Abstract:
A redundant level measuring system comprises a chamber for fluidic coupling to a process vessel whereby material level in the vessel equalizes with material level in the chamber. A float including a magnet in the chamber rises and falls with material level in the chamber. A magnet actuated visual indicator is mounted to the chamber for indicating level of the magnet in the chamber. A measurement instrument includes an antenna and a measurement circuit. The instrument is mounted atop the chamber with the antenna extending downwardly into the chamber. The measurement circuit measures time of flight of a through air signal representing level of the material in the chamber. A shield in the chamber isolates the float from the antenna.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority of provisional application No. 61/372,149 filed Aug. 10, 2010. 
     
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable. 
       MICROFICHE/COPYRIGHT REFERENCE 
       [0003]    Not Applicable. 
       FIELD OF THE INVENTION 
       [0004]    This invention relates to a level measuring system and, more particularly, to a redundant level measuring system. 
       BACKGROUND 
       [0005]    Numerous technologies exist for measuring level of liquids or solids in an industrial process environment. Among these are transmitters which measure the level and transmit a signal representing actual level. The techniques for measuring level include through air radar, guided wave radar, magnetostrictive, capacitance and the like. 
         [0006]    A magnetic level indicator is another type of commonly used level sensing device. A magnetic level indicator, also known as a flipper gauge, is constructed of a chamber, a float and a visual indicator. The chamber, also known as a cage, is essentially a pipe or similar device external to a process tank or vessel which is usually mounted vertically and which is usually connected to the tank through two or more horizontal pipes. One of the horizontal pipes is near the bottom of the chamber and the other is near the top of the cage. This arrangement allows the material level in the chamber to equalize with the material level in the tank, largely isolating the cage from agitation, mixing or other activities in the tank. The chamber, which is usually a pressure vessel, can be isolated from the tank using valves. The float is sized and weighted for the specific gravity and pressure of the application and contain magnets which actuate a visual indicator on the outside of the chamber to indicate level. 
         [0007]    In certain applications it is desirable to transmit a level signal to a remote device in addition to the local visual indication of a magnetic level indicator. Currently, magnetic level indicators are used with magnetostrictive transmitters or with a series of reed switches, either of which provides an indication of continuous level which is redundant to the primary visual indication provided by the magnetic level indicator. Both the magnetostrictive and reed switch sensors are located on and external to the chamber and are actuated by the magnet placed inside the float in the chamber. A significant drawback to these redundant systems is that the float may fail, in which case both the primary visual and secondary transmitter signals are lost. 
         [0008]    The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner. 
       SUMMARY 
       [0009]    In accordance with the invention, a redundant level measuring system includes a through air measurement instrument with an antenna mounted in the chamber. 
         [0010]    Broadly, there is disclosed herein a redundant level measuring system comprising a chamber for fluidic coupling to a process vessel whereby material level in the vessel equalizes with material level in the chamber. A float including a magnet in the chamber rises and falls with material level in the chamber. A magnet actuated visual indicator is mounted to the chamber for indicating level of the magnet in the chamber. A measurement instrument includes an antenna and a measurement circuit. The instrument is mounted atop the chamber with the antenna extending downwardly into the chamber. The measurement circuit measures time of flight of a through air signal representing level of the material in the chamber. A shield in the chamber isolates the float from the antenna. 
         [0011]    In accordance with one aspect of the invention, a redundant level measuring system comprises a chamber for fluid coupling to a process vessel whereby material level in the vessel equalizes with material level in the chamber. An elongate partition in the chamber defines a float space and an open space. A float including a magnet is in the chamber float space for rising and failing with material level in the chamber. A magnet actuated visual indicator is mounted to the chamber for indicating level of the magnet in the chamber. A through air measurement instrument includes an antenna and a measurement circuit. The instrument is mounted atop the chamber with the antenna directed downwardly to the chamber open space. The measurement circuit generates and receives a frequency signal using a radiating element supported proximate the antenna. The measurement circuit measures level of the material in the chamber. 
         [0012]    It is a feature of the invention that a body supports the radiating element in the housing proximate the antenna for rotation at any angular orientation. 
         [0013]    It is another feature of the invention that the body comprises a substrate rotatably mounted in the housing so that the radiating element can be independently oriented relative to a process vessel. The substrate may comprise a circuit board. 
         [0014]    It is a further feature of the invention that a feed bearing is attached to the body operatively engaging a feed end of the antenna so that the body is free to rotate in the housing guided by the antenna. 
         [0015]    It is a further feature of the invention that the partition physically isolates the open space from the float. 
         [0016]    It is yet another feature of the invention that the measurement instrument comprises a micropower impulse radar instrument. 
         [0017]    It is still another feature of the invention that the antenna comprises an antenna horn. 
         [0018]    It is still a further feature of the invention that the antenna comprises a dielectric rod antenna and the radiating element comprises a loop launcher. 
         [0019]    It is still another feature of the invention that the radiating element comprises an oscillator circuit including micro strip resonators. 
         [0020]    There is disclosed in accordance with a further aspect of the invention a redundant level measuring system comprising a chamber for fluidic coupling to a process vessel whereby material level in the vessel equalizes with material level in the chamber. An elongate partition in the chamber defines a float space and an open space. A float includes a magnet in the chamber float space for rising and falling with material level in the chamber. A magnet actuated visual indicator is mounted to the chamber for indicating level of the magnet in the chamber. A through air radar measurement instrument includes a housing. An antenna is secured to the housing. A process adaptor is associated with the antenna and the housing for securing the instrument to the chamber to define a process seal and with the antenna directed to the chamber open space. The control in the housing generates or receives a high frequency signal. The control comprises an electromagnetic radiating element. A body supports the radiating element proximate the antenna for rotation at any angular orientation without effecting the process seal. 
         [0021]    Other features and advantages will be apparent from a review of the entire specification, including the appended claims and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is an elevation view of an exemplary redundant level measuring system in accordance with the invention mounted to a process vessel; 
           [0023]      FIG. 2  is a perspective view of a redundant level measuring system in accordance with a first embodiment of the invention; 
           [0024]      FIG. 3  is a front elevation view of the measuring system of  FIG. 2 ; 
           [0025]      FIG. 4  is a side elevation view of the measuring system of  FIG. 2 ; 
           [0026]      FIG. 5  is an elevation view of a partition of the measuring system of  FIG. 2 ; 
           [0027]      FIG. 6  is a section view taken along the line  6 - 6  of  FIG. 3 ; 
           [0028]      FIG. 7  is a sectional view taken along the line  7 - 7  of  FIG. 4 ; 
           [0029]      FIG. 8  is an elevation view of the through air measurement instrument for the redundant level measuring system of  FIG. 2 ; 
           [0030]      FIG. 9  is a sectional view of the through air measurement instrument of  FIG. 8 ; 
           [0031]      FIG. 10  is an elevation view of an alternative through air measurement instrument for a redundant level measuring system according to the invention; 
           [0032]      FIG. 11  is an elevation view of a further alternative through air measurement instrument for a redundant level measuring system according to the invention; and 
           [0033]      FIGS. 12 and 13  illustrate orientation patterns for electrical and magnetic fields. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    An exemplary redundant level measuring system  20  in accordance with the invention is shown in  FIG. 1 . The redundant level measuring system  20  is used for providing redundant level measurement of a tank or vessel  22  having a material  24 , the level of which is to be sensed. The level measuring system includes a chamber  26  for fluidic coupling to the vessel  22  via a first horizontal pipe  28  near the top of the vessel  22  and a second horizontal pipe  30  near the bottom of the vessel  22 . The vessel  22  can be isolated from the chamber  26  using valves  32  in each of the top pipe  28  and the bottom pipe  30 . 
         [0035]    Referring to  FIGS. 2-4 , the chamber  26  comprises an elongate pipe  34  having a top flange  36  and a bottom flange  38  to define an interior space  40 , see  FIGS. 6 and 7 . A bottom plate  42  is secured to the bottom flange  38  to close a bottom end of the interior space  40 . A top plate  44  is secured to the top flange  36  to close the top of the interior space  40 . The described arrangement allows the material level in the vessel  22  to equalize with the level in the chamber  26  while largely isolating the chamber  26  from agitation, mixing or other activities in the vessel  22 . 
         [0036]    In accordance with the invention, the redundant level measuring system  20  comprises a magnetic level indicator  46  and a level transmitter  48 . 
         [0037]    Referring to  FIG. 5 , a partition  50  comprises a formed metal plate having a center wall  52  connected to opposite side walls  54  and  56 . The partition  50  has a length corresponding to length of the pipe  34 . The partition  50  is received in the chamber interior space  40 , see  FIGS. 6 and 7 , to define a float space  58  on one side of the partition  50  and an open space  60  on an opposite side of the partition  50 . 
         [0038]    The magnetic level indicator  46  includes a float  62 , see  FIG. 7 , in the chamber float space  58 , and an external visual indicator  64 . The float  62  rides up and down in the chamber  26  with the surface of the material  24 . The float  62  is typically hollow so that it rides freely on the surface of the material  24 . The float  62  may be made of stainless steel or the like and houses a magnet  66  adapted to be positioned at the surface of the material  24 . As such, the float  62  is also referred to as a magnetic float. The float  62  is sized and weighted for the specific gravity and pressure of the application. The visual indicator  64  is strapped to the chamber  26  and is totally isolated from the process material  24 . The visual indicator  64  includes rotating flags  68 . Each flag  68  contains an alignment magnet which reacts to the float magnet  68  and protects against false actuation. With raising level, the flags  68  rotate, changing color. The flags  68  are positioned alongside graduated markings  70  on the visual indicator  64  to indicate level of a material  24 . 
         [0039]    The transmitter  48  comprises a through air radar measurement instrument. Such a transmitter may be as generally described in Gard, Ser. No. 12/321,959, filed Jan. 27, 2009, owned by the assignee of the present application, the specification of which is hereby incorporated by reference herein. 
         [0040]    As described therein, the transmitter uses micro power impulse radar (MIR) in conjunction with equivalent time sampling (ETS) and ultra-wideband (UWB) transceivers for measuring a level using time domain reflectometry (TDR). Particularly, the instrument uses through air radar for sensing level. While the embodiments described herein relate to an MIR level sensing apparatus, various aspects of the invention may be used with other types of process control instruments for measuring various process parameters, such as a rangefinder, as will be apparent to those skilled in the art. 
         [0041]    The instrument  48  uses pulse-burst radar technology with ETS circuitry. Short bursts of microwave energy are emitted and subsequently reflected from a surface. The distance is calculated by the equation 
         [0000]        D =(velocity of EM propagation)*transit time(round trip)/2. 
         [0042]    Level is then calculated by applying a tank height value. ETS is used to measure the high speed, low power electromagnetic (EM) energy. The high speed EM energy (1,000 ft/Φs) is difficult to measure over short distances and at the resolutions required in the process control industry. ETS captures the EM signals in real time (nanoseconds) and reconstructs them in equivalent time (milliseconds), which is much easier to measure. ETS is accomplished by scanning the vessel to collect thousands of samples. The round trip event on a 65 ft. tank takes only 133 nanoseconds in real time. After it is reconstructed in equivalent time it measures 200 milliseconds. 
         [0043]    The radar signal produced by the instrument  48  can interact with the magnetic float. In accordance with the invention, the partition  50  is provided in the chamber for isolating the magnetic float from the antenna. 
         [0044]    Referring particularly to  FIGS. 8 and 9 , the transmitter  48  includes a housing  80 . The housing  80  comprises a base  82  and a cover  84  threaded to the base  82  to define an enclosed space  86 . The cover  84  has a top opening  88  closed by a glass plate  90 . A bottom of the base  82  includes a downwardly depending neck  92 . The neck  92  is receivable in a process adaptor  94 . The process adapter  94  includes threads  95  and is generally cylindrical and connects to an antenna housing  96  narrowing downwardly from the housing  80  to a conical closed end  98 . In the illustrated embodiment, the process adapter  94  and antenna housing  96  comprise a unitary structure. 
         [0045]    An antenna  100  comprises an antenna horn  102  embedded in potting compound  104  within the antenna housing  96  and process adapter  94 . The potting compound  104  also serves to permanently and rigidly attach the process adaptor  94  to the housing base  82 . The antenna horn  102  includes an upper feed end  106  that projects into the housing space  86 . 
         [0046]    A control  110  in the housing space  86  generates or receives a high frequency signal, as described below. The control  110  comprises a mounting bracket  112  fixedly secured to the base  82  in any known manner. First and second main circuit boards  114  and  116  are fixedly secured to the bracket  112  using fasteners  118  and  120 , respectively. A bezel  122  is secured in the housing. A rotating RF transceiver circuit board  124  is rotationally mounted relative to the fixed mounting bracket  112 , and thus antenna horn  102 , and is electrically connected to the first main circuit board  118  by a flexible cable  126 . 
         [0047]    An antenna feed bearing  128  extends downwardly from the transceiver circuit board  124  and is sized to be telescopically received in the antenna horn feed end  106 . The feed bearing  128 , and thus the transceiver circuit board  124 , are free to rotate about a vertical axis, guided by the antenna horn  102   
         [0048]    The main circuit boards  114  and  116  include electrical circuitry for supplying power to the control  110 , and a control circuit to provide measurement functions, display control, configuration, general operation and the like for sensing level and interfacing with other peripherals and control equipment, as is well known to those skilled in the art. The particular circuitry does not form part of the present invention and is not described in detail herein. 
         [0049]    The transceiver circuit board  124  contains the necessary circuitry to produce a microwave signal, transmit the signal to a liquid or other surface, receive and process the radar return signal into a so-called  A video@ wave form from which the locations of the radar echoes can be determined. In the illustrated embodiment of the invention, the main circuitry generates and sends to the transceiver circuit board  124  control signals such as time variable gain, window, run/stop and end of ramp that control the radar scanning process. 
         [0050]    As described in the &#39;959 application, a main oscillator circuit generates a square wave. The main oscillator is a crystal controlled oscillator with a typical frequency in the 3-5 MHz range. The frequency may be on the order of 3.6864 MHz. A divide-by-two circuit produces a pulse repetition frequency of 1.8432 MHz. This is done to reduce the power requirement in delay locked loop (DLL) logic gates, A DLL timing circuit generates a precise, controlled timing delay between two logic transitions, referred to as transmit, or TX, pulse and receive, or RX, pulse. The transitions are on the order of 0 to approximately 100 nanoseconds, according to the value of a ramp signal input from a ramp generator. The delay lock loop in conjunction with a sampling detector performs the function of equivalent time sampling on the transmitted RF signal in accordance with previous, well known designs. 
         [0051]    A transmit pulse generator and a receive pulse generator are controlled by the DLL timing circuit and use a single high-frequency switching transistor to generate a very fast, less than one nanosecond, pulse to excite a harmonic oscillator. The TX pulse creates the transmit pulse out the antenna  100  towards the radar target, such as a liquid level surface. The RX pulse is delayed from the TX pulse by an amount determined by the ramp generator and the DLL timing circuit. The purpose of the RX pulse is to gate the sampling detector and listen for TX pulses which have been reflected by a distant target, and have returned to the antenna  100  after a delay dependent on the target distance. 
         [0052]    The harmonic oscillator represents a Colpitts oscillator comprised of a high frequency HJFET and tuned via microstrip resonators to oscillate at approximately 13 GHz while being rich in second harmonic (26 GHz) content. It is primarily the 26 GHz component that is transmitted. This allows the use of smaller antennas which achieve smaller radiated beam widths. 
         [0053]    The antenna  100  is not electrically connected to the circuit  124 . Instead, the antenna  100  uses an antenna horn  102  that is placed over the oscillator transistor and microstrips of the harmonic oscillator, as generally depicted in  FIG. 9 , whereby the RE energy from the harmonic oscillator is directly coupled into the antenna horn  102  after radiating directly from the circuit elements themselves. As is apparent, a wave guide could also be used with, or instead of, the antenna horn  102 . 
         [0054]    The sampling detector comprises a microwave diode that is placed inside the antenna horn  102  proximate the harmonic oscillator. TX pulses that are reflected by the liquid surface are received by the antenna  100  and conducted to the sampling diode where they are mixed and detected with the delayed RX pulse to perform the equivalent time sampling function. A preamp comprises a fixed gain stage to amplify signal from the sampling detector. A variable gain bandpass amplifier comprises a bandpass amplifier tuned to the frequency of the video or down-converted signal which is a result of the equivalent time sampling process. Also, the amplifier provides a variable gain, as controlled by the signal  A time variable gain@. In radar, since the strength of echoes decreases with increasing distance, a time variable gain circuit is used to increase the gain of the receiver with increasing distance from the transmitter to offset the effect of diminishing radar signal strength with distance. 
         [0055]    Thus, the circuitry on the transceiver circuit board  124  operates to generate the transmission signal and the signal is launched directly from the transceiver circuit board  124 . By rotating the transceiver circuit board  124 , the signal pattern from the harmonic oscillator can be oriented. This allows the assembly of the housing  80  and process adaptor  94  with the antenna  100  to be installed in any rotational direction in the chamber  26  simplifying the installation process. Also, the RF signal is generated on the transceiver circuit board  124  directly into the antenna horn  102  without the need of multiple intermediate components. The bearing  128  allows the transceiver circuit board  124  to rotate relative to the housing  80 . 
         [0056]    In the illustrated embodiment of the invention, an electromagnetic radiating element comprises the transistor and microstrip resonators of the harmonic oscillator. The invention is not limited to such radiating element, but could alternatively use other elements, such as a loop launcher or the like. Moreover, the characteristics of the propagation signal described herein are by way of example only. The invention is not intended to be limited to any particular frequency or wavelength. 
         [0057]    Referring to  FIG. 6 , the top plate  44  includes a circular opening  140  located above the chamber open space  60 . A collar  142  is secured atop the top plate  44  surrounds the opening  140 . The collar includes internal threads  144  for threadable receiving the process adaptor threads  95 . As such, the antenna  100  is directed downwardly to the chamber open space  60 , as shown in  FIG. 7 . 
         [0058]    The partition  50  physically isolates the open space  60  from the float space  62 . This prevents the instrument  48  from sensing the float  62 . Instead, the instrument  48  senses the material level in a conventional manner. 
         [0059]    Radar level devices exhibit predictable results when used in standard, circular wave guides. Energy transmitted from the antenna couples nicely to the interior of the wave guide and propagates smoothly until encountering an impedance mismatch created by a medium with a significantly different dielectric. Measurement is complicated for non-contact radar due to the use of the internal partition  50  resulting in a non-standard wave guide as shown in  FIG. 7 . This irregular shape presents challenges to propagating microwave energy with respect to proper polar alignment, propagation velocity and propagation notes. The propagation of a microwave signal involves the transmission of the electrical (“E”) and magnetic (“H”) fields. In practice, a receiver will tend to respond to radar targets that lie in the E field plane more than the H field plane, which are perpendicular to each other. Proper alignment of the E and H fields optimize the application by allowing optimum response to desired targets and minimal response to false targets. By using the rotating transceiver circuit board  124 , the ability to properly align the H field is enhanced. This allows polar alignment while leaving the antenna  100  and the transmitter housing  80  in place and unchanged in their orientation. Proper alignment is achieved when the radar signal propagates cleanly and uniformly inside the open space  60  and does not produce cancellations, ghosts, pulse distortions, etc., which can all be side effects of incorrect signal propagation in the wave guide. As is apparent, the particular orientation will depend on the size and shape of the wave guide. 
         [0060]    The radiation pattern of a typical tank level radar includes the so-called ‘E’ (electric) and “H” (magnetic) fields. These fields are oriented at a right angle (90°) to each other. However, the transmitter&#39;s detector (receiver) responds to the electric (E field) component of the signal. 
         [0061]    When the radar scene is not symmetrical (uniform in all directions), the orientation of the E field relative to the radar scene gives rise to the “polarization” effect; that is, the radar&#39;s detected signal will vary as a function of the orientation (polarization) of the E field relative to the object(s) in the radar&#39;s beam. In the case of a radar transmitter installed in the chamber  26 , one can see that the chamber  26  is not symmetrical. In the top view shown in  FIG. 12 , if the transmitter  48  was oriented so that its E field is perpendicular to the walls  34  of the chamber  26  and the partition center wall  52 , there will be an undesirable “multipath” effect as a result of the E field having two direct paths from the radar transmitter, to the liquid surface and back. The first is the direct reflection from the transmitter  48 , to the surface and back. The second would be from the transmitter, off either the side wall  38  or the partition center wall  52  to the surface, and then back to the transmitter  48 . Because microwaves of this frequency have wavelengths of only a couple inches or less, there will be strong cancellation effects at certain liquid levels due to the destructive effect of the delayed multipath signal subtracting from the direct reflected signal. The result would be “nulls” or signal dropouts at several levels in the chamber  26 . 
         [0062]    In the view shown in  FIG. 13 , the transceiver circuit board  124  has been rotated no that the E field is no longer perpendicular to any surfaces that are orthogonal to the beam. When the E field is positioned as shown in  FIG. 13 , the beam will “deflect”, or scatter, off the angled surfaces of the chamber  26 . As the beam scatters its reflected amplitude to the transmitter  48  is greatly attenuated. In this case the strong multipath interferences in the first case have been avoided, and nulls in the signal strength will not be observed. The signal amplitude reflected from the surface will be much more uniform over the length of the chamber  26  and will not be subject to interferences and dropouts. 
         [0063]      FIG. 10  illustrates another through air radar level transmitter  148  which can be used as an alternative to that shown in  FIG. 9 . Such a transmitter may be as generally described in Janitch et al. U.S. Pat. No. 7,106,248, owned by the assignee of the present application, the specification of which is hereby incorporated by reference herein. The transmitter  148  includes a control housing  150 , an antenna  152  and a universal connector  154  for connecting the antenna  152  to the housing  148 . The antenna  152  comprises a dielectric rod  156 . A loop launcher  158  is located in the connector  154 . The universal connector  154  allows for the loop launcher  158  to be rotated independently of the antenna  152  and the housing  150 . Alternatively, the transmitter  148  may use a horn antenna  160 , such as shown in  FIG. 11 . 
         [0064]    As is apparent, other forms of though air transmitters may be used in connection with the redundant level measurement system in accordance with the invention. For example, the transmitter could instead be an ultrasound transmitter including a source and a transducer, as is known. 
         [0065]    As is apparent, the shape of the chamber may be different from that shown. Likewise, the chamber may be connected to the vessel by only one pipe. The vessel may be pressurized or nonpressurized. The present invention is not directed to any particular tank or vessel configuration or chamber configuration. 
         [0066]    Thus, in accordance with the invention, there is provided a redundant level measuring system comprising a through air radar measuring transmitter and a magnetic level indicator. 
         [0067]    It will be appreciated by those skilled in the art that there are many possible modifications to be made to the specific forms of the features and components of the disclosed embodiments while keeping within the spirit of the concepts disclosed herein. Accordingly, no limitations to the specific forms of the embodiments disclosed herein should be read into the claims unless expressly recited in the claims. Although a few embodiments have been described in detail above, other modifications are possible. For example, the logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. Other steps may be provided, or steps may be eliminated, from the described flows, and other components may be added to, or removed from, the described systems. Other embodiments may be within the scope of the following claims.