Fluid Flowmeter

The present invention is an improvement over the flow measuring apparatus disclosed in U.S. Pat. No. 3,443,434-William C. Baker et al -- which was issued on May 13, 1969. More particularly, the aforesaid patent relates to an arrangement wherein a portion of the fluid streaming through a main supply line is diverted in a shunt path by a conduit which is electrically heated from a constant power source. A thermoelectric device is associated with the conduit, and the temperature sensed by the thermoelectric device is used to provide an indication of the flow through the shunt path and the main supply line.

The relationship that exists between fluid flow and the variables encountered in a temperature sensing arrangement for measuring fluid flow is given by the formula:

where M is the mass flowrate, H is the heat input per unit time, Cp is the heat capacity of the fluid at constant pressure and .DELTA.t is the temperature differential between the temperature detected by a sensing device responsive to the fluid flow and a reference temperature, such as that of a heat sink.

The heat capacity term Cp cannot indiscriminately be considered a constant since it varies with fluid composition, temperature and pressure. In most fluids the variation with pressure is small and can be neglected. However, the variation with temperature can be quite large for some fluids.

The Cp for carbon dioxide, for example, varies 19.5% for a 200.degree. C. change, and such a variation is typical of the variations of the operating temperatures of some constant power flowmeters. Therefore, the accuracy of flow measurement is adversely affected by variations of Cp encountered with such a flowmeter.

Furthermore, in constant power type flowmeters a relatively large temperature change must be detected to provide good sensitivity. A typical constant power flowmeter using thermocouples to sense temperature requires a 213.degree. C. temperature differential for a 10 mvdc output.

By comparison, when operating a flowmeter in a constant temperature mode as contemplated by the present invention, variations in Cp due to flow are substantially eliminated and Cp is relatively constant for any given fluid. Additionally, it is only necessary to develop a temperature differential of sufficient magnitude to provide a stable error signal capable of maintaining the differential constant. As a result, the constant temperature flowmeter operates with improved accuracy, increased range, reduced operating temperature and improved response time.

Briefly, the invention comprises a main supply line carrying the fluid, the flow characteristics of which are to be measured. A restrictive head element is provided in the main line to develop a differential pressure in the line. An external shunt line, or bypass, is connected to the main supply line to divert a portion of the fluid through the shunt from the high pressure side of the head element to the low pressure side. A constant temperature flowmeter is associated with the shunt line, the flowmeter including a conduit having a first electrically heat portion and an unheated portion, each portion having a thermoelectric device associated therewith to sense the temperature at the locations of the devices. The outputs of the thermoelectric devices are applied to circuitry which responds to the temperature differential detected by the devices so as to control the voltage applied to the electrical heater thereby maintaining the temperature differential constant. The voltage required to accomplish this is a function of the fluid flow characteristics of the system.

The invention will become more fully apparent when considered in light of the following detailed description of illustrative embodiments of the invention and from the appended claims.

The illustrative embodiments may be best understood by reference to the accompanying drawings, wherein:

FIG. 1 is a schematic drawing, partially in section, illustrating a typical constant temperature flow sensor which may be used in practicing the present invention;

FIG. 2 is a schematic drawing, partially in section, illustrating a first embodiment of the invention;

FIG. 3 is a schematic drawing, partially in section, illustrating a second embodiment of the invention;

FIG. 4 is a schematic drawing, partially in section, illustrating a third embodiment of the invention.

Referring first to FIG. 1, a constant temperature flow sensor 10 is illustrated which is suitable for use with the several embodiments of the invention disclosed herein. The sensor comprises an electrically and thermally conductive heat sink having end walls 12 and 14, a partition 16, and bottom portions 18 and 20 which connect the partition 16 with the end walls 12 and 14, respectively. A conduit 22 passes through the end walls and the partition in contacting relationship therewith, the partition dividing the conduit into portions 24 and 26. Conduit portions 24 and 26 are thermally conductive, and at least conduit portion 26 is electrically conductive. Power is inductively coupled in conduit portion 26 by a wound toroid 28 energized by a variable voltage source 30. When toroid 28 is energized, the loop comprising conduit portion 26, end wall 14, bottom portion 20 and partition 16 permits a current flow thereby heating conduit portion 26. A first thermocouple TC-1 is formed at the junction of the unheated portion 24 of conduit 22 and an attached conductor 32 of a dissimilar material. A second thermocouple TC-2 is formed at the junction of the heated conduit portion 26 and an attached conductor 34 of a dissimilar material. The outputs of thermocouples TC-1 and TC-2 are coupled by conductors 32 and 34, respectively, to a differential amplifier 36. The output of the amplifier is joined to a comparator 38 as is a fixed reference voltage source 40. The comparator output is a control signal which is connected as an input to the variable voltage source 30. An indicator 42 is controlled by the output of source 30 to develop a reading which is a function of the voltage produced by source 30.

In operation, reference 40 is set to produce an output from source 30 sufficient to heat conduit portion 26 via toroid 28 by an amount sufficient to create a desired temperature differential .DELTA.t between the locations of TC-1 and TC-2. Any variation in .DELTA.t causes the differential amplifier output to vary thereby altering the output of comparator 38. This in turn adjusts the voltage output of source 30 so as to change the amount of electrical heating of conduit portion 26 until the predetermined .DELTA.t is reached. The indicator 42 provides an indication which is a function of the voltage necessary to restore .DELTA.t to its desired constant value.

Referring now to FIG. 2 a first embodiment of the invention will be described. Within a main supply line 52 fluid moves on the direction indicated by the arrowheads. The main flow is represented by M.sub.1. A primary restrictive element 54 is positioned in the supply line to develop a differential pressure .DELTA.p, across the element which is proportional to the square of the flow M.sub.1 through it. Element 54 may take many forms such as an orifice plate, a flow nozzle, a venturi nozzle, etc. The same differential pressure exists across a shunt line, or bypass 22, and causes a flow M.sub.2 therethrough. The by-pass is sized to limit the flow to a small fraction of the main flow M.sub.1 and to insure that it is essentially laminar throughout its intended range. Under these conditions the by-pass flow M.sub.2 is directly proportional to the differential pressure .DELTA.p, across element 54 and therefore proportional to the square of the main flow M.sub.1.

A constant temperature flow sensor is included in the shunt path. As indicated previously in the discussion of the formula:

the mass flow through a heated tube type of flow sensor is directly proportional to the heat input per unit time (H) into the tube, and inversely proportional to the heat capacity of the fluid (Cp) and to the temperature differential in the fluid as it passes through the heated tube (.DELTA.t). The heat input is proportional to the electrical power supplied to the tube, which is proportional to the square of the applied voltage (E.sup.2 /R). With the temperature differential held constant, and the heat capacity of the fluid and the electrical resistance of the heated tube being substantially constant, the mass flowrate is proportional to the square of the applied voltage. The flow M.sub.2 in the by-pass is measured by the constant temperature flow sensor and thus is proportional to the square of the voltage applied to the heater portion of sensor 10. Since flow M.sub.2 is also proportional to the square of the main flow M.sub.1, the heater voltage squared is also proportional to the square of the main flow M.sub.1. Thus, the heater voltage is directly proportional to the flow M.sub.1 in the main supply line 52.

A second embodiment of the invention illustrated in FIG. 3 differs from the preceding embodiment by including in the shunt line 22 a second restrictive element 56. The purposes of element 56 are (1) to match the pressure drop .DELTA.p across the by-pass at the rated flow of the constant temperature flow sensor 10 to that across the primary restrictive element 54 at its rated flow and (2) to insure that flow through the by-pass varies directly with the pressure differential. Again, the heater voltage is proportional to the flow in the main supply line 52.

A third embodiment of the invention is shown in FIG. 4. In this case the primary restrictive element is a pitot tube 58. The pressure drop .DELTA.p across the tip of the pitot is proportional to the square of the velocity of the fluid passing the pitot tube. The pressure differential .DELTA.p causes a small flow M through the conduit 22 of the constant temperature flow sensor connected across the pitot tube. With laminar flow through conduit 22, the flow is proportional to the pressure differential .DELTA.p. The square of the voltage applied to the heater portion of sensor 10 is also proportional to the flow M. Therefore, the voltage applied to the heater is directly proportional to the velocity of the fluid past the pitot.