THERMAL MASS FLOW SENSOR

Abstract: An apparatus for sensing a mass flow rate of a fluid in a thermal mass flow meter has a sensor tube. The sensor tube has a tube for carrying the fluid flow at the flow rate to be sensed. A temperature responsive member is disposed in good heat transfer relation with the tube. The temperature responsive member, when energized, produces at a sensor signal node a sensor signal responsive to the rate of mass flow of the fluid through the tube. The sensor signal is offset from a null signal condition by an unsymmetrical temperature distribution along the sensor tube when the mass flow rate of the fluid is zero. A means for modifying the temperature distribution along the sensor tube alters the unsymmetrical temperature distribution and causes the sensor signal node to produce a substantially null signal condition when the mass flow rate of the fluid through the tube is zero.

Description: THERMAL MASS FLOW SENSOR BACKGROUND OF THE INVENTION The invention relates to an apparatus for measuring the mass rate of flow of a fluid. More particularly, the present invention relates to an apparatus for thermally measuring the mass rate of flow of a fluid wherein the apparatus or thermal mass flow sensor has a thermal biasing device or devices associated with a sensor tube for modifying the thermal characteristics of the sensor tube so that center of the temperature profile is centered with the signal tap and the sensor signal is thereby standardized to reduce the effect of manufacturing tolerances. In the manufacture of integrated circuits, it is necessary to perform multiple process steps including epitaxial growth steps, vapor deposition steps, diffusion steps and etching steps. All of these process steps are dependent upon chemical reactions with a silicon wafer in a reaction chamber or chemical reactions between or among the process gases in the reaction chamber. For instance, dichlorosilane and trichlorosilane are used in the epitaxial growth of silicon upon a substrate. If silicon is to be doped, phosphorus oxychloride, diborane, arsine and/or phosphine may be employed as dopant carrying gases in combination with silane compounds. Oxygen may be metered into a heated reaction chamber to cause thermal oxidation of silicon to take place to form silicon oxide. Silicon nitride may be deposited upon a silicon wafer by the reaction of ammonia and dichlorosilane. Wafers may be etched by gases such as sulphur hexafluoride excited in plasma reactors. The thickness and electrical characteristics of the substances deposited or grown on the wafer are influenced in part by the amount of reactant gas within the reactor. In order to control the amount of reactant gas in the reactor, mass flow controllers are connected between gas sources and the reactor to meter the flow of gases into the reactor to insure that the semiconductor manufacturing processes are performed properly. Failure to meter the gases properly may result in defective integrated circuits which must be scrapped. Mass flow controllers also may be used to meter anesthetic gases for use in medicine and for other precision process and analysis operations. In the generally accepted terminology of the flow measurement industry a flow meter is an instrument for measuring the rate of flow of a gas, and a flow controller is a flow meter with a control valve and a feedback circuit combined to not only measure the flow but regulate it to a value that can be electronically set or manipulated. A flow sensor is a transducing element used within a flow meter or flow controller. It produces an electrical signal indicative of the rate of flow of a flow of fluid such as gas flowing through the sensor. Specifically such sensors include a small diameter tube with a centrally heated region and means for sensing the temperature of the heated region at a pair of points along the tube. A flow of fluid such as gas through the heated region lowers the temperature of the upstream portion of the heated region and raises the temperature of the downstream portion of the heated region. The temperature differential results in a change in the signal, typically from a bridge including windings positioned about the heated region of the tube. A problem with such sensors is that the gas within the tube is heated which may give rise to gas convection currents within the tube. Typically only a fraction of the total fluid flowing through the flow meter passes through the tube. The remainder passes through a path called a bypass which restricts the flow of gas so that the mass flow rate of gas through the bypass has a known relationship to the mass flow rate of gas through the tube. Usually the bypass flow rate is a linear multiple of the flow rate through the tube . Many times the mass flow controllers are incorporated in gas shelves as part of other processing equipment such as diffusion furnaces, chemical vapor deposition equipment, sputtering equipment, plasma etchers and the like. Most mass flow controllers are mounted so that the primary gas flow path through their bypass conduits is in a substantially horizontal direction. Typical mass flow controllers include a U-shaped sensor tube having a pair of vertically oriented legs connected by a substantially horizontal leg about which a temperature sensitive heater/sensor wire is wound. The heater/sensor wire comprises a portion of an electrical bridge which is excited from a source of electrical energy and whose differential voltage changes as the flow of gas through the sensor tube preferentially cools the upstream portion of the sensor winding with respect to the downstream portion of the sensor winding causing a voltage shift therein. In the past, various attempts have been made to speed up the thermal response of the sensor of a mass flow controller, see for instance, U.S. Patent No. 4,686,856 to Vavra, et al. for Mass Flow Meter, assigned to the instant assignee, which teaches the use of a band of thermally conductive material wrapped around the sensor tubing unit, as may best been seen in FIGS. 2, 4, 5 and 7, to produce an isothermal band around the sensor unit tending to substantially improve the performance of the unit. See also FIGS. 11 and 12 which show an encasement 110 made of thermally conductive material connected to the tube 12 immediately adjacent to the ends of the coil 14. The encasement 110 is evacuated so that thermal convection does not occur relative to the coil. The isothermal strap 18 has been replaced by the encasement 110. Also see U.S. Patent No. 4,548,075 to Mariano for Fast Responsive Flow Meter Transducer which discloses a thermal shunt 50 comprised of a wire 50 including a notch 52 at each end for mounting and thermally conductive contact against the sensor tube 34 while a coiled wire spring clip 54 secures the shunt wire in position. Preferably the shunt wire is made of commercially pure aluminum having 5/64" diameter and providing a faster response to the sensor tube than would normally be obtained without the shunt. British Patent Specification No. 1248 563 for Electrical Flow Meters discloses a heat sink 2 having its ends connected to a measuring duct 1 of a thermal flow meter. U.S. Patent No. 4,815,280 to Tujimura, et al. for Thermal Flow Meter also discloses a thermal mass flow sensor having a thermal shunt connected to a sensor tube to speed up its response. While all such devices improve the performance of the flow meters of which they are a part by speeding up their thermal response they suffer from the problem that after having been constructed, the maximum or center point temperature of the sensing coil may not be located symmetrically with respect to the voltage signal being measured. As a result, the voltage signal may be slightly off null at zero flow due to the manufacturing and materials tolerances which arise in the fabrication of the sensor. These may arise from variations in the sensor wire, either as to its composition or diameter, crystal structure or the like from one spool of wire to another. They may result from wire insulation flaws from thickness variations or variations in the material or fabrication of the sensor tubing which may result in alterations in the inside or outside diameters of the tubing, the wall thickness and the like. They may arise from alterations in tension and pitch of bobbin winders employed for winding the sensor wire about the tube to form the sensor tube and may result from variations in the sensor tubing inside wall finish and in the degree of contaminant control during manufacture and subsequent use. As a result of all of these variations, the industry has been faced with the fact that the sensors employed in thermal mass flow meters may vary from mass flow controller to mass flow controller. A sensor cannot simply be removed and have another substituted for it. The entire mass flow meter or controller must be recalibrated. Further, due to the nonlinearities in the flow characteristic, which are to be compensated for in the circuitry of the mass flow controller, shifts from the zero point in the mass flow controller sensor lead to additional calibration problems. Problems from the same factors lead to shifts in the effective gain of the sensors from sensor to sensor which will, of necessity, have to be compensated in the electronics of the mass flow controller. Manufacturing tolerances can also result in the variation of the sensor's output signal with ambient temperature, and in significant variability of its dynamic response to changes in flow rate. Thus, if a sensor is replaced in a mass flow controller its gain cannot be assumed to be standardized and it is necessary to recalibrate the electronics with the new sensor in the mass flow controller leading to expensive down-time and the like. What is needed is a method and apparatus for rapidly and easily producing a flow sensor having known gain characteristics and a preselected null or thermal zero. SUMMARY OF THE INVENTION A thermal mass flow sensor embodying the present invention includes a sensor tube having a tube for carrying a flow of gas therethrough whose ma _s rate of flow is to be measured and a temperature responsive member in this embodiment, usually a winding about the tube having a relatively large thermal coefficient of resistance. The winding is adapted to be energized from a source of electrical energy in a mass flow meter or mass flow controller. The sensor tube is mounted upon a base which is connected to a bypass of a mass flow meter or mass flow controller. In a mass flow controller downstream of the bypass is a valve for controlling the flow of fluid through the mass flow controller. An important aspect of the invention relates to the fact that while the prior art has shown various thermal shunting arrangements in association with sensor tubes, for speeding up the response of the sensor tubes, there is no teaching or suggestion in the prior art of the use of a thermal shunting arrangement or other temperature biasing apparatus or means for effecting substantial thermal standardization of the sensor itself. The thermal standardization often removes the need for additional electrical nulling of the signal processing circuitry which receives the sensor output. This desirable end is achieved by manufacturing the thermal mass flow sensor with a moveable thermal shunt placed thereon, which thermal shunt is shifted with respect to the windings and the sensor tube until a maximal or null signal reading is obtained from the sensor signal indicative of the fact that the temperature profiles extending along the sensor tube from both sides of the center are symmetrical and thus that the temperature distribution is substantially balanced. The thermal shunt is also movable in translation with respect to the sensor winding in order to alter the sensitivity of the winding to changes in flow rate. As the thermal shunt is displaced farther from the winding, the sensitivity of the sensor increases but its speed decreases. As the thermal shunt is moved closer to the winding, the sensitivity decreases but the response becomes faster. The thermal shunt is moved until the desired sensitivity and time response is obtained from the winding, as well as the proper nulling, and at that point, the thermal shunt is permanently attached to the sensor tube at the desired location by silver epoxy adhesives or the like. In various alternative embodiments the shunt can be an open loop or can be a closed container positioned about the windings to prevent convection effects of gas outside the windings from perturbing the sensor readings. Further, if a housing type thermal shunt is used, the housing may have located therein and possibly in contact with the sensor tube, various insulators such as mica washers and the like for limiting the heat transfer from the upstream winding to the downstream winding of the thermal mass flow sensor. In further embodiments, the thermal mass flow sensor may be nulled by the use of guard or auxiliary electric heaters placed on the sensor tube and driven by energy sources whose power levels may be separately and independently regulated. The mass flow controller embodying the present invention also comprises a metal base block or other relatively large metal structural element having a large thermal mass that remains essentially at ambient temperature regardless of small heat inputs from the sensor tube or signal amplification and conditioning circuit. The base block is a heat sink that acts as a thermal ground. In various embodiments of the invention the thermal shunt may be attached to the thermal ground so that its function is not only to influence the temperatures at defined positions on the sensor tube, but also to control and reduce the magnitude of the temperature differences between the thermal ground and the thermal shunt attachment zones or points on the sensor tube. By reducing the temperature differences t "ween those selected portions of the sensor tube and the -.henna1 shunt attachment points temperature perturbations due to external heat sources, whether above or below the temperature of the sensor tube, and temperature transients associated with those heat sources are reduced. The effect of the thermal shunt on the sensor tube is adjusted to reduce the effect of variations of sensor tube thermal characteristics due to manufacturing variations. More specifically, the sensor tube variations affect null balance at no flow, sensitivity to flow rate, and time response to flow transients. The sensor tube and thermal shunt being adjusted are mounted on a standard thermal mass flow sensor base block and electrically connected with a standard thermal mass flow controller signal amplifying and conditioning circuit. The standard signal amplifying and conditioning circuit is connected electrically to a UCAL-1000 mass flow controller calibrator system, available from Unit Instruments, Inc., 1247 West Grove Avenue, Orange, California including an IBM compatible personal computer executing UICALSYS software provided by Unit Instruments as part of the calibrator system. The calibrator is also connected to a standard thermal mass flow controller plumbed in series with the sensor tube and thermal shunt under test in the gas line. Two or more standard thermal mass flow controllers may be plumbed in parallel with appropriate valving so that they can receive fixed flows of gas, such as dry nitrogen or other gases, and provide output signals related to those gas flows to the calibrator and computer. The standard mass flow controllers may then be switched into the test sensor gas circuit by means of the valves to cause rapid dynamic or time-dependent flow rate changes in the sensor tube under test. The calibrator and the computer are programmed to operate the system at no flow and at various flow rates in sequence, and to record the sensor's signal output. Both transient and steady-state signals from the thermal flow controller under test are recorded. Following a programmed run the computer displays the results in comparison with established flow standards. The test may be repeated with the test sensor stabilized at a second ambient temperature if the temperature coefficient is being adjusted. The person calibrating the sensor tube and thermal shunt, if it is not producing a null signal, then moves both attachment points of the thermal shunt a small amount, one toward the sensor winding and the other away from the sensor winding, viz. upstream if the no-flow signal is positive or above the standard null signal and downstrea if it is negative or below the standard null signal. If the sensitivity of the sensor tube is in error, both ends of the thermal shunt are moved farther away from the sensor winding to increase sensitivity, or toward the sensor winding to reduce it. If the speed of response is being adjusted rather than the sensitivity, then the ends of the thermal shunt are moved closer to the sensor winding to increase speed, or farther away to slow the response as identified by the time constant of the dominant pole in the computer software. The programmed test is then re-run to determine the effect of the thermal shunt adjustments and to indicate the magnitude of further adjustments if necessary. In mass flow controllers that include thermal insulation associated with the sensor tube or other parts that could influence the performance and that might have been moved for adjustment, all such parts must be returned to their proper position while the test is being run. Other combinations of test procedures and adjustment perfection criteria may be devised to suit particular combinations of manufacturing tolerances and application requirements. All, however, will benefit from the capability of the present invention to adjust for symmetry-related anomalies in the inherent thermal characteristics of the sensor tube by moving the thermal shunt attachment points to the sensor tube, or their equivalents in a common direction relative to the flow direction, and to adjustment for sensitivity-related anomalies by moving the shunt attachment points to the sensor tube in opposing directions. It is a principal aspect of the present invention to provide a thermal mass flow sensor, which is substantially thermally nulled. It is another aspect of the instant invention t provide a thermal mass flow sensor having a preselected sensitivity. It is a further aspect of the instant invention to provide a thermal mass flow sensor having a preselected dynamic response for standardization during mass flow rate transients. Other aspects of the present invention will be apparent to one of ordinary skill in the art upon a perusal of the specification and claims in light of the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric view of a first thermal mass flow sensor embodying the present invention and having portions broken away to show details thereof; FIG. 2 is a sectional view of a thermal mass flow controller having the thermal mass flow sensor shown in FIG. 1 incorporated therein; FIG. 3 is a sectional view of the first thermal mass flow sensor shown in FIG. 1; FIG. 4 is an isometric view of a second thermal mass flow sensor embodying the present invention and having portions broken away to show details thereof; FIG. 5 is a sectional view of the thermal mass flow sensor shown in FIG. 4; FIG. 6 is a sectional view taken substantially along line 6—6 of FIG. 5 showing details of the second mass flow sensor; FIG. 7 is an isometric view of a third thermal mass flow sensor embodying the present invention and having portions broken away to show details thereof; FIG. 8 is a sectional view of the third thermal mass flow sensor shown in FIG. 7; FIG. 9 is a sectional view of the third thermal mass flow sensor shown in FIG. 8 taken substantially along line 9—9; FIG. 10 is an isometric view of a fourth thermal mass flow sensor embodying the present invention and having portions broken away to show details thereof; FIG. 11 is a sectional view of the fourth thermal mass flow sensor shown in FIG. 10; FIG. 12 is a sectional view of the fourth thermal mass flow sensor shown in FIG. 11 taken along line 12—12 of FIG. 11; FIG. 13 is an isometric view ox a fifth thermal mass flow sensor embodying the present invention and having portions broken away to show details thereof; FIG. 14 is a sectional view of the fifth thermal mass flow sensor shown in FIG. 13; FIG. 15 is a sectional view of the fifth thermal mass flow sensor shown in FIG. 14 taken substantially along line 15—15 of FIG. 14; FIG. 16 is an isometric view of a sixth thermal mass flow sensor embodying the present invention and having portions broken away to show details thereof; FIG. 17 is a sectional view of the sixth thermal mass flow sensor shown in FIG. 16 having portions broken away; FIG. 18 is a sectional view of the sixth thermal mass flow sensor shown in FIG. 17; FIG. 19 is an isometric view of a seventh thermal mass flow sensor embodying the present invention and having portions broken away to show details thereof; FIG. 20 is a sectional view of the seventh thermal mass flow sensor shown in FIG. 19; FIG. 21 is a sectional view of the seventh thermal mass flow sensor shown in FIG. 20 taken substantially along line 21—21 of FIG. 20; FIG. 2 is a side elevational view, partially in section, of an eighth thermal mass flow sensor embodying the present invention; FIG. 23 is a sectional view of the eighth thermal mass flow sensor shown in FIG. 22 taken substantially along line 23—23 of FIG. 22; FIG. 24 is a side elevational view, partly in section, of a ninth thermal mass flow sensor embodying the present invention; FIG. 25 is a sectional view of the ninth thermal mass flow sensor shown in FIG. 24 taken substantially along line 25—25 of FIG. 24; FIG. 26 is an isometric view of a tenth thermal mass flow sensor embodying the present invention and having portions broken away to show details thereof; FIG. 27 is a side elevational view, partially in section, of the tenth thermal mass flow sensor shown in FIG. 26; FIG. 28 is a sectional view of the tenth thermal mass flow sensor shown in FIG. 27 taken substantially along line 28—28 of FIG. 27; FIG. 29 is a schematic diagram of the mass flow controller shown in FIG. 2; FIG. 30 is a side elevational view, partially in section, of an eleventh thermal mass flow sensor embodying the present invention; FIG. 31 is a sectional view of the eleventh thermal mass flow sensor shown in FIG. 30 taken substantially along line 31—31 of FIG. 30; FIG. 32 is a isometric view of a twelfth thermal mass flow sensor embodying the present invention and having portions broken away to show details thereof; FIG. 33 is a sectional view of the twelfth mass flow sensor shown in FIG. 32 taken substantially along line 33—33 of FIG. 32; FIG. 34 is a sectional view of the twelfth mass flow meter shown in FIG. 32 taken substantially along line 34—34 of FIG. 32; FIG. 35 is a schematic showing of a thirteenth thermal mass flow sensor embodying the present invention; FIG. 36 is a schematic diagram of the thirteenth thermal mass flow sensor shown in FIG. 35 having dual adjustable constant current sources for effecting thermal biasing of a sensor tube of the thirteenth thermal mass flow sensor; and FIG. 37 is a schematic diagram of a calibration system for the thermal mass flow sensor. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and especially to FIG. 2, a mass flow controller 10 having a thermal mass flow meter 12 and a valve 14 is shown therein. The thermal mass flow meter 12 has a thermal mass flow sensor 16, shown in FIGS. 1, 2, and 3, embodying the present invention. The thermal mass flow sensor 16 is connected to a bypass 18 of the mass flow meter 12 and has a sensor tube 20 for receiving a portion of a flow of a gas from a bypass of the mass flow meter 12. The sensor tube 20 includes a unitary stainless steel tube 22, having a center point 23, for carrying the flow of gas and a thermally responsive member 24 wound about the tube 22 symmetrically with the tube center point 23. The thermally responsive member 24, when energized with an electrical current from end to end, produces an electrical signal at a central signal node 26, in response to the flow of gas through the sensor tube 20. The sensor tube 20, and more particularly the thermally responsive member 24, generally do not produce a sufficiently accurate null electrical signal at zero rate of mass flow of gas therethrough due to the manufacturing assymetries which unbalance the thermal characteristic of the sensor tube 20. Means for modifying a thermal characteristic of the sensor tube 20, comprising a thermal shunt 28, is connected to the sensor tube 20 in good heat conducting relationship. The thermal shunt 28 reduces the time required for the sensor tube 20 to respond to a change in the rate of flow of the gas. The thermal shunt 28 is positioned on the sensor tube 20 to alter its thermal characteristic so that the sensor tube 20 produces a null signal at zero flow. The sensor tube 20 is mounted on a sensor tube base 30 of the thermal mass flow sensor 16 so that the sensor tube 20 is in communication with the bypass 18 of the mass flow meter 12 of the mass flow controller 10. The bypass 18 includes a bypass body 32 having connected to it an inlet 34 and an outlet 36. An internal pressure dropping device 38 is positioned within a bore 40 of the bypass 18 with the sensor tube 20 substantially straddling the pressure dropping device 38. The bore 40 of the bypass 32 feeds gas into the valve 14 and delivers the gas through a bore 42 to the outlet 36 for delivery to a reactant chamber or the like. The base 30 includes a lower metal base element 44 and an upper metal base element 46. The lower metal base element 44 is adapted to engage the bypass 18 and form a portion thereof. A metal convection reducing cap 48 is fitted snugly over the upper metal base portion 46 to reduce external convective disturbances of the sensor tube 20. In order to communicate with the bypass 18 and receive a portion of the flow of gas therefrom, the stainless steel tube 22, which is substantially U-shaped and has an upstream or inlet leg 50, a downstream or outlet leg 52 and a cross-piece or flow measuring portion 54, has its inlet leg 50 connected in communication with the upstream portion of the bypass 18 and its outlet leg 52 connected in communication with the downstream portion of the bypass 18 so that a portion of the gas flowing into the bypass 18 flows into the inlet leg 50 across the measuring portion 54 and out through the outlet leg 52. The tube 22 is composed of 316 stainless steel alloy. An insulating blanket 56 is disposed in a generally U-shape around the thermally responsive member 24 in order to further limit convective disturbance of the sensor tube 22. The thermal blanket 56 is held in place by a wire clip 58. Underneath the thermal blanket 56 is the temperature responsive member 24 which includes an upstream heater/sensor winding 60 and a downstream heater/sensor winding 62, both composed of multiple turns of 1.5 mil nickel wire having a high temperature coefficient of resistivity. The wire is connected in an electrical bridge and energized, as will be discussed hereinafter. In order to reduce the thermal response time of the sensor tube 20, in particular its rate of temperature change when changes in gas flow occur through the tube 22, the thermal shunt 28 is provided. As is well known in the flow controller art, the mass rate of flow of the gas through the tube 22 causes the upstream winding 60 to be cooler than the downstream winding 62, all other things being equal. This assumes that at no-flow condition, the temperature gradient along the tube 22 is symmetric about its center point 23. In this embodiment, the center point 23 of the tube 22 is at a region 66 immediately adjacent the signal node 26 for sensing the signal of a voltage divider comprised of the upstream winding 60 and the downstream winding 62. In this embodiment, the thermal shunt 28 comprises a printed circuit board 70 having an insulating layer 72 covered with a layer of copper foil 74. The printed circuit board is mounted on the large thermal mass metal base 46 with the copper foil 74 in good heat conducting contact therewith. A plurality of wire windings 76, 78, 80 and 82 are attached to the legs 50 and 52 of the U-shaped tube 22 in good heat conducting contact. These windings are soldered or silver epoxied to the legs 50 and 52, as well as to the copper foil 74 of the thermal shunt 28, in order to effect good thermal conduction from the upstream leg 50 to the downstream leg 52 to speed up the response of the temperature responsive member 24 and thermal grounding to the metal base 46. During the manufacture of the thermal mass flow sensor 16, the windings 76, 78, 80 and 82 are shifted along the legs 50 and 52. In particular, upper windings 78 and 80 are shifted along the legs 50 and 52 in order to adjust the effective contact points of the thermal shunt 28 immediately adjacent the upstream winding 60 and the downstream winding 62, respectively. This shifts the thermal bias in the center point temperature at zero flow rate. The result is that the no-flow signal, obtained at the signal node 26, is also shifted, thereby achieving a thermal mass flow sensor which has been thermally nulled rather than electrically nulled. Once the thermal null condition has been detected, the windings 78 and 80 are permanently connected to the tubes 50 and 52 by soldering or by silver epoxy or the like. In order to further isolate the sensor tube 20 and increase its speed, the windings 76 and 82 provide a thermal ground due to the fact that they carry heat to the copper foil layer 76, which is in good contact with the upper metal base portion 46 which comprises a heat sink. This also decreases the sensitivity of the sensor tube 22 In addition, the sensitivity of the thermal mass flow sensor 14, that is its electrical output in response to changes in the flow rate through the sensor, may also be adjusted by the relative position of the windings 78 and 80 with respect to the center point 23. Once the thermal mass flow sensor 14 has been nulled, it may be appreciated that there are a number of positions along the inlet leg 50 and the outlet leg 52 which may provide such a null. If both windings 78 and 80 are located relatively far from the center point 23, the sensor tube 22 will have relatively high sensitivity but slow thermal response. As the windings 78 and 80 are moved closer to the center point 23, the sensitivity will decrease, but the thermal response time will also shorten. As a result, by movement of the windings 78 and 80 as the center point 23 is maintained at the thermal null, the sensitivity also may be adjusted to a standardized value. Thus, the instant thermal mass flow sensor 14, by being manufactured with the initially movable thermal shunt contact points 78 and 80, as well as to some extent the windings 76 and 82, allows a thermally balanced and sensitivity standardized thermal mass flow sensor to be produced despite manufacturing tolerances. Referring now to FIGS. 2 and 29, the temperature responsive member 24 comprising the windings 60 and 62 is energized by a constant current generator 90. The constant current generator 90 is coupled to an electrical bridge 92 which includes windings 60 and 62, as well as the signal node 26, as portions thereof. A differential amplifier circuit 94 is coupled to the bridge 92. A speed-up amplifier 96 is connected to the differential amplifier circuit 94 and the valve module 98, which includes the valve 14, is connected to the speed-up amplifier 96 to be driven thereby. The constant current generator 90 includes a node 100 which is coupled to a positive 15-volt DC source, as well as a node 102 coupled to a negative 15-volt DC source. A voltage dropping resistor 104 is coupled to the node 100. A first Zener diode 106 is coupled to the resistor 104. A second Zener diode 108 is coupled between the Zener diode 106 and a resistor 110, which is connected to the node 102. A node 112 is connected to the resistor 104, as is a lead 114 which has a node 116 therein. The node 116 is coupled through the lead 118 to the Zener diode 106. A node 120 exists between the Zener diodes 106 and 108. A potentiometer 122, having a resistor 124 and a sweep-arm 126, has the resistor 124 connected across or in parallel with the Zener diodes 106 and 108 with the sweep-arm 126 being coupled to a resistor 128. A selected regulated voltage is supplied by moving the arm 126 with respect to the resistor 124 in effect by adjusting the potentiometer 122. This potential is delivered to the resistor 128. An operational amplifier 130, having a non- inverting input 132, an inverting input 134 and an output 136, has the non-inverting input 132 connected to the node 112 to receive a regulated voltage therefrom. The irtput 136 is connected to the inverting input 134 by a capacitor 138, which provides an integrating function to remove any voltage changes or noise from the output 136. An NPN transistor 138, having a base 140, a collector 142 and an emitter 144, has the collector 142 connected to the node 100 to receive the 15-volt positive potential therefrom, and to deliver a current through the base emitter circuit to the bridge 92, which is connected to the emitter 144 at a node 146. The regulated current supplied by the transistor 138 to the bridge 92 flows through one side of the bridge comprising the winding 60 and 62, and the other side of the bridge comprising a potentiometer 148 having a resistance element 150 and a sweep-arm 152. The resistance element 150 is connected between the node 146 and a node 154 formed by the junction of the resistance element 160 and the winding 62. Also connected to the node 154 is a node 156 which has connected to it a resistor 158, which is connected to the junction of the capacitor 138 and the input terminal 134. A resistor 160 is connected to the node 120 through a lead 162 to receive a selected zero-volt signal therefrom. Thus, the amount of current flowing to ground through the resistor 160 causes a voltage offset to occur at the node 156, which is sensed by the inverting input terminal 134 of the high impedance operational amplifier 130, thereby biasing the current flow through the npn transistor 138 with negative feedback to cause the sensed current through the bridge to remain constant despite temperature changes in the elements of the bridge. As stated above, with the constant current flowing through the bridge 92, changes in the flow rate through the tube 22 result in temperature shifts in the windings 60 and 62, causing concomitant shifts in the voltage at the node 26 to occur. Further, zero adjustment of the electrical characteristics of the circuit may be achieved by movement of the tap or sweep-arm 152 with respect to the resistor 150, both being elements of the potentiometer 148. The tap 152 is connected to the amplifier 94 which comprises an operational amplifier 166 having an inverting input terminal 168, a non-inverting input terminal 170 and an output terminal 172. The non- inverting terminal 170 receives the sensor signal from the node 26 and a D.C. bias signal from the sweep arm 126. A capacitor 172a is connected to the node 26 and to ground as well as the resistors 172b and 172c act in conjunction with the other passive elements connected to the operational amplifier 166 to provide the differential response function. The inverting input terminal 168 receives the potential from the sweep-arm 152. A capacitor 174 is connected between the output terminal 172 and the inverting input terminal 168 to provide an integrating function. A resistor 176 is connected to the junction of the capacitor 174 and the inverting input terminal 168. A resistor 178 is connected to the output terminal 172. A potentiometer 180, having a resistor 182, has the resistor 182 connected to the resistor 178 and to a resistor 184 to comprise a voltage divider network 186, which is connected to ground. As stated above, the circuit is grounded at zero volts through the node 120. A selected signal between the output voltage 172 and zero volts is fed back through a sweep-arm 188 of the potentiometer 180 to the resistor 176, which is connected to the sweep-arm. Thus, effectively speaking, a reduced voltage is fed back through the resistor 176 to the inverting input terminal 168 of the operational amplifier 166, allowing the time constant of the differential amplifier circuit 94 to be altered by adjustment of the sweep-arm 188, thereby allowing the time response of the entire mass flow controller 10 to be adjusted. The amplifier 96 includes a resistor 190 connected to the output terminal 172 and a capacitor 192 connected in parallel with the resistor 190. The resistor 190 and capacitor 192 combination are connected to an operation amplifier 194 having an input terminal 196, an output terminal 198 and an inverting input terminal 200. A feedback loop 202 is connected between the output terminal 198 and the input terminal 200. The feedback loop 202 includes a resistor 204 and a capacitor 206 in parallel with the resistor 204. Connected to the junction of the input terminal 200 and the resistor 204 and the capacitor 206 is a resistor 208 and a capacitor 210, which is grounded. The resistor 208 and the capacitor 210, as well as the other passive elements of the amplifier 96, configure the operational amplifier 194 so that the entire circuit 96 functions as a speed-up circuit, which provides an output voltage at the node 198 to drive the valve operating circuitry and valve of the valve system 98. Referring now to FIGS. 4, 5 and 6, a thermal mass flow sensor 216, which is a second embodiment of the present, is shown therein. The thermal mass flow sensor 216 may be substituted in the mass flow meter 12 of the mass flow controller 10 for the thermal mass flow meter 16. The thermal mass flow meter 216 may be connected to the bypass 18 in the same manner in which the thermal mass flow meter 16 is connected to the bypass 18. The thermal mass flow meter 216 has a sensor tube 220 for receiving a portion of a flow of gas from the bypass 18 of the mass flow meter 12. The sensor tube 220 is a unitary stainless steel tube 222, having a center point 223, for carrying the flow of the portion of the gas and a thermally responsive member 224, wound about the tube 222, symmetrically with the tube center point 223. The thermally responsive member 224 produces an electrical signal at a central signal node 226, located at the tube center point 223, in response to the flow of gas through the tube 222. The sensor tube 220, and more particularly the thermally responsive member 224, generally do not produce a sufficiently accurate null electrical signal at zero rate of mass flow of gas therethrough due to the manufacturing assymetries which unbalance the thermal characteristic of the sensor tube 220. Means for modifying a thermal characteristic of the sensor tube 220, comprising a thermal shunt 228, is connected to the sensor tube 220 in good heat conducting relationship. The thermal shunt 228 reduces the time required for the sensor tube 220 to respond to a change in the rate of flow of the portion of the gas. The thermal shunt 228 is positioned about the sensor tube 220 and in contact therewith in good heating relationship in order to alter the thermal characteristic of the sensor tube 220 so that the sensor tube 220 produces a null signal at zero mass flow rate. The sensor tube 220 is mounted on a sensor tube base 230 of the thermal mass flow sensor 216 so that the sensor tube 220 is in communication with tne bypass 18 of the mass flow meter 12 of the mass flow controller 10. The base 230 includes a lower plastic base element 244 and an upper metal base 246 having a large thermal mass. The lower plastic base element 244 is adapted to engage the bypass 18 and form a portion thereof. A metal convection reducing cap 248 is fitted snugly over the upper metal base portion 46 to reduce external convective disturbances of the sensor tube 220. In order to communicate with the bypass 18 and receive a portion of the flow of gas therefrom, the tube 222, which i-_ substantially U-shaped and has an upstream or inlet leg 250 and a downstream or outlet leg 252 and a cross-piece or flow measuring portion 254 connected therebetween, has its inlet leg 250 connected in communication with the upstream portion of the bypass 18 and its outlet leg 252 connected in communication with the downstream portion of the bypass 18 so that a portion of the gas flowing into the bypass 18 flows into the inlet leg 250, across the measuring portion 254, and out through the outlet leg 252. The tube 222 is composed of 316 sta less steel alloy. An insulating blanket 256 is disposed around the thermally responsive member in order to further limit convective disturbance of the sensor tube 222. Underneath the thermal blanket 256 is the temperature respon. ? e member 224 which includes an upstream heater/sensor winding 260 and a downstream heater/sensor winding 262, both composed of multiple turns of 1.5 mil nickel wire havir" a high temperature coefficient of resistivity. Nickel wire is connected in an electrical bridge and energized in the same fashion as is the upstream sensor 60 and the downstream sensor 62. In order to reduce the thermal response time of the sensor tube 220, in particular its rate of temperature change when changes in gas flow occur through the tube 222, the thermal shunt 228 is provided. As is well known in the flow controller art, the mass rate of flow of the gas through the tube 222 causes the upstream winding 260 to be cooler than the downstream winding 262, all other things being equal. This assumes that at no-flow condition, the temperature gradient along the tube 222 is symmetric about its center point 223. In this embodiment, the center point 223 of the tube 222 is at a region 266 immediately adjacent the signal node 226 for sensing the signal of a voltage divider comprised of the upstream winding 260 and the downstream winding 262. In this embodiment, the thermal shunt comprises a nested tube 270 having a first open-ended cylindrical can-like element 272 nested slidably within a second open- ended cylindrical can element 274. Both the can elements 272 and 274 are comprised of copper. The can element 272 has a substantially circular cross-section outer wall 276 and defines a slot 278 therein to allow leads to be taken from the windings 260 and 262. Likewise, the can 274 has a substantially circular cross-section can wall 280 defining a slot 282 therein through which bridge leads pass. The can 276 is slidably nested within the can 274. The can 272 also includes a circular end wall 286, which is in good thermal conduction with the cross-piece 254 as is a circular end wall 288, which is connected to the cylindrical wall 276. The circular end walls 286 and 288 may be moved with respect to the cross-piece 254 of the tube 222 either simultaneously or together. During manufacture of the thermal mass flow sensor team, the thermal shunt 228 is adjusted to provide a thermal null at the center point 223 so that an electrical signal null is generated at the node 226. In order to adjust the center point, the entire nested can configuration 270 may be slid transversely with respect to the center point 223. If sensitivity is to be adjusted, for instance if sensitivity is to be reduced, the can portions 272 and 274 are pulled away from each other, thereby lengthening the overall can, which reduces the response time but increases the sensitivity of the thermal sensor. Likewise, if the can portions 272 and 274 are forced together, the thermal response time shortens while the sensitivity increases. Thus, the sensitivity and the thermal null and response time may be simultaneously adjusted using the nested can configuration of the type embodied in the thermal mass flow sensor 216. Thermal grounding of the thermal mass flow sensor is provided by a plurality of windings 290 which are connected to the upstream and downstream legs 250 and 252 by soldering and the like. The windings are attached to a printed circuit board 292 having a copper layer 294 thereon mounted in good thermal conduction with the metal base 246. Once the thermal shunt 228 has been properly adjusted, the thermal conductivity between the circular end walls 286 and 288 is enhanced and the walls are fixed to the sensor tube 222 by the use of silver epoxy. The windings 260 and 266, as well as the signal node 226, are directly substituted for the windings 60 and 62 as well as the signal node 26, respectively, in the circuit set forth in FIG. 29. Referring now to FIGS. 7, 8 and 9, a third embodiment of a thermal mass flow sensor embodying the present invention is shown therein and generally identified by reference number 316. The thermal mass flow sensor 316 is connected to the bypass 18 of the mass flow meter 12 and has a sensor tube 320 for receiving a portion of the flow of gas from the bypass 18 of the thermal mass flow meter. The sensor tube 320 includes a unitary stainless steel tube 322 having a center point 323, for carrying a portion of the flow of gas, and a thermally responsive member 324, wound about the tube 322, symmetrically with the tube center point 323. The thermally responsive member 324 produces an electrical signal at a central signal node 326 located at the tube center point 323 in response to the flow of gas through the tube 322. The sensor tube 320, and more particularly the thermal responsive member 324, generally do not produce a sufficiently accurate null electrical signal at zero rate of mass flow of gas therethrough due to the manufacturing assymetries which unbalance the thermal characteristic of the sensor tube 320. Means for modifying a thermal characteristic of the sensor tube 320, comprising a thermal shunt 328, is connected to sensor tube 320 in good heat conducting relationship. The thermal shunt 328 reduces the time required for the sensor tube 320 to respond to a change in the rate of flow of the gas. The thermal shunt 328 is positioned on the sensor tube 320 to alter its thermal characteristic so that the sensor tube 320 produces a null signal at zero flow. The sensor tube 320 is mounted on a sensor tube base 330 of the thermal mass flow sensor 316 so that the sensor tube 320 is in communication with the bypass 18 of the mass flow meter 12 of the mass flow controller 10. The base 330 includes a lower plastic base element 344 and an upper metal base element 346 having a large thermal mass. The lower plastic base element 344 is adapted to engage the bypass 18 and form a portion thereof. A metal convection reducing cap 348 is fitted snugly over the upper metal base portion 346 to reduce external convective disturbances of the sensor tube 320. In order to communicate with the bypass 18 and receive a portion of the flow of gas therefrom, the sensor tube 320, which is substantially U- shaped and has an upstream or inlet leg 350, a downstream or outlet leg 352 and a flow measuring portion 354, has its inlet leg 350 connected in communication with the upstream portion of the bypass 18 and its outlet leg 352 connected in communication with the downstream portion of the bypass 18 so that a portion of the gas flowing into the bypass 18 flows into the inlet leg 150 across the measuring portion 354 and out through the outlet leg 352. The tube 322 is unitary and composed of 316 stainless steel alloy. An insulating blanket 56 is disposed in a generally U-shape around the thermally responsive member 324 in order to further limit convective disturbances of the sensor tube 320. The insulating blanket 356 is retained in place partially by the thermal shunt 328 and partially by a wire clip 358, which is connected to two wire windings 359 of a plurality of wire windings 359 connected to the upstream or inlet leg 350 and the downstream or outlet leg 352. Underneath the thermal blanket 356 is the temperature responsive member 324, which includes an upstream heater/sensor winding 360 and a downstream heater/sensor winding 362, both composed of multiple turns of 1.5 mil nickel wire having a high temperature coefficient of resistivity. The nickel wire is connected in the electrical bridge 392 having been substituted for the winding 60 and 62 shown therein. Between the windings 360 and 362 is the signal node 326, which is connected to the input terminal 170 of the differential amplifier 94. In order to reduce the thermal response time of the sensor tube 320, in particular its rate of temperature change when changes in gas flow occur through the sensor tube 320, the thermal shunt 328 is provided. As is well known in the flow controller art, the mass rate of flow of the gas tarough the sensor tube 320 causes the upstream winding 360 to be cooler than the downstream winding 362, all other things being equal. This assumes that at no-flow co* ;ion, the temperature gradient along the sensor tube 32_ _s symmetric about its center point 323. In this embodiment, the center pc int 323 of the sensor tube 320 is at a region 366 immediately adjacent the signal node 326 for sensing the signal of the voltage divider comprised of the upstream winding 360 and the downstream winding 362. In this embodiment, the thermal shunt 328 comprises a copper strip which is substantially rectangular in shape and has a first end 370 wrapped about the outlet leg 352 and a second end 372 wrapped about the inlet leg 350. Movement of the shunt 328 generally downwardly along the legs 350 and 352 will increase the sensitivity and increase the thermal response time of the sensor tube 320. Movement of the entire thermal shunt upwardly along the legs will reduce the response time and the sensitivity. Thus, a particular sensitivity and response time may be selected by precisely positioning the thermal shunt for the measured sensitivity from the sensor. In addition, tilting of the thermal shunt, that is placement of one of the ends above the other, will cause a shift in the center point temperature at no-flow condition in order to bring the maximum temperature when no gas is flowing into coincidence with the center point 323 thus providing the proper null signal at the signal node 326. Once the sensor tube 320 has thus been nulled and its sensitivity selected by the proper positioning of the ends 370 and 372 of the copper thermal shunt strip 328, the ends 370 and 372 are permanently attached to the outlet leg 352 and the inlet 350, respectively, by silver epoxy. The sensor tube 320 is thermally grounded by the windings which are silver epoxied to a copper foil 380 of a printed circuit board 382. The printed circuit board 382 is mounted in the metal base 346 with the copper foil 380 in good thermal conduction therewith. Referring now to FIGS. 10, 11 and 12, a fourth embodiment of a thermal mass flow sensor, embodying the present invention and generally indicated by reference numeral 416, is shown therein. The thermal mass flow sensor 416 is connected to the bypass 18 of the thermal mass flow meter 12 and has a sensor tube 420 for receiving a portion of the flow of gas from the bypass 18. The sensor tube 420 includes a unitary stainless steel tube 422 having a center point 423, for carrying the flow of gas, and a thermally responsive member 424 wound about the tube 422 symmetrically with the tube center point 423. The thermally responsive member 424 produces an electrical signal at a central signal node 426, located at the tube center point 423, in response to the flow of gas through the tube 422. The sensor tube 420, and more particularly the thermal responsive member 424, generally do not produce a sufficiently accurate null electrical signal at zero rate of mass flow of gas therethrough due to the manufacturing assymetries which unbalance the thermal characteristic of the sensor tube 420. Means for modifying a thermal characteristic of the sensor tube 420, comprising a thermal shunt 428, is connected to the sensor tube 420 in good heat conducting relationship. The thermal shunt 428 reduces the time required for the sensor tube 420 to respond to a change in the rate of flow of the gas. The thermal shunt 428 is positioned on the sensor tube 420 to alter its thermal characteristic so that the sensor tube 420 produces a null signε at zero flow. The sensor tube 420 is mounted on a sensor tube base 430 of the thermal mass flow sensor 416 so that the sensor tube 420 is in communication with the bypass 18 of the mass flow meter 12 of the mass flow controller 10. The base 430 includes a lower plastic base element 444 and an upper metal base element 446 having a large thermal mass. The lower plastic base element 444 is adapted to engage the bypass 18 and form a portion thereof. A metal convection reducing cap 448 is fitted snugly ov r the upper metal base portion 446 to reduce external convective disturbances of the sensor tube 420. In order to communicate with the bypass 18 and receive a portion of the flow of gas therefrom, the tube 422, which is substantially U-shaped and has an inlet leg 450, an outlet leg 452 and a flow measuring portion 454 therebetween, has its inlet leg 450 connected in communication with the upstream portion of the bypass 18 and its outlet leg 452 connected in communication with the downstream portion of the bypass 18 so that a portion of the gas flowing into the bypass 18 flows into the inlet leg 450 across the measuring portion 454 and out through the outlet leg 452. The tube 422 is composed of 316 stainless steel alloy. An insulating blanket 456 is disposed in a generally U-shape around the thermally responsive member 424 in order to further limit convective disturbances of the sensor tube 422. The thermal blanket 456 is held in place by a wire clip 458. Underneath the thermal blanket 456 is the temperature responsive member 424. The wire clip 458 is connected to two of a plurality of windings 459. Two of the windings 459 are connected to the inlet leg 450. The other two windings 459 are connected to the outlet leg 452. The temperature responsive member 424 includes an upstream heater/sensor winding 460 and a downstream heater/sensor winding 462, both composed of multiple turns of 1.5 mil nickel wire having a high temperature coefficient of resistivity. The nickel wire is connected in the electrical bridge 92 having been substituted for the windings 60 and 62, shown in FIG. 29. The node 426 is substituted for the node 26 of FIG. 29. In order to reduce the thermal response time of the sensor tube 420, in particular its rate of temperature change when changes in gas flow occur through the tube 420, the thermal shunt 428 is provided. As is well known in the flow controller art, the mass rate of flow of the gas through the sensor tube 420 causes the upstream winding 460 to be cooler than the downstream winding 462, all other things being equal. This assumes that at no-flow condition, the temperature gradient along the sensor tube 420 is symmetric about its center point 423. In this embodiment, the center point 423 of the sensor tube 420 is at a region 466 immediately adjacent the signal node 426 for sensing the signal of a voltage divider comprised of the upstream winding 460 and the downstream winding 462. In this embodiment, the thermal shunt 428 comprises a copper strip having a first end 472 wrapped about inlet leg 450 and a second end 474 wrapped about the outlet leg 452. The ends 472 and 474 are attached by silver epoxy to the inlet leg 450 and outlet leg 452, respectively. A copper wiper arm 476, which is attached to a printed circuit board 478 having a front copper foil layer 480 and a back foil layer 482, is attached at the back foil layer 482. The arm 476 is also in good thermal conductive contact with the strip 470 comprising a portion of the thermal shunt 428. The adjustment of the shunt strip 470 vertically along the legs 450 and 452 controls the sensitivity of the sensor tube 420. The adjustment of the sweep-arm 476, which forms a thermal ground in conjunction with the foil 482, shifts the center point bias of the thermal shunt with respect to the sensor tube 420 in order to bring the overall thermal center point into conjunction with the center point 423 of the tube 422. Once the sensor tube 420 has been nulled by movement of the copper sweep-arm 476, the sweep-arm * 476 is permanently attached to the strip 470 by silver epoxy or the like. It should also be appreciated that additional thermal grounding is provided by the attachment of the windings 459 to the foil 480 via silver epoxy or the like. The sensor tube 420 is thermally grounded by the windings which are silver epoxied to a copper foil 480. The printed circuit board 478 is mounted in the metal base 446 with the copper foils 480 and 482 in good thermal conduction therewith. Referring now to FIGS. 13, 14 and 15, a thermal mass flow sensor comprising a fifth embodiment of the present, invention is shown therein and generally identified by reference numeral 516. The thermal mass flow sensor 516 is connected to the bypass 18 of the thermal mass flow meter 12 and has a sensor tube 520 for receiving a portion of the flow of gas from the bypass 18 of the thermal mass flow meter 12. The sensor tube 520 includes a unitary stainless steel tube 522, having a center point 523, for carrying the flow of a portion of gas, and a thermally responsive member 524 wound about the tube 522 symmetrically with the tube center point 523. The thermally responsive member 524 produces an electrical signal at a central signal node 526, located at the tube center point 523, in response to the flow of gas through the tube 522. The sensor tube 520, and more particularly the thermally responsive member 524, generally do not produce a sufficiently accurate null electrical signal at zero rate of mass flow of gas therethrough due to the manufacturing assymetries which unbalance the thermal characteristic of the sensor tube 520. Means for modifying a thermal characteristic of the sensor tube 520, comprising a thermal shunt 528, is connected to the sensor tube 520 in good heat conducting relationship. The thermal shunt 528 reduces the time the required for the sensor tube 520 to respond to a change in the rate of flow of the portion of the gas. The thermal shunt 528 is positioned on the sensor tube 520 to alter its thermal characteristic so that the sensor tube 520 produces a null signal at zero mass flow rates. The sensor tube 520 is mounted on a sensor tube base 530 of the thermal mass flow sensor 516 so that the sensor tube 520 is in communication with the bypass 18 of the mass flow meter 12 of the mass flow controller 10. The base 530 includes a lower plastic base element 544 and an upper metal base element 546 having a large thermal mass. The lower plastic base element 544 is adapted to engage the bypass 18 and form a portion thereof. A metal convection reducing cap 548 is fitted snugly over the upper metal base portion 546 to reduce external convective disturbances of the sensor tube 520. In order to communicate with the bypass 18 and receive a portion of the flow of gas therefrom, the sensor tube 520, which is substantially U- shaped and has an inlet leg 550, an outlet leg 552 and a flow measuring portion 554, has its inlet leg 550 connected in communication with the upstream portion of the bypass 18 and its outlet leg 552 connected in communication with the downstream portion of the bypass 18 so that a portion of the gas flowing into the bypass 18 flows into the inlet leg 550, across the measuring portion 554 and out of the outlet leg 552. The tube 522 is composed of 316 stainless steel alloy. An insulating blanket 556 is disposed in a generally U-shape around the thermally responsive member 524 in order to further limit convective disturbance of the sensor tube 520. Underneath the thermal blanket 556 is the temperature responsive member 524 which includes an upstream heater/sensor winding 560 and a downstream heater/sensor winding 562, both composed of multiple turns of 1.5 mil nickel wire having a high temperature coefficient of resistivity. The nickel wire is connected in the electrical bridge 92 having been substituted for the windings 60 and 62 and the signal node 526, as will be discussed hereinafter. In order to reduce the thermal response time of the sensor tube 520, in particular its rate of temperature change when changes in gas flow occur in the tube 522, the thermal shunt 528 is provided. As is well known in the flow controller art, the mass rate of flow of the gas through the sensor tube 520 causes the upstream winding 560 to be cooler than the downstream winding 562, all other things being equal. This assumes that at no-flow condition, the temperature gradient along the sensor tube 520 __s symmetric about its center point 523. In this embodiment, the center point 523 of the sensor tube 520 is at the region 566 immediately adjacent the signal node 526 for sensing the signal of a voltage divider comprised of the upstream winding 560 and the downstream winding 562. In this embodiment, the thermal shunt 528 comprises a U-shape member 570 composed of copper and having a pair of legs 572 and 574 formed integrally with a cross-piece portion 554. The leg 572 has a slot 580 formed therein. The leg 574 has a slot 582 formed therein. The slots 580 and 582 are snap-fitted over the cross-piece 554 of the sensor tube 520 in good . .. rmal engagement therewith. The thermal shunt 528 may be shifted along the cross-piece 554 or the legs even bent in order to thermally bias the sensor tube 520 so that the thermal center point at no-flow condition is at the tube center point 523 and, thus, a true null signal condition is generated at the signal node 526. This is achieved, as it is for the other embodiments, by measuring the electrical characteristics of the sensor at zero flow rate and adjusting the legs 572 and 574 to achieve the desired null signal. In addition, the total spread of the legs 572 and 574 will control the amount of sensitivity achieved from the sensor tube 520. The sensor tube 520 is also supported by means of multiple windings 590 which are attached to the edges 592 of a printed circuit board 594 mounted in the metal base 546. Referring now FIGS. 16, 17 and 18, a thermal mass flow sensor, which is a sixth embodiment of the present invention, is shown therein and is generally identified by reference numeral 616. The thermal mass flow sensor 616 is connected to the bypass 18 of the mass flow meter 12 in substitution for the thermal mass flow sensor 616. The thermal mass flow sensor 616 has a sensor tube 620 for receiving a portion of a flow of a gas from the bypass 18 of the thermal mass flow meter 12. The sensor tube 620 includes a unitary stainless steel tube 622, having a center point 623, for carrying the flow of gas, and a thermally responsive member 624 wound about the tube 622 symmetrically with the tube center point 623. The thermal responsive member 624 produces an electrical signal at a central signal node 626 located at the tube center point 623, in response to the flow of gas through the tube 622. The sensor tube 620, and more particularly the thermally responsive member 624, generally do not produce a sufficiently accurate null electrical signal at zero rate of mass flow of gas therethrough due to the manufacturing assymetries which unbalance the thermal characteristic of the sensor tube 620. Means for modifying a thermal characteristic of the sensor tube 620, comprising a thermal shunt 628, is connected to the sensor tube in good heat conducting relationship. The thermal shunt 628 reduces the time required for the sensor tube 620 to respond a change in the rate of flow of the gas. The thermal shunt 628 is positioned on the sensor tube 620 to alter its thermal characteristic so that the sensor tube 620 produces a null signal at zero flow. The sensor tube 620 is mounted on a sensor tube base 620 of the thermal mass flow sensor 616 so that the sensor tube 620 is communication with the bypass 18 of the mass flow meter 12 of the mass flow controller 10. The base 630 includes a lower plastic base element 644 and an upper metal base element 646. The lower plastic base element 644 is adapted to engage the bypass 18 and form a portion thereof. A metal convection reducing cap 648 is fitted snugly over the metal base portion 646 to reduce external convective disturbances of the sensor tube 620. In order to communicate with the bypass 18 and receive a portion of the flow of gas therefrom, the tube 620, which is substantially U-shaped and has an inlet leg 650, a downstream or outlet leg 652 and a flow measuring portion 654, has its inlet leg 650 connected in communication with the upstream portion of the bypass 18 and its outlet 652 connected in communication with the downstream portion of the bypass 18 so that a portion of the gas, flowing into the bypass 18, flows into the inlet leg 650, across the measuring portion 644 and out through the outlet leg 652. The tube 622 is composed of 316 stainless steel alloy. The temperature responsive member 624 includes an upstream heater/sensor winding 660 and a downstream heater/sensor winding 662, both composed of multiple turns of 1.5 mil nickel wire having a high temperature coefficient of resistivity. The nickel wire is connected in the electrical bridge 92 in substitution for the windings 60 and 62 shown therein. Likewise, the signal node 626 is substituted for the signal node 26 in FIG. 29. In order to reduce the thermal response time of the thermal response time of the sensor tube 620, in particular its rate of temperature change when changes in gas flow occur through the sensor tube 620, the thermal shunt 628 is provided. As is well known in the flow controller art, the mass rate of flow of the gas through the sensor tube 620 causes the upstream winding 660 to be cooler than the downstream winding 662, all other things being equal. This assumes that at no-flow condition, the temperature gradient along the tube 622 is symmetric about its center point 623. In this embodiment, the center point 623 of the sensor tube 620 is at a region 666 immediately adjacent the signal node 626 for sensing the signal of the voltage divider comprised of the upstream winding 660 and the downstream winding 662. In this embodiment, the thermal shunt 628 comprises a housing 670 comprising a housing body 672 and a cover wall 674 attached thereto. Mere particularly, the housing 670 includes a first vertical wall 674 adjacent and parallel to the inlet leg 650. A second vertical wall 676 is adjacent and parallel to the outlet leg 652 and an upper wall 678, connected to the walls 674 and 676, is immediately adjacent the flow measuring portion 654. A back wall 680 is in contact with the walls 674, 676 and 678, and is mounted upon the metal base 646. In order to properly adjust the center point temperature of the sensor tube 620, blobs of silver epoxy are injected between the walls 674, 676 and/or 678, as shown at points 682 and 684, so that thermal conduction is achieved between portions of the sensor tube 620 and the thermal shunt 628. Furthermore, the thermal shunt 628 acts similarly to the previous thermal blankets 56, 256, 356, 456 and 556 in that it completely encloses the substantial portion of the sensor tube 620, thereby reducing convective currents immediate adjacent the windings 660 and 662. Further, the placement of the blobs of silver epoxy 682 and 684, with respect to the windings 660 and 662, control the sensitivity of the sensor tube 620, as was set forth previously. Referring to FIGS. 19, 20 and 21, a thermal mass flow sensor comprising a seventh embodiment of the present invention is generally identified therein by reference numeral 716. The thermal mass flow sensor 716 is connected to the bypass 18 of the thermal mass flow meter 12 in substitution for the thermal mass flow sensor 16. The thermal mass flow sensor 716 has a sensor tube 720 for receiving a portion of a flow of gas from the bypass 18 of the thermal mass flow meter 12. The sensor tube 720 includes a unitary stainless steel tube 722, having a center point 723, for carrying the flow of gas and a thermally responsive member 724 wound about the tube 720 symmetrically with the tube center point 723. The thermal responsive member 724 produces an electrical signal at a central signal node 726, located at the tube center point 723, in response to the flow of gas through the tube 722. The sensor tube 720, and more particularly the thermally responsive member 724, generally do not produce a sufficiently accurate null electrical signal at zero rate of mass flow of gas therethrough due to the manufacturing assymetries which unbalance the thermal characteristic of the sensor tube 720. Means for modifying a thermal characteristic of the sensor tube 720, comprising a thermal shunt 728, is connected to the sensor tube 720 in good heat conducting relationship. The thermal shunt 728 reduces the time required for the sensor tube 720 to respond to a change in the rate of flow of the portion of the gas. The thermal shunt 728 is positioned on the sensor tube 720 to alter its thermal ch« acteristic so that the sensor tube 720 produces a null signal at zero flow. The sensor tube 720 is mounted on a sensor tube base 730 of the thermal mass flow sensor 716 so that the sensor tube 720 is in communication with the bypass 18 of the thermal mass flow meter 12 of the thermal mass flow controller 10. The base 730 includes a lower plastic base element 744 and an upper metal base element 746 having a large thermal mass. The lower plastic base element 744 is adapted to engage the bypass 18 and form a portion thereof. A metal convection reducing cap 748 is fitted snugly over the upper metal base portion 746 to reduce external convective disturbances of the sensor tube 720. In order to communicate with the bypass 18 and receive a portion of the flow of gas therefrom, the sensor tube 720, which is substantially U-shaped and has an inlet leg 750, an outlet leg 752 and a flow measuring portion 754 connected therebetween, has its inlet leg 750 connected in communication with the upstream portion of the bypass 18 and its outlet leg 752 connected in communication with the downstream portion of the bypass 18, so that a portion of the gas flowing into the bypass 18 flows into the inlet leg 750, across the measuring portion 754 and out through the outlet leg 752. The sensor tube 720 is composed of 316 stainless steel alloy. An insulating blanket 756 is disposed about the thermally responsive member 724 in order to further limit convective disturbance of the sensor tube 720. Underneath the thermal blanket 756 is the temperature responsive member 724, which includes an upstream heater/sensor winding 760 and a downstream heater/sensor winding 762, both composed of multiple turns of 1.5 mil nickel wire having a high temperature coefficient of resistivity. The nickel wire is connected in the electrical bridge 92 with the upstream winding 760 substituted for winding 60 and the downstream winding 762 substituted for the winding 62. In order to reduce the thermal response time of the sensor tube 720, in particular its rate of temperature change when changes in gas flow occur through the tube 722, the thermal shunt 728 is provided. As is well known in the flow controller art, the mass rate of flow of the gas through the sensor tube 720 causes the upstream winding 760 to be cooler than the downstream wiηding 762, all other things being equal. This assumes that at no-flow condition, the temperature gradient along the sensor tube 720 is symmetric about its center point 723. In this embodiment, the center point 723 of the sensor tube 720 is at a region 766 immediately adjacent the signal node 726 for sensing the signal of a voltage divider comprised of the upstream winding 760 and the downstream winding 762. In this embodiment, the thermal shunt 728 comprises a tube 770. The tube 770 includes a cylindrical circular tube wall 772 having a pair of end caps 774 and 776 fitted into the ends thereof. The plugs 774 and 776 have recesses 778 and 780 formed respectively therein for receiving silver epoxy or solder for affixing the shunt 728 to the sensor tube 720. In order to adjust the temperature center point at no-flow conditions to coincide with the center point 723, the end caps 774 and 776 may be slid in and out of the tube wall 772 as well as the entire shunt 770 being slid along the sensing portion 754 of the sensor tube 720. This is done at no-flow conditions while the null signal is measured in order to achieve a true null signal at zero flow. Furthermore, the sensitivity may be adjusted by the effective length of the tube 770 being adjusted by the movable caps 774 and 776. Once the desired adjustment has been achieved, silver epoxy is inserted into the recesses 778 and 780 to firmly attach the tube 770 to the sensor tube 720 permanently in good heat conducting relation. Thermal grounding for the sensor tube 720 is provided by a printed circuit board 782. The sensor tube 720 is attached via windings 784 to a copper foil layer 786 of the printed circuit board 782. The copper foil layer is thermally connected to the metal base 746. It may also be appreciated that the cylinder 772 has a slot 790 formed therein to allow the signal node 726 to be taken outside the shunt 728. Further, the thermal blanket 756 is enclosed within the thermal shunt 728 to limit convective disturbances to the thermally responsive member 724. Referring now to FIGS. 22 and 23, a thermal mass flow sensor comprising an eighth embodiment of the present invention is shown therein and generally identified by reference numeral 816. The thermal mass flow sensor 816 may be connected to the bypass 18 of the thermal mass flow meter 12 in substitution for the thermal mass flow 16. The thermal mass flow sensor 816 has a sensor tube 820 for receiving a portion of a flow of a gas from the bypass 18 of the thermal mass flow meter 12. The sensor tube 820 includes a unitary stainless steel tube 822, having a center point 823, for carrying the flow of gas, and a thermally responsive member 824 wound about the sensor tube 820 symmetrically with the sensor tube 820 center point 823. The thermally responsive member 824 produces an electrical signal at a central signal node 826, located at the sensor tube 820 center point 823, in response to the flow of gas through the tube 822. The sensor tube 820, and more particularly the thermally responsive member 824, generally do not produce a sufficiently accurate null electrical signal at zero rate of mass flow of gas therethrough due to the manufacturing assymetries which unbalance the thermal characteristic of the sensor tube 820. Means for modifying a thermal characteristic of the sensor tube 820, comprising a thermal shunt 828, is connected to the sensor tube 820 in good heat conducting relationship. The thermal shunt 828 reduces the time required for the sensor tube 820 to respond to a change in the rate of flow of the gas therethrough. The thermal shunt 828 is positioned on the sensor tube 820 to alter its thermal characteristic so that the sensor tube 820 produces a null signal at zero flow. The sensor tube 820 is mounted on a sensor tube base 830 of the thermal mass flow sensor 816 so that the sensor tube 820 is communication with the bypass 18 of the thermal mass flow meter 12 of the thermal mass flow controller 10. The base 830 includes a lower plastic base element 844 and an upper metal base element 846 having a large thermal mass. The lower plastic base element 844 is adapted to engage the bypass 18 and form a portion thereof. A metal convection reducing cap 848 is fitted snugly over the upper metal base portion 846 to reduce external convective disturbances of the sensor tube 820. In order to communicate with the bypass 18 and receive a portion of the flow of gas therefrom, the sensor tube 820, which is substantially U-shaped and has an inlet leg 850, an outlet leg 852 and a flow measuring portion 854 connected therebetween, has the inlet leg 850 connected in communication with the upstream portion of the bypass 18 and its outlet leg 852 connected in communication with the downstream portion of the bypass 18 so that the portion of the flow of the gas flowing into the bypass flows into the inlet leg 850, through the measuring portion 854 and out through the outlet leg 852. The sensor tube 820 is composed of 316 stainless steel alloy. An insulating blanket 856 is disposed in a generally circular shape around the thermally responsive member 824 in order to further limit convective disturbance of the sensor tube 820. The thermal blanket 856 is held in place by the thermal shunt 828. Underneath the thermal blanket 856 is the temperature responsive member 824 which includes an upstream heater/sensor winding 860 and a downstream heater/sensor winding 862, both composed of multiple turns of 1.5 mil nickel wire having a high temperature coefficient of resistivity. The nickel wire is connected in the electrical bridge 92 in substitution for the windings 60 and 62. In order to reduce the thermal response time of the sensor tube 820, in particular its rate of temperature change when changes in gas flow occur through the tube 822, the thermal shunt 828 is provided. As is well known in the flow controller art, the mass rate of flow of the gas through the tube 822 causes the upstream winding 860 to be cooler than the downstream winding 862, all other things being equal. This assumes that at no-flow condition, the temperature gradient along the tube 822 is symmetric about its center point 823. In this embodiment, the center point 823 of the tube 822 is at a region 866.immediately adjacent the signal node 826 for sensing the signal of a voltage divider comprised of the upstream winding 860 and the downstream winding 862. In this embodiment, the thermal shunt 828 comprises a cylinder 870 having a first cylinder half 872 and a second cylinder half 874, both of which may be comprised of good thermal conductors such as copper or aluminum. The cylinder half 872 has formed integrally with it a substantially circular end wall 876. The cylinder half 874 has formed integrally with it a substantially circular end wall 878. The end walls 876 and 878 are placed in good thermal conduction via silver epoxy or solder with the flow measuring portion 854. The cylinder 872 terminates in a reduced diameter portion 880 which interfits with an enlarged diameter portion 882 of the cylinder 874 in telescopic sliding condition to allow the thermal shunt 828 to have its length varied in order to control the sensitivity of the sensor tube 820 while the entire thermal shunt 828 can be translated with respect to the center point 823 to control the null signal condition at the node 826, as was stated above for other embodiments of the invention. A mica washer 884 is positioned about the center point 823 to provide convection control and reduce convective disturbances of the windings 860 and 862 in combination with the thermal blanket 856. A plurality of windings 888 are attached in good heat conduction by silver epoxy to the legs 850 and 852, and to a copper foil layer 892 of a printed circuit board 890 to thermally ground the sensor tube 820 to the metal base 846 that is in good heat conduction relation with the copper foil layer 889. Good thermal conductivity may also be achieved by the use of solder or the like. Referring now to FIGS. 24 and 25, a ninth embodiment of the thermal mass flow sensor embodying the present invention is shown therein and generally identified by reference numeral 916. The thermal mass flow sensor 916 may be connected to the bypass 18 of the thermal mass flow meter 12 in substitution for the thermal mass flow sensor 16. The thermal mass flow sensor 916 has a sensor tube 920 for receiving the sensed portion of the flow of the gas from the bypass 18 of the thermal mass flow meter 12. The sensor tube 920 includes a unitary stainless steel tube 922, having a center point 923, for carrying the flow of gas and a thermally responsive member 924 wound about the tube 922 symmetrically with the tube center point 923. The thermally responsive member 924 produces an electrical signal at a central signal node 926, located at the tube center point 923, in response to the flow of gas through the tube 922. The sensor tube 920, and more particularly the thermally responsive member 924, generally do not produce a sufficiently accurate null electrical signal at zero rate of mass flow of gas therethrough due to the manufacturing assymetries which unbalance the thermal characteristic of the sensor tube 920. Means for modifying a thermal characteristic of the sensor tube 920, comprising a thermal shunt 928, is connected to the sensor tube 920 in good heat conducting relationship. The thermal shunt 928 reduces the time required for the sensor tube 920 to respond to a change in the rate of flow of the gas. The thermal shunt 928 is positioned on the sensor tube 920 to alter its thermal characteristic so that the sensor tube 920 produces a null signal at zero flow. The sensor tube 920 is mounted on a sensor tube base 930 of the thermal mass flow sensor 916 so that the sensor tube 920 is in communication with the bypass 18 of the thermal mass flow meter 12 of the thermal mass flow controller 10. The base 930 includes a lower plastic base element 944 and an upper metal base element 946 having a large thermal mass connected thereto. The lower plastic base element 944 is adapted to engage the bypass 18 and form a portion thereof. A metal conrection reducing cap 948 is fitted snugly over the upper metal base portion 946 to reduce external convective disturbances of the sensor tube 920. In order to communicate with the bypass 18 and receive a portion of the flow of gas therefrom, the tube 922, which is substantially U-shaped and has an inlet leg 950, an outlet leg 952 and a flow measuring portion 954 connected therebetween, has its inlet leg 950 connected in communication with the upstream portion of the bypass 18 and its outlet leg 952 connected in communication with the downstream portion of the bypass 18 so that the portion of the gas flowing into the bypass 18 also flows into the inlet leg 950, through the measuring portion 954 and out the outlet leg 952. The tube 922 is composed of 316 stainless steel alloy. An insulating blanket 956 is disposed in a generally circular configuration about the thermally responsive member 924 in order to further limit convective disturbance of the sensor tube 920. The thermal blanket 956 is held in place by the thermal shunt 928. Underneath the thermal blanket 956 is the temperature responsive member 924 which includes an upstream heater/sensor winding 960 and a downstream heater/sensor winding 962, both composed of multiple turns of 1.5 mil nickel wire having a high temperature coefficient of resistivity. The nickel wire is connected in the electrical bridge 92 with the elements 960 and 962 respectively having been substituted for windings 60 and 62. In order to reduce the thermal response time of the sensor tube 920, in particular its rate of temperature change when changes in gas flow occur through the tube 922, the thermal shunt 928 is provided. As is well known in the flow controller art, the mass rate of flow of gas through the tube 922 causes the upstream winding 960 to be cooler than the downstream winding 962, all other things being equal. This assumes that at no-flow condition, the temperature gradient along the tube 922 is symmetric about its center point 923. In this embodiment, the center point 923 of the tube 922 is at a region 966 immediately adjacent the signal node 926 for sensing the signal of the voltage divider comprised of the upstream winding 960 and the downstream winding 962. In this embodiment, the thermal shunt 928 comprises a cylinder 970 having a slotted cylindrical wall 972 and a pair of end walls 974 and 976 engaging via silver epoxy or solder or other good thermal conductor of the flow measuring portion 954 of the tube 922. During manufacture, the thermal shunt 928 is attached to the sensor tube 922 positioned symmetrically about the center point 923. It may be appreciated that a mica washer 980 is positioned within the tube 922 to provide further convective isolation of the windings 960 and 962 from convective disturbances. The sensitivity and the nulling by the shunt 928 are achieved by a movable copper wiper 980 in good thermal conductivity with the tube wall 982 and attached to a foil layer 984 of a printed circuit board 986 in thermal grounding contact with the metal base 946. The nulling is achieved by moving the copper arm in sweeping fashion along the tube 972 until a null condition is achieved at zero flow rate at the signal node 926. Sensitivity is then controlled by nibbling pieces of the copper tube away to control the amount of thermal grounding of the sensor tube 920 to the foil layer 984. The sensor tube 920 is further thermally grounded to the metal base by a plurality of windings 988 which are silver epoxied to the inlet leg 950 and the outlet leg 952 and the foil layer 984. Referring now to FIGS. 26, 27 and 28, a tenth embodiment of a thermal mass flow sensor embodying the present invention is shown therein and identified by reference numeral 1016. The thermal mass flow sensor 1016 may be connected to the bypass 18 of the thermal mass flow meter 12 in substitution for the thermal mass flow sensor 16. The thermal mass flow sensor 1016 has a sensor tube 1020 for receiving as portion of a flow of the gas from the bypass 18 of the thermal mass flow meter 12. The sensor tube 1020 includes a unitary stainless steel tube 1022, having a center point 1023, for carrying the flow of gas and a thermally responsive member 1024 wound about the tube 1022 symmetrically with the tube center point 1023. The thermally responsive member 1024 produces an electrical signal at a central signal node 1026, located at the tube center point 1023, in response to the flow of gas through the tube 1022. The sensor tube 1020, and more particularly the thermally responsive member 1024, generally do not produce a sufficiently accurate null electrical signal at zero rate of mass flow of the gas therethrough due to the manufacturing assymetries which unbalance the thermal characteristic of the sensor tube 1020. Means for modifying a thermal characteristic of the sensor tube 1020, comprising a thermal shunt 1028, is connected to the sensor tube 1020 in good heat conducting relationship. The thermal shunt 1028 reduces the time required for the sensor tube 1020 to respond to a change in the rate of flow of the gas. The thermal shunt 1028 is positioned on the sensor tube 1020 to alter its thermal characteristic so that the sensor tube 1020 produces a null signal at zero flow. The sensor tube 1020 is mounted on a sensor tube base 1030 of the thermal mass flow sensor 1016 so that the sensor tube 1020 is in communication with the bypass 18 of the thermal mass flow meter 12 of the mass flow controller 10. The base 1030 includes a lower plastic base element 1044 and an upper metal base element 1046 having a large thermal mass. The lower plastic base element 1044 is adapted to engage the bypass 18 and form a portion thereof. A metal convection reducing cap 1048 is fitted snugly over the upper metal base portion 1046 to reduce external convective disturbances of the sensor tube 1020. In order to communicate with the bypass 18 and receive a portion of the flow of gas therefrom, the tube 1022, which is substantially U-shaped and has an inlet leg 1050, a downstream or outlet leg 1052, and a flow measuring portion 1054, has its inlet leg 1050 connected in communication with the upstream portion of the bypass 18 and its outlet leg 1052 connected in communication with the downstream portion of the bypass 18 so that a portion of the gas flowing into the bypass 18 flows into the inlet leg 1050, through the measuring portion 1054 and out of the outlet leg 1052. The tube 1022 is composed of 316 stainless steel alloy. An insulating blanket 1056 is disposed in a generally U-shape around the thermally responsive member 1024 in order to further limit convective disturbance of the sensor tube 1020. The thermal blanket 1056 is held in place by the thermal shunt 1028. Underneath the thermal blanket 1056 is the temperature responsive member 1024 which includes an upstream heater/sensor winding 1060 and a downstream heater/sensor winding 1062, both composed of multiple turns of 1.5 mil nickel wire having a high temperature coefficient of resistivity. The wire is connected in the electrical bridge 92 in substitution for the heater sensor windings 60 and 62, respectively. In order to reduce the thermal response time of the sensor tube 1020, in particular its rate of temperature change - en changes in gas flow occur through the tube 1022, tht. thermal shunt 1028 is provided. As is well known in the flow controller art, the mass rate of flow of gas through the tube 1022 causes the upstream winding 1060 to be cooler than the downstream winding 1062, all other things being equa? . This assumes that at no-flow condition, the temperature gradient along the tube 1022 is symmetric about its center point 1023. In this embodiment, the center point 1023 of the tube 1022 is at a region 1066 immediately adjacent the signal node 1026 for sensing the signal of voltage divider comprised of the upstream winding 1060 and the downstream winding 1062. In this embodiment, the thermal shunt 1028 comprises a single wire piece 1070 having ends 1072 and 1074 wound about portions of the flow measuring tube 1054. In order to adjust the overall sensitivity of the sensor tube 1020, the ends 1072 and 1074 may be spread apart to increase the sensitivity and reduce the rate of response. They may be moved closer together towards the center point 1023 to reduce the sensitivity but increase the speed of response, thereby achieving the desired sensitivity/response time preselected value. In order to adjust the sensor tube 1020 thermal characteristic so that a null signal is provided at the node 1026 upon zero flow, the windings 1060 and 1062 are energized and with no-flow and are measured. More specifically, the signal node output at signal node 1026 is measured and the ends adjusted one at a time until the signal node output represents a true null flow condition. In order to thermally ground the sensor tube 1020, a printed circuit board 1080, having a copper foil layer 1082, has soldered or silver epoxied to it a plurality of windings 1084 which are connected to the legs 1050 and 1052. The printed circuit board 1080 is slidably interfitted with the upper metal base member 1046 with the copper foil layer 1082 in good heat conductivity therewith so that the sensor tube 1020 is thermally grounded to the metal base 1046. Referring now to FIGS. 30 and 31, an eleventh embodiment of the thermal mass flow sensor embodying the present invention is shown therein and generally identified by reference numeral 1116. The thermal mass flow sensor 1116 may be connected to the bypass 18 of the thermal mass flow meter 12 in substitution for the thermal mass flow sensor 16. The thermal mass flow sensor 1116 has a sensor tube 1120 for receiving a portion of a flow of the gas from the bypass 18 of the thermal mass flow meter 12. The sensor tube 1120 includes a unitary stainless steel tube 1122, having a center point 1123, for carrying the flow of gas and a thermally responsive member 1124 wound about the tube 1122 symmetrically with the tube center point 1123. The thermally responsive member 1124 produces an electrical signal at a central signal node 1126, located at the tube center point 1123, in response to the flow of gas through the tube 1122. The sensor tube 1120, and more particularly the thermally responsive member 1124, generally do not produce a sufficiently accurate null electrical signal at zero rates of flow of gas therethrough due to the manufacturing assymetries which unbalance the thermal characteristic of the sensor tube 1120. Means for 5 modifying a thermal characteristic of the sensor tube 1120, comprising a thermal shunt 1128, is connected to the sensrr tube 1120 in good heat conducting relationship. The thermal shunt 1128 reduces the time required for the sensor tube 1120 to respond to a change in the rate of flow of the 10 gas. The thermal shunt 1128 is positioned on the sensor tube 1120 to alter its thermal characteristic so that the sensor tube 1120 produces a null signal at zero flow. The sensor tube 1120 is mounted on a sensor tube base 1130 of the thermal mass flow sensor 1116 so that the 15 sensor tube 1120 is in communication with the bypass 18 of the thermal mass flow meter 12 of the thermal mass flow controller 10. The base 1130 includes a lower plastic base element 1144 and an upper metal base element 1146 having a large thermal mass. The lower plastic base element 1144 is 20 adapted to engage the bypass 18 and form a portion thereof. A metal convection reducing cap 1148 is fitted snugly over the upper metal base portion 1146 to reduce external convective disturbances of the sensor tube 1120. In order to communicate with the bypass 18 and receive a portion of 25 the flow of gas therefrom, the tube 1122, which is substantially U-shaped and has an upstream or inlet leg 1150, a downstream or outlet leg 1152, and a flow measuring portion 1154, has its inlet leg 1150 connected in communication with the upstream portion of the bypass 18 3. and its outlet leg 1152 connected in communication with the downstream portion of the bypass 18 so that a portion of the gas flowing into the bypass 18 flows into the inlet leg 1150, through the measuring portion 1154 and out through the outlet leg 1152. The tube 1122 is composed of 316 35 stainless steel alloy. An insulating blanket 1156 is disposed in a generally square shaped configuration around the thermally responsive member 1124 in order to further limit convective disturbance of the sensor tube 1120. The thermal blanket 1156 is held in place by the thermal shunt 1128. Underneath the thermal blanket 1156 is the temperature responsive member 1124 which includes an upstream heater/sensor winding 1160 and a downstream heater/sensor winding 1162, both composed of multiple turns of 1.5 mil nickel wire having a high temperature coefficient of resistivity. The upstream winding 1160 and the downstream winding 1162 are substituted for the windings 60 and 62 of FIG. 29. The wire is connected in the electrical bridge 92 and energized. In order to reduce the thermal response time of the sensor tube 1120, in particular its rate of temperature change when changes in gas flow occur through the tube 1122, the thermal shunt 1128 is provided. As is well known in the flow controller art, the mass rate of flow of the through the tube 1122 causes the upstream winding 1160 to be cooler than the downstream winding 1162, all other things being equal. This assumes that at no-flow condition, the temperature gradient along the tube 1122 is symmetric about its center point 1123. In this embodiment, the center point 1123 of the tube 1122 is at a region 1166 immediately adjacent the signal node 1126 for sensing a signal of a voltage divider comprised of the upstream winding 1160 and the downstream winding 1162. In this embodiment, the thermal shunt 1128 comprises a unitary copper or aluminum box-like shunt 1170 having a rectangular cross-section box member 1172 formed integrally with a slotted tail member 1174. The box member includes an upper wall 1176, a pair of side walls 1178 and 1180, a partial bottom wall 1182 and a pair of end walls 1184 and 1186, which end walls are in good thermal conduction with the measuring portion 1154 of the sensor tube 1120. The box member 1172 is open at an opening 1188 so that leads may be taken out to the processing circuitry shown in portions of FIG. 29. The box member 1172 is affixed to the sensing tube 1120 in a substantially symmetric position with respect to the center point 1123, and then may be adjusted by laser trimming slots into the tail member 1174 to move the center point temperature into coincidence with the center point 1123 at no-flow conditions in order to provide a null signal at 1126. Likewise, the length of the slots and total area of material removed controls the amount of thermal grounding from the tail portion 1174 to its thermal grounding attachment on a printed circuit board 1190, more particularly its attachment to a copper foil layer 1192 of the printed circuit board 1190, and, thus, controls the thermal sensitivity of the sensor tube 1120. The sensor tube 1120 has a plurality of windings 1194 attached to the inlet leg 1150 and the outlet leg 1152 by silver epoxy, which silver epoxy is also bonded to the foil layer 1192 to provide support for the sensor tube 1120. The printed circuit board 1190 is positioned in the upper metal base portion 1146. Referring now to FIGS 32, 33 and 34, a twelfth thermal mass flow sensor embodying the present invention is shown therein and generally identified by reference numeral 1216. The thermal mass flow sensor 1216 is connected to the bypass 18 of the thermal mass flow meter 12 in substitution for the thermal mass flow sensor 16. The thermal mass flow sensor 1216 has a sensor tube 1220 for receiving a portion of a flow of the gas from the bypass 18 of the thermal mass flow meter 12. The sensor tube 1220 includes a unitary stainless steel tube 1222, having a center point 1223, for carrying the flow of gas and a thermally responsive member 1224 wound about the tube 1222 symmetrically with the tube center point 1223. The thermally responsive member 1224 produces an electrical signal at a central signal node 1226, located at the tube center point 1223, in response to the flow of gas through the tube 1222. The sensor tube 1220, and more particularly the thermally responsive member 1224, generally do not produce a sufficiently accurate null electrical signal at zero rate of mass flow of gas therethrough due to the manufacturing assymetries which unbalance the thermal characteristic of the sensor tube 1220. Means for modifying a thermal characteristic of the sensor tube 1220, comprising a thermal shunt 1228, is connected to the sensor tube 1220 in good heat conducting relationship. The thermal mass flow sensor includes a lower metal body block portion 1230 and an upper metal body block portion 1232, both having large thermal masses, interfitted therewith. The upper and lower body block portions 1230 and 1232 are connected together by threaded fasteners 1234 which extend through apertures 1236 in the upper body block portion. The lower body block portion 1230 includes an interior conforming wall 1244 for receiving the thermal shunt 1228. A similar interior conforming wall 1246 defines the interior portion of the upper body block portion 1232. The tube 1222 terminates at a cavity 1250 and a cavity 1252 in the ends of the body block portions 1230 and 1232. The termination also comprises a thermal ground for the tube 1222. More particularly, the lower body portion 1230 has a gas inlet bore 1254 and a gas outlet bore 1256 in communication therewith so that gas may be received from the bypass 18 through the inlet bore 1254, travel into the cavity 1250, through the tube 1222, through the cavity 1252 and then out through the outlet bore 1256 to the downstream portion of the bypass 18. The thermally responsive member 1224 includes an upstream winding 1260 and a downstream 1262, both composed of 1.5 mil nickel wire having a high temperature coefficient of resistivity. The windings 1260 and 1262 are substituted for the windings 60 and 62 of FIG. 29, as is the node 1226 substituted for the node 26 of FIG. 29. The windings 1260 and 1262 are attached to a foil 1268 on a printed circuit board 1269, which foil 1268 comprises the node 1226. A pair of foils 1270 and 1272 are connected to the constant current source 90 of FIG. 29 to be energized thereby. The center point of the tube 1222 has a mica washer 1274 resting thereon to reduce convective flow in the region of the windings 1260 and 1262, and thereby reduce thermal perturbation of the windings 1260 and 1262. The thermal shunt includes a cylindrical canister 1276 having tapering ends 1278 and 1280 which are in good thermal contact with wire windings 1282 and 1284, respectively. The wire windings 1282 and 1284 are movable along the tube 1222 in order to adjust the center point temperature of the tube at no-flow conditions to provide a true null flow representation at the node 1226 without the need f-x electrical biassing. Once the null flow condition has been detected, the windings 1282 and 1284 are permanently attached at the null points so selected by solder or silver epoxy. Referring now to FIGS. 35 and 36, a thirteenth embodiment of the thermal mass flow sensor embodying the present invention : shown therein and is generally identified by reference numeral 1316. The thermal mass flow sensor 1316 is connected to a bypass 18 of the thermal mass flow meter 12 and has a sensor tube 1320 for receiving a portion of a flow of a gas from the bypass 18 of the thermal mass flow meter 12. The sensor tube includes a unitary stainless steel tube 1322, having a center point 1323, for carrying the flow of gas and a thermally responsive member 1324 wound thereabout. The thermally responsive member 1324 produces an electrical signal at a signal node 1326 in response to the flow of gas through the tube 1322. The sensor 1320, and more particularly the thermally r*. ponsive member 1324, generally do not produce a sufficiently accurate null electrical signal at zero rate of mass flow of gas therethrough due to the thermal characteristic of the sensor tube 1320. Means for modifying a thermal characteristic of the sensor tube 1320 comprising a current controlling circuit 1328 is connected to the sensor tube 1320, in particular to the temperature responsive member 1324. The temperature biassing circuit 1328 includes a node 1330 connected to a positive voltage source and a node 1332 connected to a negative voltage source. It may be appreciated that the temperature responsive member 1324 includes an upstream winding 1340 and a downstream winding 1342, each composed of 1.5 mil nickel wire having a high temperature coefficient of resistivity, nickel wire being wound about portions of a flow measuring portion 1344 of the tube 1322. An inlet tube 1346 and an outlet tube 1348 are connected to the flow measuring portion 1344. Electric current for the upstream winding 1340 is received from a first constant current generator 1350. Electric current for the downstream winding 1342 is received from a constant current generator 1352. T h e constant current generator 1350 and the constant current generator 1352 receive regulated voltages from a regulated supply 1354 connected between the nodes 1330 and 1332. The regulated supply 1354 includes a resistor 1356 connected to the node 1330. A first Zener diode 1358 and second Zener diode 1360 are connected in series with a grounding node 1362 located therebetween. A resistor 1364 is connected to the Zener diode 1360 and to the constant current generator 1350. A voltage dividing resistor 1366, comprising a potentiometer 1366 having a sweep-arm 1368, is connected in parallel with the Zener diode 1358 and 1360 so that a regulated voltage ranging between the node voltages at the cathode of Zener diode 1358 and the anode of the Zener diode 1360 may be placed upon a lead 1370 connected to the sweep-arm 1368. An input resistor 1372 is connected to an operational amplifier 1374 at its non-inverting node 1376. The operational amplifier 1374 also has an output node 1378 and an inverting input node 1380. The output node 1378 has an npn transistor 1382 having a collector 1384 connected to receive current from the node 1330. An emitter 1386 of the transistor 1382 delivers controlled current to the downstream winding 1342. The downstream winding 1342 is connected to a grounding resistor 1400 and to a feedback resistor 1402, which is connected to the input node of the operational amplifier 1374 so that the current through the downstream winding is controlled by the independent current source 1352. The current source 1350 is identical to the current source 1352 and includes an operational amplifier 1410 having an inverting input terminal 1412, a non- inverting input terminal 1414 and an output terminal 1416. The non-inverting input terminal is connected through a resistor 1420 to the junction of Zener diode 1360 and a resistor 1364 to receive regulated voltage therefrom. The output terminal drives a PNP transistor 1422 at its base 1424. An emitter 1426 of the PNP transistor 1422 is connected to the winding 1340 to control the current flow therethrough. The collector 1428 is connected to the node 1332. A feedback loop comprising a resistor 1436 is connected to the upstream winding 1340 to supply a potential to the non-inverting terminal 1412. A grounding resistor 1438 is also connected to the upstream winding 1340 to provide a voltage offset from ground. A zeroing signal is fed through a resistor 1450 to a non-inverting summing input node 1452 of a non-inverting amplifier 1454. The non-inverting amplifier 1454 has an inverting node 1456 connected through an integrating network 1460 comprising a voltage divider 1462, a grounded resistor 1464 and a capacitor 1466. An output node 1468 of the amplifier 1454 may drive other portions of the circuit such as the speed¬ up circuit 96 of FIG. 29 or the differential amplifier circuit 94 of FIG. 29 as the case may be. It may be appreciated that the current through the upstream winding 1340 and the downstream winding 1342 may be varied independently of each other so that the effective thermal center point at no-flow conditions may be brought into coincidence with the center point 1323 of the mass flow sensor 1316 in order to provide true thermal nulling so that the electrical signal produced at null conditions is reflective of a zero flow rate with no offset. Referring now to FIGS. 1, 2, 3 and 37, the effect of the thermal shunt 28 on the sensor tube 20 is adjusted to reduce the effect of variations of sensor tube thermal characteristics due to manufacturing variations. More specifically, the sensor tube variations affect null balance at no flow, sensitivity to flow rate and time response to flow transients. The sensor tube 20 and the thermal shunt 28 being adjusted are mounted on a standard thermal mass flow sensor base block 1500 and electrically connected with a standard thermal mass flow controller signal amplifying and conditioning circuit to form a thermal mass flow controller 1504. The standard signal amplifying and conditioning circuit of the flow controller 1504 is connected electrically to a UCAL-1000 mass flow controller system including a calibrator 1506, available from Unit Instruments, Inc., 1247 West Grove Avenue, Orange, California and an IBM compatible personal computer 1508 executing UICALSYS calibrator software from Unit Instruments, Inc. The calibrator 1506 is also connected to a standard thermal mass flow controller 1510 plumbed in series with the sensor tube 20 and thermal shunt 28 under test in a gas line 1512. Two or more standard thermal mass flow controllers including a thermal mass flow controller 1514, may be plumbed in parallel with appropriate valves 1516 and 1518 to receive metered flows of gas, such as dry nitrogen or other gases, from a gas source 1520 and provide output signals related to those gas flows to the calibrator 1506 and computer 1508. The standard mass flow controllers 1510 and 1514 may then be switched into a test sensor gas circuit 1522 by means of the valves 1516 and 1518 to cause rapid dynamic or time-dependent flow rate changes in the sensor tube 20 under test. The time-dependent flow rate changes cause the output voltage of the winding 24 to exhibit a dynamic or time-dependent electrical characteristic. The calibrator 1506 and the computer 1508 are programmed to operate the system at no flow and at various flow rates in sequence, and to record the sensor's signal output. Both transient and steady-state signals from the thermal flow controller 1504 under test are recorded. Following a programmed run the computer 1508 displays the results in comparison with established flow standards. The test may be repeated with the test sensor 16 stabilized at a second ambient temperature if the temperature coefficient is being adjusted. The person calibrating the sensor tube 20 and thermal shunt 28, if it is not producing a null signal, then moves both attachment points of the thermal shunt 28 to the tube 22 a small amount, one toward the sensor winding 24 and the other away from the sensor winding 24 with both ends moving upstream if the no-flow signal is positive or above the standard null signal and downstream if it is negative or below the standard null signal. If the sensitivity of the sensor tube 20 is in error, both ends of the thermal shunt 28 are moved away or toward the winding 24 along the legs 50 and 52. They are both moved farther away from the sensor winding 24 to increase sensitivity, or toward the sensor winding 24 to reduce it. If the speed of response is being adjusted rather than the sensitivity, then the ends of the thermal shunt 28 are moved closer to the sensor winding 24 to increase speed, or farther away to slow the response as identified by the time constant of the dominant pole of the sensor response function in the computer software. The programmed test is then re-run to determine the effect of the thermal shunt adjustments and to indicate the magnitude of further adjustments if necessary. In mass flcrf controllers that include thermal insulation associated with the sensor tube or other parts that could influence the performance and that might have been moved for adjustment, all such parts must be returned to their proper position while the test is being run. Other combinations of test procedures and adjustment perfection criteria may be devised to suit particular combinations of manufacturing tolerances and application requirements. All, however, will benefit from the capability of the present invention to adjust for symmetry-related anomalies in the inherent thermal characteristics of the sensor tube by moving the thermal shunt 28 attachment points to the sensor tube 20, or their equivalents in a common direction relative to the flow direction, and to adjustment for sensitivity-related anomalies by moving the shunt attachment points to the sensor tube 20 in opposing directions. While there has been illustrated and described a particular embodiment of the present invention, it will be appreciated that numerous changes and modifications will occur to those skilled in the art, and it is intended in the appended claims to cover all those changes and modifications that fall within the true spirit and scope of the present invention.

Claims: WHAT IS CLAIMED IS: 1. A sensor for sensing a mass flow rate of a fluid in a thermal mass flow meter, comprising: a sensor tube having a tube for carrying a flow of fluid whose mass flow rate is to be measured, and having a temperature responsive member disposed in good heat conducting relation with the tube and which is electrically energized, said temperature responsive member having a signal node from which a sensor signal responsive to the mass flow rate is generated, said sensor signal being offset from a null signal condition by a thermal characteristic of the sensor tube when the mass flow rate in the tube is zero; and means for modifying the thermal characteristic of said sensor tube to cause said signal node of said thermally responsive member to produce a substantially null signal condition when the mass flow rate in the tube is zero. 2. A sensor for sensing a mass flow rate of a fluid in a thermal mass flow meter according to claim 1, wherein said means for modifying the thermal characteristic comprises a thermal conductor having a pair of ends in good thermal contact with the tube. 3. A sensor for sensing a mass flow rate of a fluid in a thermal mass flow meter according to claim 1, wherein said means for modifying the thermal characteristic comprises a thermal shunt having a pair of ends in good thermal contact with the tube. 4. A sensor for sensing a mass flow rate of a fluid in a thermal mass flow meter according to claim 1, wherein said temperature responsive member comprises a temperature responsive winding wound about the tube and having a first tap, a second tap and a third tap, the first tap and the second tap being energized from a source of electrical energy, said third tap comprising the sensor signal node. 5. A sensor for sensing a mass flow rate of a fluid in a thermal mass flow meter according to claim 1, wherein said means for modifying the thermal characteristic of said sensor tube comprises an extendible thermal shunt having a first end connected to the tube on a first side of the winding and a second end connected to the tube at a second side of the winding. 6. A sensor for sensing a mass flow rate of a fluid in a thermal mass flow meter according to claim 1, wherein said means for modifying the thermal characteristic of said sensor tube comprises a thermal shunt substantially enclosing said winding and having a first end connected to the tube on a first side of the winding and a second end connected to the tube at a second side of the winding. 7. A sensor for sensing a mass flow rate of a fluid in a thermal mass flow meter according to claim 1, wherein said means for modifying the thermal characteristic of said sensor tube comprises a nulling electrical heater connected to the tube to shift the maximum temperature point to be substantially coincident with the sensor signal node. 8. A sensor for measuring a rate of flow of a mass of fluid in a thermal mass flow meter, comprising: a sensor tube having a tube for carrying the mass of fluid moving at the rate to be measured, said tube having an outside portion, said sensor tube having a temperature responsive member disposed around said outside portion of said tube, a rate of change of a temperature dependent electrical characteristic of said temperature responsive member with respect to the rate of flow of the mass of fluid being offset from a selected value by a thermal characteristic of said sensor tube; and means for modifying the rate of change of the thermal characteristic of said sensor tube with respect to the rate of flow of the mass of fluid to cause said temperature dependent electrical characteristic of said thermally responsive member to be substantially at the selected value. 9. An apparatus for sensing a mass flow rate of a fluid in a thermal mass flow meter, comprising: a sensor tube having a tube for carrying a flow of fluid whose mass flow rate is to be measured, and having a temperature responsive member disposed around an outside portion of the tube and which is electrically energized, said temperature responsive member having a node from which a sensor signal responsive to the mass flow rate is generated, said signal being offset from a null signal condition by a thermal characteristic of the sensor tube when a zero mass flow rate is present in the sensor tube; and means for modifying the thermal characteristic of said sensor tube to cause said signal node of said thermally responsive member to produce a substantially null signal condition when the zero mass flow rate is present in the tube. 10. An apparatus for measuring a rate of flow of a mass of fluid in a thermal mass flow meter, comprising: a sensor tube having a tube for carrying the mass of fluid moving at the rate to be measure. , said tube having an outside portion, said sensor tube having a temperature responsive member disposed around said outside portion of said tube, a temperature dependent electrical characteristic of said temperature responsive member being offset from a preselected condition indicative of a preselected sensitivity of the apparatus by a thermal characteristic of said sensor tube when the mass of fluid is moving; and means for modifying the thermal characteristic of said sensor tube to cause said temperature dependent electrical characteristic of said thermally responsive member to be at the preselected condition indicative of the preselected sensitivity of the apparatus when the mass of fluid is moving through the tube. 11. A method of making a sensor for a thermal mass flow meter, comprising the steps of: contacting a tube with a temperature responsive member; energizing the temperature responsive member; measuring an electrical characteristic of the temperature responsive member when a mass flow rate of a fluid through the tube is substantially zero; and altering a thermal characteristic of the tube and the temperature responsive member to cause the temperature responsive member to produce a substantial null condition in the electrical characteristic of the temperature responsive member under substantially zero mass flow rate conditions. 12. A method of making a sensor for a thermal mass flow meter, comprising the steps of: winding a temperature responsive winding about a tube; energizing the temperature responsive winding; measuring an electrical characteristic of the temperature responsive winding when a mass flow rate of a fluid through the tube is zero; and altering a thermal characteristic of the tube and the temperature responsive winding to cause the temperature responsive winding to produce a null condition in the electrical characteristic of the temperature responsive winding under zero mass flow rate conditions. 13. A method of calibrating a sensor for a thermal mass flow meter, comprising the steps of: energizing a temperature responsive member in contact with a tube for carrying a flow of fluid; thermally biasing the temperature responsive member; determining a first magnitude of an electrical characteristic of the temperature responsive member; and altering the thermal bias on the temperature responsive member to cause the temperature responsive member to exhibit a null magnitude of the electrical characteristic. 14. A method of calibrating a sensor for a thermal mass flow meter, comprising the steps of: energizing a temperature responsive member in contact with a tube for carrying a flow of fluid; thermally biasing the temperature responsive member; flowing a selected amount of fluid through the tube; determining a first magnitude of an electrical characteristic of the temperature responsive member in response tc the flow of the selected amount of fluid through the tube; and altering the thermal bias to cause the temperature responsive member to exhibit a second magnitude of the electrical characteristic in response to the flow of the selected amount of fluid through the _._ibe. 15. A method of alibrating a sensor for a thermal mass flow meter, comprising the steps of: energizing a temperature responsive member in contact with a tube for carrying a flow of fluid with a temperature responsive member; thermally biasing the temperature responsive member; changing a flow rate of a fluid through the tube; determining a first magnitude of a time-dependent electrical characteristic of the temperature responsive member in response to change in flow of the selected amount of fluid through the tube; and altering the thermal bias to cause the temperature responsive member to exhibit a second magnitude of the time-dependent electrical characteristic in response to the change in flow of the selected amount of fluid through the tube.

Inventors: DOYLE MICHAEL JOHN, LEMAY DAN BROWN, VAVRA RANDALL JAMES, VU KIM NGOC, DOYLE, MICHAEL, JOHN, LEMAY, DAN, BROWN, VAVRA, RANDALL, JAMES, VU, KIM, NGOC
Applicants: UNIT INSTR INC, UNIT INSTRUMENTS, INC.
Application year: 1992
Main classification: G01F25
IPC codes: G01F1, G01F5, G01F25