Abstract:
A differential pressure sensor for sensing changes in pressure at a desired location, which sensor includes a sensing portion and a reference portion to produce an output indicative of the difference therebetween, both the sensing portion and the reference portion being open to the pressure around the sensor until the sensor is located in the desired sensing location and then the reference portion is closed to capture the pressure then existing at the desired location and any pressure changes thereafter producing signals indicative of the pressure differences.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention is in the field of pressure sensors and, more particularly, differential pressure sensors.  
           [0003]    2. Description of the Prior Art  
           [0004]    In order to measure pressure differences, it has long been common to use an absolute pressure sensor and determine two consecutive readings, the difference therebetween being the pressure differential. It is also common to use a pressure sensor which employs a predetermined reference pressure and detects the difference between the sensed pressure and the reference pressure. Honeywell makes pressure sensors that operate on both principles and are known as PPTR3000AP2VB (absolute pressure sensor) and PPTR3000GP2VB (reference pressure is ambient atmosphere) respectively.  
         SUMMARY OF THE INVENTION  
         [0005]    In some situations where it is very expensive or difficult to place and replace pressure sensors or, where high absolute pressures are encountered and small pressure differences are to be detected, such as, for example, in oil well applications where using a plurality of pressure sensors located at various locations along a pipe may be used to determine the flow rate of the oil being pumped from a deep well, neither type of pressure sensor may be able to provide an accurate and easily usable differential pressure sensor. For example, in the oil-pumping situation, the pressure on the oil at various positions along the pipe may be as high as 25,000 psi, and a change of 1 psi may be considered significant. Providing a sensor that can accurately measure a difference of 1 psi between two consecutive readings, i.e., one part in 25,000, is extremely difficult and, at least, would be very expensive. When using a pressure sensor that operates with respect to a reference pressure the problem is eased because the reference pressure can be pre-set to about the same as the sensed pressure. Thus, a sensor with an accuracy of 1 part in 100 can be used to detect a small psi change in a high absolute pressure environment. While more practical for measuring small pressure differences, use of a reference pressure becomes extremely cumbersome and expensive when they are required to be used at various locations in the pipe, particularly when the absolute pressure to which the sensor is to be subjected is not known, or which varies along the pipe length. Under such conditions, having a pre-set reference pressure source is rather impractical. While trial and error may be used, frequent withdrawing of the sensor from a desired location in the pipe in order to replace the reference pressure source to match the ambient pressure at the desired location and then re-inserting it back to the desired location is prohibitively difficult and costly.  
           [0006]    The present invention avoids the above problems by providing a reference chamber that, like the sensing chamber, is open to the ambient pressure as the sensor is positioned at the desired location. The fluid filling the reference chamber is thus automatically equal to the ambient pressure at that location. Once the reference pressure is established, the reference chamber may then be closed or sealed off from the ambient pressure and the pressure in the reference chamber remains fixed while the sensing chamber continues to sense the ambient pressure. The difference between the sensed pressure and the reference pressure is thereafter measured and small changes (e.g., 1 psi) are easily detected with a far less expensive detector (e.g., one with an ability to accurately measure 1 part in 100). 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 shows a cross-section of a simplified oil pumping arrangement;  
         [0008]    [0008]FIG. 2 shows a cross-section of a differential pressure sensor with a sensing and reference chamber both open to the ambient pressure;  
         [0009]    [0009]FIG. 3 show a cross-section of the differential pressure sensor of FIG. 2 with the reference chamber sealed off from the ambient pressure; and,  
         [0010]    [0010]FIG. 4 shows a schematic representation of a device for controlling the flow in a conduit. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0011]    Referring to FIG. 1, a pump  10  is shown connected to an underground pipe  12  which extends down from the earth surface  14  to a pool of fluid  16 , which will be considered a pool of oil in the following description. The oil is usually mixed with sand or other earth materials. It should be noted that oil can be removed from an underground location by other methods, such as application of pressures, without the use of a pump and the word “pump” herein should be read to include other forms of removal. Also, the invention may be used during the drilling process to detect pressure changes and pipe  12  may be a drill hole. As used herein, “pipe” should be read to include any conduit or orifice including a hole. In any event, pipe  20  may change directions such as at bend  18  and thereafter extend through the oil pool  16  along a pipe (or hole) portion  20 . In some cases, pockets of water such as shown by reference numeral  22  may lie adjacent portions of oil pool  16 . Pump  10  operates to pump oil from pocket  16  through pipes  12  and  20 , and out through an outlet pipe  24  to a down stream receiver (not shown). However, oil exhausted too quickly may cause water to rush into any void created by the oil migration and accordingly, it is extremely important to control the rate of pumping and avoid any water from entering pipes  12  and  20 . Thus, a very accurate measure of flow is needed.  
         [0012]    In order to measure the flow of oil in pipes  12  and  20 , a plurality of pressure sensors  30  are positioned at various positions or locations  31  along pipes  12  and  20 . While a half dozen such sensors are shown in FIG. 1, different numbers of sensors may be used in actual practice. Furthermore, as will be shown, the positions of the sensors may be changed from time to time. The pressure sensors  30  (an example of which will be described in connection with FIGS. 2 and 3) operate to detect pressure changes, i.e., pressure differences at the various locations, and these signals are sent to a processor  32  which is shown in FIG. 1 connected to pump  10  by a line  34  so that, using well-known techniques, the rate of flow or flow field along the pipes  12  and  20  can be determined and used to control the flow rate. It will be understood, that the oil pressure along the pipes may be from around 6000 psia to as high as about 25,000 psia and that it may be desired to detect pressure variations of less than 1 psi to measure flow accurately. It will also be understood that placing a large number of sensors at desired locations is a rather difficult and time-consuming procedure which makes it desirable, in many cases, to use sensors that will not have to be removed and reinstalled.  
         [0013]    [0013]FIGS. 2 and 3 show a cross-section of an example of one of the pressure sensors  30  of the present invention. In the figures, an upper housing  40  and a lower housing  42  are shown providing upper and lower chambers  44  and  46  respectively. A ceramic, silicon, or other deformable material diaphragm  48  is held between the upper housing  40  and lower housing  42  and the shape of chambers  44  and  46  allows material  48  to bend upward and downward as the pressure difference therebetween changes. Sensors such as one or more piezoresistive devices  49  are fixed, etched, or otherwise connected to the surface, or integrated into the diaphragm of material  48 , and may be connected to form a Wheatstone bridge that produces electrical signals indicative of the deformation of the material  48  and thus of the pressure differential between chambers  44  and  46 . These signals may be conducted such as by wires  50 ,  52 ,  54 , and  56  leading out of the pipes  20  and  12  through pump  10  and line  34  to processor  32  in FIG. 1 or, alternately, the signals may be fed to a transmitter  58  as shown by dashed line connection  59  to send sonic, or r-f signals as shown be arrow  60  directly to the processor  32 . In either case, the processor  32  operates on the signals in a well-known manner to determine the pressures being sensed and the flow field involved. Processor  32  may also provide a visual pressure/flow signal, for example at an indicator  58 , and/or may provide control signals to alter the operation of pump  10  via connection  34  in such a way as to control the flow rate through pipes  12  and  20 . Alternate methods for controlling the flow in pipes  12  and  20  may include use of choke devices for causing variation in the cross sectional area of the pipes or sphincter valves controlled by signals from the processor  32  as shown by arrow  61 . One such device will be explained in connection with FIG. 4.  
         [0014]    As mentioned, pipes  12  and  20  could also represent boreholes in an oil field and the ambient portion of the differential pressure could be opened and closed while drilling to give pressure changes over time at various locations. This would provide for taking pressure measurements at different locations. For such applications, a pump is not necessary.  
         [0015]    In FIGS.  2 , and  3 , one or more pipes such as pipes  62  and  63  are shown passing through upper and lower housings  40  and  42  respectively and into chambers  44  and  46  to permit the flow of fluid from the ambient oil therein. Sensors of this general type are well known in the industry and an example of such a sensor is found in the above-mentioned Honeywell sensor PPTR3000GP2VB.  
         [0016]    In the present invention, both pipes  62  and  63  are exposed to the same ambient pressure, P1. Then, after an equilibrium has been reached, one of the pipes (say pipe  62 ) is closed, as, for example, by an on-board means such as a battery pack  64  and a switch  65 , that may be opened and closed by a surface command as, for example, by a wire through conduits  12  and  20 , or by a signal from a control transmitter  66  operable to transmit activation signals shown by arrows  67  in FIGS. 1 and 3. Activation of switch  65  then operates to energize an actuator  68 , which is shown in FIG. 3 as operable to activate a closer such as a valve  69  that blocks pipe  62  and seals the chamber  44  from the ambient, thereby making pressure P1 in chamber  44  fixed so that no further changes in pressure will occur therein. From then on, the pressure changes will effect chamber  46  and not chamber  44  so that the differential pressure between P1 and P1± a small variation x is measured. It will be noted that both chambers are subject to the same temperature and pressure environment and both use the same sensor, thereby significantly reducing many temperature, pressure, and hysteresis errors. Pressure sensors able to withstand very high temperatures may also be employed thereby allowing the use of the present invention in very high temperature environments. If it is desirable to reopen pipe  68  at a later time so that a new reference pressure may be used in chamber  44 , (for example to allow the sensor  30  to be moved to an alternate location) then actuator  68  could be activated by another signal  67  from transmitter  66  to open valve  68 , thus allowing a new reference pressure to enter chamber  44 . While the manner of closing the pipe  60  has been shown with a battery pack  64 , switch  65 , actuator  68 , and valve  69 , any suitable methods may be employed. For example, if reopening of pipe  62  is not needed, then an explosive charge might be activated by the surface transmitter  66  which would deform pipe  60  by bending or crimping it.  
         [0017]    In addition to controlling the pump  10 , FIG. 4 shows another possible way of controlling the flow through pipes  12  and  20 . In FIG. 4, a section of pipe  20 A is shown with a flow shown by arrow  70  going therethrough. A clutch or valve mechanism  72  is shown around pipe  20 A and has associated therewith a closing member  74  extending into pipe  20 A, and movable into and out of the flow as shown by double-ended arrow  76 . A desired flow signal from the processor  32 , as shown by arrow  61 , activates valve mechanism  72  to move members  74  into or out of the flow  70  to thereby change the cross-sectional area of pipe  20 A and thus control the flow. One or more valve mechanisms like  72  may be employed along the lengths of pipes  12  and  20 .  
         [0018]    It is thus seen that we have provided a novel, reliable pressure, and highly accurate differential sensor suitable for use in various difficult situations. Many changes will occur to those having skill in the art. For example, while an oil-pumping situation is used to describe the invention in a preferred environment, other situations such as chemical production plants, food-processing plants, paint mixing and production plants could also find the present invention useful. At least two sensors are need for a determination of flow, but a single sensor could be used for determining variations in fluid level. For example, a sensor such as sensor  30  may be lowered into a container in which the fluid level varies. The sensor could be positioned at a desired depth in the container and chamber  44  sealed off to provide a reference pressure. Then as fluid level changed, the pressure in chamber  46  would change (but not in chamber  44 ) and the output representing the differential pressure would be indicative of the change in fluid level.  
         [0019]    Many changes to the present invention will occur to those skilled in the art. For example, in addition to those mentioned above, other uses for the invention, other ways of transmitting the signals from the sensor to the remote signal processing equipment and from the transmitter or processor to the sensor may be devised and, other methods for sealing pipe  62  may be employed and while single pipes  62  and  63  have been shown for admitting the ambient pressure into chambers  44  and  46 , two or more conduits may be used for each chamber. Accordingly, we do not wish to be limited to the specific structures shown in connection with the preferred embodiments. The following claims define the scope of the present invention.