Pressure measuring systems

Drawings

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United States Patent Office 3,364,749 Paiented Jan. 23, 1968

3,364,749 applied directly as a pressure and that an additional nov PRESSURE MEASURNG SYSTEMS able member is not essential to separate the unknown Anatole J. Sipin, 17 E. 77th St., New York, N.Y. 602 fluid from the environment. For example, the electro Filed Oct. 18, 1965, Ser. No. 497,000 magnetic driver can be assembled between the branches 11. Claims. (CI. 73-398) of a U-tube mercury manometer; and level switches or This invention relates to a system for measuring fluid other sensors used to detect motion of the mercury column from a reference position when pressure is applied. The pressure over a wide range with high accuracy. manometer can be armored so that small values of differ Need exists in industrial processes and iaboratory ap ential pressure can be detected by the level sensor at plications for an instrument that can measure absolute O high values of absolute pressure. If a permanent magnet or differential pressure over a wide range with high sensi or other constant field is applied, the current supplied to tivity and high accuracy and that provides an electrical the electromagnetic driver is directly related to unknown output for remote transmission that is not affected by pressure. This current can be measured by an annineter at transmission losses and interference. long distances from the sensor; and it is unaffected by Existing pressure transducers using the extension of an 15 transmission line resistance.

elastic member as a measure of applied pressure have a Different embodiments of the invention are described resolution which is a constant fraction of the full scale hereinafter in connection with the following drawings, output, limiting the range of measurement. Also, such disclosing various specific features and advantages. transducers often have delicate electrical elements with In the drawings,

Very Sinail voltage outputs or high internal impedances, 20 FIG. 1 is a schematic diagram of the pressure measur making it necessary to shield signal leads, use amplifiers ing system showing a preferred form of the invention. with very high input impedances of take other measures FIG. 2 is a sectional elevation view of a preferred to avoid spurious effects. Automatic manometers with construction for the pressure balancing means shown in follow-up systems have been constructed having high F.G. 1.

accuracy and satisfactory electrical outputs. These in 25 FIG. 3 is a fragmentary view showing a detail of the struments, however, are complex and expensive; and pressure balancing device shown in FIG. 2. their range is limited by a practical height for the manom FIG. 4 is a schematic diagram of another embodiment eter tubes. Force balance instruments are known in which of the pressure measuring System. the unknown pressure is applied as a force on a member The pressure measuring system shown in FIG. 1 in and an electrically generated restoring force is applied 30 cludes a first pressure sensing member 6, a second pres to balance the member. The restoring force provides a sure sensing member 12, and an electromagnetic driver measure of the unknown pressure. The pressure sensing 54 connected to the pressure sensing members. Member member is typically an elastic member such as a bellows 18 consists of a housing 16 with an internal cavity, a or diaphragm. It is difficuit to provide high accuracy at high pressures with such devices. The restoring force is 35 chambers 2358,andseparating diaphragm the cavity into two pressure electrical contact 24 protruding applied in relation to the deflection of the member. To into charaber 20, a lead 25 connecting contact 24 into an withstand high pressure the elastic sensing member is external circuit, a first tube 26, connecting chamber 22 stiff; and the spring force opposed to deflection intro to a source of unknown pressure, and a second tube 28 duces errors associated with other transducers using connecting chamber 20 to the electromagnetic driver. elastic elements. 40 Member 2 includes housing 30, pressure chamber 34, It is an object of this invention to provide a system for contact 33 connected to lead 40 and fluid pressure tubes measuring fluid pressure over a wide range with high 42 and 44. The continuous channel formed by tubes accuracy.

it is another object of this invention to provide a 28 and 44 and electromagnetic driver 14 is filled with a conductive fiuid, shown in FIG. 1 as a liquid, which aiso

System for measuring high pressures with high accuracy. 4 5 partially

It is a third object of this invention to provide a sys fills chambers 20 and 34. Electrode 46, insulated tem for measuring Small differential pressures at high from tube 44, is immersed in the conductive liquid and values of absolute pressure. connected into an external circuit through lead 48. Leads It is a fourth object of this invention to provide a sys 25, 46 and 48 provide inputs to amplifier circuit 56, which supplies tem for measuring fluid pressure having an electrical out 50 turn supplies an imbalance signal to integrator 52, which in put which is not affected by transmission losses. driving current to the electromagnetic driver. It is a further object of this invention to provide a The driving current is measured by indicator 54, which is calibrated in units of pressure.

small and simply constructed device to meet the fore Assuming that the pressure applied to chamber 22 going objectives.

The invention is a pressure measuring system of the 55 through tube 26 is higher than that applied to chamber 34, the level of conductive liquid in chamber 20 will fall force balance type in which the restoring force is applied while that in chamber 34 will rise. When the level in electromagnetically to a conductive fluid. Unknown pres chamber 34 has increased a Srinall amount, the circuit sure is applied to a line containing electrically conductive between contact 33 and electrode 46 will be closed, caus fluid, moving the fluid. Motion of the conductive fluid is ing a constant voltage of given polarity to be applied to sensed by a pickup; and a pressure balancing device 80 integrator 52. The integrator delivers a continuously in applies an electromagnetic force to the conductive fluid creasing value of electric current to the driver, which in accordance with the output of the pickup in a direc electromagnetically applies an increasing pressure to the tion to null the fluid motion and restore a pressure bal conductive liquid in a direction opposite to its motion ance. The force applied by the electro-magnetic device until the null levels in both chambers are restored, and is measured to indicate the unknown pressure. The pres 65 the circuit between Switch 38 and electrode 46 is again sure balancing device can be an electromagnetic driver open. At this point the electromagnetic pressure exactly of known configuration in which magnetic flux and elec balances the unknown pressure differential applied to tric current are passed through the conductive fluid in chambers 22 and 34. The electromagnetic pressure is also orthogonal directions transverse to the line to apply a pressure force to the conductive fluid in the line by 70 islinearly related to the electrical current; and ammeter 54 calibrated directly to indicate the unknown pressure.

motor action. The principal advantage of this System. Over previous force balance devices is that the feedback is 22,ifthe the pressure in chamber 34 exceeds that in chamber circuit between contact 24 and electrode 46 is

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3 of increasing flux density. Thus, high balancing pressures closed, a current of opposite polarity is fed to the electro can be attained with a relatively small magnetic structure magnetic driver and meter 54 indicates the negative pres and relatively low values of current. The advantage of Sure differential. the invention can be appreciated by comparing the bal If one of the chambers 22 and 34 is exposed to atmos ancing means with those of conventional force feedback phere, meter 54 will indicate gauge pressure or vacuum. Systeins, where the balance pressure depends on the much The meter will indicate absolute pressure if one of the larger cross-sectional area of an elastic member. chambers is evacuated. An AC source as well as a DC source can be utilized As shown in FIG. 1 the space between the liquid and for the electrical current if the magnetic field is created diaphragm in chamber 20 is charged with gas. It is to by a coil energized by a constant voltage at the same be understood that the diaphragm is slack and that the 0 frequency.

gas volume varies so that its pressure is always that of The structure for an electromagnetic driver shown in the fluid in the chamber separated by the diaphragm. FIGS. 2 and 3 has advantages in other applications be Alternatively, the space can be filled with a non-conduc sides pressure measurement. It can be used as a calibrated tive liquid of lower specific gravity than the conductive pressure source for test purposes. It can also be used in liquid, in which event there will be insignificant volume a modified circuit of FIG. 1 as a pressure switch. The change. Also the conductive liquid can be made to entirely amplifier and integrator are removed, and the bias force fill chamber 20 and closure made between movable con tacts on the slack diaphragm and fixed contact in for switching the contacts is adjusted by varying electrical chamber 22. It is to be understood that pressure sensing current to driver 14. This device can be used as a high members 0 and 12 are only illustrative of means to con current relay.

tain the volume of conductive fluid within the system and The pressure measuring system shown schematically in FIG. 4 is advantageous for flow measurements as it in to apply an unknown pressure to the line. Both pressure dicates sensing members can include chambers sealed by flexible the square root of applied pressure. The system diaphragms, of which chamber 20 in sensing member 10 2 includes two pressure sensing members 96 and 98 and an is an example. Or the chambers can be replaced by bel 5 electromagnetic of a housing driver 100. The first member 96 consists 102, with an internal cavity 104, a bellows lows. If the conductive fluid is a liquid, both pressure 186, sealed from the internal cavity, with one end fixed sensing members can be constructed as is member 12, to the housing, a strain gage 108 with one end fixed to with a single chamber 34 to act as a reservoir, contain the housing and the other end attached to the movable ing the conductive fluid in the line. Here the unknown fluid pressure is applied directly to the conductive liquid 30 end of the belows, a tube 110 leading unknown fluid pressure into cavity 104 and another tube 112 leading in the chamber. conductive fluid to the interior of the bellows from the It is to be understood that all elements shown merely electromagnetic driver. Pressure sensing member 98 simi illustrate the function performed. Amplifier 50, for ex larly consists of housing 114 with cavity 18, bellows 116, ample, could be a relay; and integrator 52 could be a Straing gage 128 and tubes 122 and 124. Strain gages 108 motor-driven potentiometer.

FIGS. 2 and 3 show details of construction of a pre and 120 are connected into a bridge circuit with resistors ferred form of electromagnetic driver 4 in FiG. 1. The The electromagnetic driver consists of an electromag driver includes a magnet 56 having an axis with two co net 40 with a field coil 142 and poles 144 and 146. A axial poles, 58 with an external cylindrical face, and 60 40 channel 148 leads conductive fluid past the poles with with an internal cylindrical face, and an annular air gap a flow path perpendicular to the direction of magnetic 62 between the pole faces. A ribbon element 64 is spirally fiuX. Electrodes 50 and 152 apply electric current to placed about the axis in the air gap, providing a continu the conductive fluid in a direction perpendicular to both ous channel for conductive liquid in the air gap. Elec the flow path and the direction of flux, thus applying trically insulating cover 56 seals one end of the air gap and insulating cover 68 seals the other end of the air gap. 45 pressure

Leads force to the conductive fluid along the flow path.

154 and 56 apply the bridge imbalance voltage

A continuous passage for conductive liquid between inlet to amplifier 158, which applies the same electric current tube 44 and outlet tube 28 is provided by spiral channel to the field coil through lead 162 and thence to the elec 70, formed between adjacent layers of element 64 and trode, which are series connected to the field coil through connecting with channel 72 in cover 68 and channel 74 in lead i64. Ammeter 160 measures the current and can be cover 66. The ribbon element is an assembly of a nega 50 calibrated in units of flow or the square root of pressure. tive electrical conductor 76 and a positive electrical con This is so because the balance pressure is proportional ductor 78 separated by an electrical insulator 80. Pole 58 to the product of magnetic flux and current. Since the is insulated by layer 82 and pole 60 is insulated by layer magnetic flux is here proportional to the current, the bal 84. Element 64 is bonded to both pole faces to seal the liquid channel between conductors of adjacent turns. The 55 ance and pressure is proportional to the square of the current;

since the unknown pressure equals the balance pres positive terminal of a source of electric current is con Sure, the current measurement indicates square root of nected to the positive conductor 78 of element 64 at one pressure.

end by lead 88 and at the other end by lead 959. The The System of FIG. 4 operates essentially the same as negative terminal of the source is connected to the nega that of FIG. 1 except that the output of the strain gage tive conductor 80 of element 64 at one end by lead 92 and 60 bridge, sensing bellows displacement, is proportional to at the other end by lead 94. applied pressure. Other proportional displacement sensors Assuming that the electrical polarities are as shown in can be used, such as variable capacitors, differential trans FiG. 3, electrical current will flow from the positive con formers and potentiometers. A carbon pile can also be ductor to the negative conductor in a direction parallel to the length of the air gap and perpendicular to the direc 65 used to sense unbalanced pressure force applied by the bellows. Here the carbon pile acts as a variable resistor tion of flux. Force is thereby applied to move the con directly in series with the leads of the electromagnetic ductive fiuid in a spiral direction parallel to the con driver and the amplifier can be eliminated. ductors of element 64. The conductors are connected to One advantage of the invention is that the diaphragm the electrical source at both ends to obtain a uniform distribution of current denstiy. The applied force per unit 70 isandSubstantially bellows sealing members can be very flexible as there no pressure across them in operation.

length of the channel depends only on the flux density This reduces pressure sensing error due to stiffness of and the magnitude of electric current. By making the cross the sealing member to a negligible level, whereas such an Sectional area of the flow channel small, the pressure error is significant in devices where the member must achieved can be very high. This can be done by minimiz ing the width of the air gap, which has ihe beneficial effect 75 seal the fluid from the atmosphere.

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The system herein described can be utilized in many character claimed in claim in which the motion sensing applications requiring detections of changes in fluid pres means includes an electrical switch and the pressure bal Sure. It can be used as a pressure control System, in ancing means includes an integrator. which case a selected value of current is applied to the electromagnetic driver to establish a pressure reference, 5 ter5.claimed

A system for measuring fluid pressure of the charac in claim in which the conductive fluid con and the pressure imbalance signal is fed to a control de taining means at one end of the line is a chamber sealed vice in a direction to restore pressure balance. The system by a flexible diaphragm.

can also be used to measure or control temperature, in 6. A system for measuring fluid pressure of the charac which case the unknown pressure, proportional to tem ter claimed in claim in which the conductive fluid con perature, is supplied from a temperature sensing element, () taining means at one end of the line is a bellows. such as a fluid-charged bulb. The system can also be used 7. A System for measuring fluid pressure of the charac to measure or control liquid level, where the unknown ter claimed in claim i in which the conductive fluid is a pressure is directly related to the level of liquid being liquid and the containing means at one end of the line sensed. is a reservoir.

While the invention has been described in its preferred i 5 8. A system for measuring fluid pressure of the charac embodiments, it is to be understood that the words which ter claimed in claim in which the motion sensing means have been used are words of description rather than of provides an output which is proportional to the motion limitation and that changes within the purview of the of the conductive fluid in the line from a reference. appended claims may be made without departing from 9. A system for measuring fluid pressure of the char the true scope and spirit of the invention in its broader 20 acter claimed in claim 3 in which the motion sensing aspects.

What is claimed is: means includes a strain gage bridge circuit and the bridge 1. A system for measuring fluid pressure including, a imbalance voltage controls the pressure balancing means. volume of electrically conductive fluid in a line having it. A System for measuring fluid pressure including, a pair of pressure sensing elements, each element having two ends, means at each end to contain the conductive 25 an interior cavity divided by a flexible sealing member fluid within the line, means for applying an unknown into two chambers, means to apply pressure to one cham pressure to one end of the line to move the conductive ber of each element, means to connect the other cham fluid therein, means for sensing the motion of the con bers to opposite ends of an electromagnetic driver, said ductive fluid having an output, electromagnetic means responsive to the output of the sensing means to apply 3) chambers, a conductive driver and connecting means being filled with fluid, means to detect change in position a force to the conductive fluid balancing the unknown of the conductive fluid when the applied pressure changes pressure force, and means for measuring the force applied having an output, amplifying means responsive to the out by the electromagnetic means to provide an indication of put of the detecting means to apply an electric current the unknown pressure. to the electromagnetic driver, and a meter to measure 2. A system for measuring fluid presure of the character the current as an indication of the pressure. claimed in claim 1 in which the pressure balancing means 11. A System for measuring fiuid pressure of the char is an electromagnetic driver with a magnet to apply mag acter claimed in claim 10 in which the electromagnetic netic flux in a direction transverse to the line and with driver has a field coil connected in series to the fluid elec electrodes in contact with the conductive fluid to apply an electric current through the fluid in a direction trans 40 trodes and the meter provides an indication of the square Toot of the pressure.

verse to the line and perpendicular to the direction of flux, and the electric current is measured to provide an References Cited indication of the unknown pressure. UNITED STATES PATENTS 3. A system for measuring fluid pressure of the charac ter claimed in claim 2 in which the magnet of the elec 45 2,782,369 2/1957 Werner et al. -------- 73-194 tromagnetic driver is an electromagnet with a coil, and 3,034,002 5/1962 Carlson.

the coil and electrodes are electrically connected so that the same current flows through each. LOUIS R. PRINCE, Primary Examiner. 4. A system for measuring fluid pressure of the D. O. WOODIEL, Assistant Examiner.