diameter of the tube is smaller, an inverting interval of the temperature difference curve increasing with changes in the mass flow rate appears earlier. That is, the mass flow measurement sensor operating in a range of small mass flow rate can perform more precise measurement but may have a reduced upper limit of measurable mass flow rate as the diameter of the tube is smaller. On the contrary, the mass flow measurement sensor operating in a range of large mass flow rate can have an increased upper limit of measurable mass flow rate by increasing the diameter of the tube but increment of the temperature difference is reduced over the entire range, resulting in a lowered measurement accuracy.

On the other hand, the conventional mass flow measurement sensor having the sample fluid flowing tube with the disk-type cross-section has a problem in that as the diameter of the tube is increased, the response time is considerably slowed. However, proper response time can be maintained in the mass flow measurement sensor having the sample fluid flowing tube with the annular cross-section according to the present invention even though the diameter of the tube is increased. This is because the fluid flowing characteristics and thermal characteristics vary with the gap between the inner and outer diameters of the tube in the mass flow measurement sensor of the present invention.

Therefore, when the sample fluid flowing tube defines the annular flow path in the same manner as the present invention, the sectional flow area through which the sample fluid flows is enlarged and thence the linear flow range is also increased as compared with

the conventional disk-type cross-section. Further, according to the present invention, the linear flow range can be controlled by adjusting the diameter of the tube under the condition that the gap of the annular cross-section is kept constant. Moreover, according to the present invention, as the gap between the inner and outer diameters is smaller, the linear flow range is increased and the response time is also improved. Furthermore, since the present invention has the sectional flow area larger than the conventional disk-type cross-section, there is an advantage in that it is less sensitive to fine particles contained in the sample fluid and change in pressure.

Although the configuration of the mass flow measurement according to the present invention has been described in connection with the preferred embodiments, this is merely illustrative and does not intend to limit the present invention. It will be apparent to those skilled in the art that changes, modifications and adjustments can be made thereto without departing from the technical spirit and scope of the invention.