Abstract:
A vibration attenuator, for an ultrasonic transducer having a transducer shaft, having: a compressible sleeve mountable on said shaft wherein said sleeve comprises vibration attenuating material; a housing for said sleeve mountable on the shaft, and a compression device attachable to the housing and for compressing the sleeve, wherein the sleeve snugly abuts against said shaft when compressed.

Description:
BACKGROUND OF THE INVENTION 
   Ultrasonic flow meters typically have a pair of transducers which makes a direct contact with the process fluid. This type of flow meter configuration is referred to as “wetted” application. Each transducer is mounted on a transducer holder which is attached to a nozzle or port of a pipe or other flow channel (collectively referred to here as a “flow cell”). Flow meters transmit ultrasonic pulses into the flow cell and through the flow stream passing through the flow cell. The flow stream affects the transmitted pulses by, for example, altering the travel time of the pulses between a transmitter and receiver. By measuring the effect on the received pulses the flow meters can determine the flow rate of the stream. 
   Ultrasonic transit time flow meters generally have dual transducers that both emit and receive ultrasonic signal pulses. The dual transducers include an upstream transducer and a downstream transducer which are positioned such that the ultrasonic signal pulse or beam is at an inclined angle with respect to the axis of the flow cell. The upstream transducer emits an ultrasonic signal pulse that propagates through the flow cell and flow stream in a generally downstream direction. The signal pulse is received by the downstream transducer. In addition, the downstream transducer emits an ultrasonic pulse in an upstream direction that is received by the upstream transducer. The flow meter determines the difference in travel time between the ultrasonic pulses passing in a downstream direction and those passing in an upstream direction. The difference in pulse travel time can be used to determine the rate of the flow stream. 
   The transducer receives ultrasonic pulses and generates an electrical signal indicative of when the pulses are received. The transducer may convert to electrical signals only those pulses having a frequency similar to that of the transmitted pulse. The ultrasonic signals received by the receiving transducer includes the pulses that were originally emitted by the transmitting transducer. The received signals also include ultrasonic noise, so-called “acoustic short-circuit” noise, which consists of unwanted non-fluid-borne signals having a frequency within the range of frequency that the transducer detects. Short circuit noise typically arises from vibrations generated by the transmitting transducer and imparted or coupled mechanically to the transducer support and/or flowcell. These vibrations travel from the transmitting transducer by way of transducer shafts, transducer holders, transducer nozzles and the metal wall of the flow cell. These vibrations may have the same frequency as do the ultrasonic pulses that are transmitted through the fluid flow stream. However, these vibrations “short-circuit” the fluid flow by passing through the solid structures associated with the transducer and flow cell wall, and do not pass through the flow stream. Short circuit vibrations that are received by the transducer contribute to electrical signal noise in the measuring system. Short circuit noise interferes with achieving high accuracy and may obscure the data from the received signals from the ultrasonic pulses that pass through the flow stream. 
   To detect fluid flow through a pipe, ultrasonic flow meters typically use acoustic waves or vibrations having a frequency greater than 20 kHz (kilohertz). These flow meters preferably have a high SNR (signal-to-noise ratio) of greater than 20 decibels (dB). A high SNR promotes reliable signal detection, robust performance of the flow meter, and accurate readings of the flow rate by the flow meter. Conventional transit-time ultrasonic flow meters have attainted SNRs of greater than 20 dB for most liquid flow applications and in some high pressure gas flow applications. The SNRs tend to be low, e.g. less than 10 dB, for conventional transit-time ultrasonic flow meters measuring: low pressure gas flows, e.g. atmospheric flows; gases at high flow rates; high temperature gas flows that require transducers tolerant of high temperatures, and gas flows including saturated steam with condensed water. Accordingly, there is a long felt need for transit-time ultrasonic flow meters that have high SNRs when sensing gas flows having low pressures, high gas temperatures, high gas flow rates, and gas flows having saturated steam. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In a first embodiment, the invention is a vibration attenuator for an ultrasonic transducer having a transducer shaft, said attenuator comprising: a compressible sleeve mountable on said shaft wherein said sleeve comprises vibration attenuating material; a housing for said sleeve mountable on said shaft, and a compression device attachable to said housing and for compressing said sleeve, wherein said sleeve snugly abuts against said shaft when compressed. 
   In a second embodiment, the invention is a vibration attenuator for an ultrasonic transducer having a shaft, said dampening device comprising: a cylindrical housing having an internal cylindrical surface, an closed end with an aperture and an open end opposite to the closed end, wherein said internal cylindrical surface has a screw thread adjacent the open end; a plurality of compressible rings arranged in a stack within the internal cylindrical surface of the housing, wherein said rings are coaxial with the housing, and a screw plug having a screw thread to screw into the screw thread of the housing, an end which abuts against the stack of rings in the housing, and a conduit coaxial with the housing and rings, wherein the plug compresses the stack of ring as the plug screws into the housing, wherein the transducer shaft extends through the housing, rings and plug, and the rings fit snugly against the shaft when the rings are compressed in the housing. 
   In a third embodiment, the invention is a method of attenuating vibration in an ultrasonic instrument having an ultrasonic transducer and a transducer shaft, said method comprising: clamping a sleeve of compressible material around the shaft, and attenuating ultrasonic vibrations travelling through the shaft by dampening the vibration with the sleeve. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded view of an dampening mechanism the present invention. 
       FIG. 2  is a perspective view of the embodiment shown in  FIG. 1  in an assembled state. 
       FIG. 3  is side view of a flowcell, shown partially cut-away, having a pair of noise-abated transducers each with a dampening mechanism. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2  show an exploded view and an assembled view, respectively, of a dampening mechanism  10  for mounting as a sleeve on the shaft  28  of an ultrasonic gas transducer  12 . The dampening mechanism includes a stand-alone, cylindrical housing  14 , a stack of damping rings  16  and a plug screw  18  to hold the rings in the housing. The plug screw fits into an open end  20  of the housing, which has an internal thread to receive the plug. 
   The cylindrical housing  14  has cylindrical wall  22 , and a closed end  24 , opposite to the open end, with an aperture  26  to slidably receive the transducer shaft  28 . The interior surface  30  of the annular wall  22  of the housing may be a relatively smooth cylindrical surface, except for the threaded surface adjacent the open end  20  of the housing. The interior wall surface  30  has a constant diameter of about the same as or slightly larger than the diameter of the rings  16 . The cylindrical wall  30  provides a snug fit around the perimeter of the rings  16 , when the rings are compressed in the housing. The cylindrical housing  14  may have a pair of outer flat surfaces  32  to receive a wrench used to tighten the screw plug  18  in the housing  14 . 
   The plug  18  may be a cylinder with a threaded outer surface  33  which mates with the interior threaded surface adjacent the open end  20  of the cylindrical housing  14 . The plug may also have a hexagonal cap  34  which provides a grip for a tightening wrench. The depth to which the plug is screwed into the housing  14  depends on the amount of compression to be applied to the rings  16 . The screw plug  18  has an axial conduit  36  to slidably receive the transducer shaft. 
   A plurality rings  16 , such as seven, are stacked in the cylindrical housing. Each ring is an annulus having an outside diameter approximately the same as the interior diameter of the interior wall  30  of the housing  14 . The rings have an inner circular diameter slightly larger than the outer diameter of the transducer shaft. The rings are deformable and may be formed of a packing material, such as a pre-formed stack of 9000 EVSP Simplified manufactured by Garlock of Palmyra, N.Y. The rings may also be formed of other valve stem packing materials, a rope material helically arranged in the housing, or other deformable materials that provide high frequency vibration absorption and dampening. 
   The dampening mechanism may comprise a stand-alone housing, a threaded plug and a stack of damping rings  16 . The damping rings abate the vibration noise. The rings may be a pre-formed stack of packing material or a valve stem rope material helically arranged or cut and formed in rings  16  in the housing  20 . The rings  16 , when compressed axially inside the housing  20 , expand radially inward and squeeze tightly around the transducer shaft  28 . The vibration noise abatement in the transducer shaft is realized by the squeezing action of the packing material on the transducer shaft. 
   As shown in  FIG. 2 , the dampening mechanism  10  is assembled by stacking the rings  16  in the cylindrical housing  14  such that the interior apertures  38  of the rings are coaxial with the cylindrical housing  14 . The transducer shaft  28  is also coaxial with the housing and extends through the housing, rings and the plug. The shaft  28  is inserted into the housing and through the packing material  16  before the plug is tightened on the housing. 
   As the plug  18  screws into the housing  14 , the end  40  of the plug compresses the rings  16  in the housing. As the rings compress, they expand radially inward and squeeze tightly around the transducer shaft. Packing gland material is conventionally compressed by thirty percent (30%). Applying such compression to the ring stack, the length of the compressed stack of rings is approximately seventy percent (70%) of the uncompressed length of the stack. For example, a ring stack having a one inch (2.54 cm) uncompressed length will be compressed to 0.7 inches (1.8 cm). Accordingly, the length of the screw threads  33  on the plugs should be sufficient, e.g., 0.5 inches (1.25)cm, to apply the desired amount of compression to the ring stack. 
   The dampening mechanism  10  may be a stand-alone sleeve mounted on the transducer shaft  28 . The mechanism may be unconnected to any structure other than the transducer shaft. In a dual transducer flow meter, a dampening mechanism  10  may be mounted on one or both transducer shafts. 
   The snug fit between the rings  16  and transducer shaft  28  allows the rings to dampen the vibration travelling through the shaft. The rings  16  attenuate the vibration travelling from a transmitting transducer, through the transducer shaft  28  and to the receiving transducer. By dampening vibrations in the transducer shaft, the vibrations travelling from the shaft to the walls of the flow cell are reduced. If the dampening mechanism  10  is applied to both shafts of a dual transducer flow meter, then the mechanism  10  on the shaft  28  of the transmitting transducer will attenuate short circuit noise before the noise is imparted to the flow cell wall. The mechanism  10  on a shaft  28  of the receiving transducer will dampen vibrations traveling from the flow cell wall to the receiving transducer. If the dampening mechanism  10  is applied to only one shaft, then it will still attenuate short circuit noise—although less so than if two dampening mechanisms were present 
   The dampening mechanism may be applied to: gas and liquid ultrasonic flowmeters; contrapropagation vibration attenuators; other ultrasonic transducers used for non-destructive testing; acoustic level detection instruments, and other vibration sensitive devices operating in high temperature applications. The components of the dampening mechanism may be tolerant of high temperatures, such as 500° F. 
     FIG. 3  shows a flowcell  42  having a cylindrical flow path  44  which is internal to the cell and has inlet and outlet ports  46  at opposite ends of the path of the flow cell. The flow cell has a pair of transducer nozzle mounts  48  on opposite sides of the path  44 . The nozzle mounts are arranged at an angle to the path so that an upstream transducer  50  is upstream of a downstream transducer  52 . The upstream transducer is shown mounted in the nozzle mount  48 . The downstream tranducer is shown in an exploded view, to better illustrate the transducer  12 , dampening mechanism  10  and shaft  28 , which are held in the nozzle by a transducer holder  54  which attaches as an end cap to an end of the nozzle mount  48 . A gasket  56  provides a seal between the transducer holder and the nozzle mount. 
   The dampening mechanism  10  is externally mounted on the shaft of the transmitting transducer to abate vibration noise propagating on the transducer shaft from the transducer head  12  to the transducer holder  54 . Similarly, the dampening mechanism  10  is externally mounted on the shaft of the receiving transducer to abate the vibration noise propagating on the transducer shaft from the transducer holder to the transducer head. 
   The dampening mechanism  10  is a short-circuit noise abatement device applicable to, for example, a gas ultrasonic transducer  12 . The mechanism  10  includes a stand-alone cylindrical housing  20 , a sleeve of damping rings  16  and a threaded plug  18 , all of which having internal diameters for receiving the transducer shaft. As the threaded plug screws into the cylindrical housing, the sleeve of damping rings  16  is compressed. As the rings compress, they expand radially inward and squeeze tightly around the transducer shaft  28 . As a result, the vibration noise in the transducer shaft is abated. This short-circuit noise abatement device can be applied on the shaft of the transmitting and/or the receiving transducers. For optimum result a dampening mechanism should be applied to both transducer shafts, one on the transmitting transducer and the other on the receiving transducer. 
   A pair of dampening mechanisms  10  were applied to both transducer shafts in a dual transducer transit time ultrasonic flow meter. The presence of the dampening mechanisms increased the SNR of the flowmeter by more than 5.3 dB. In particular, the flow meter with dampening mechanisms  10  had an SNR of 22.5 dB and had an SNR of only 17.2 dB without the mechanisms. The test conditions included a gas flow at atmospheric pressure (0 psig) and at room temperature. Increasing the SNR by 5.3 dB nearly doubled the transducer output voltage ratio of actual signal verses noise. 
   While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.