Abstract:
An isolation assembly for connection to a process transmitter and for mitigating high temperature effects of a process fluid includes a process coupling face having an isolation diaphragm configured to contact process fluid. A transmitter coupling has a pressure coupling configured to couple to a pressure port of the process transmitter. A temperature isolation fluid conduit extends between the process coupling face and the transmitter coupling and carries an isolation fluid which couples a pressure applied to the isolation diaphragm to the pressure coupling to minimize high temperature effects of the process fluid on the process transmitter.

Description:
The present application is a Continuation-In-Part and claims the benefit of U.S. patent application Ser. No. 10/876,816, filed Jun. 25, 2004, now U.S. Pat. No. 7,036,381 the contents of which are hereby incorporated by reference in their entirety. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to process control transmitters of the type used in industrial process monitoring and control systems. More specifically, the present invention relates to transmitters which measure process variables in high temperature environments. 
   Process monitoring and control systems are used to monitor and control operation of industrial processes. Industrial processes are used in manufacturing to produce various products such as refined oil, pharmaceuticals, paper, foods, etc. In large scale implementations, these processes must be monitored and controlled in order to operate within the desired parameters. 
   “Transmitter” has become a term which is used to describe the devices which couple to the process equipment and are used to sense a process variable. Example process variables include pressure, temperature, flow, and others. Frequently, a transmitter is located at a remote location (i.e., in the “field”), and transmits the sensed process variable back to a centrally located control room. Various techniques are used for transmitting the process variable including both wired and wireless communications. One common wired communication technique uses what is known as a two wire process control loop in which a single pair of wires is used to both carry information as well as provide power to the transmitter. One well established technique for transmitting information is by controlling the current level through the process control loop between 4 mA and 20 mA. The value of the current within the 4-20 mA range can be mapped to corresponding values of the process variable. Example digital communication protocols include the HART® protocol in which a digital signal is superimposed upon an analog 4-20 mA signal, a FIELDBUS protocol in which only digital communication is employed, Profibus communication protocol, wireless protocol, or others. 
   One type of transmitter is a pressure transmitter. In general, a pressure transmitter is any type of a transmitter which measures a pressure of a fluid of the process. (The term fluid includes both gas and liquids and their combination.) Pressure transmitters can be used to measure pressures directly including differential, absolute or gauge pressures. Further, using known techniques, pressure transmitters can be used to measure flows of the process fluid based upon a pressure differential in the process fluid between two locations. 
   Typically, a pressure transmitter includes a pressure sensor which couples to the pressure of the process fluid through an isolation system. The isolating system can comprise, for example, a isolation diaphragm which is in physical contact with the process fluid and an isolation fill fluid which extends between the isolation diaphragm and the pressure sensor. The fill fluid preferably comprises a substantially incompressible fluid such as an oil. As the process fluid exerts a pressure on the isolation diaphragm, changes in the applied pressure are conveyed across the diaphragm, through the isolation fluid and to the pressure sensor. Such isolation systems prevent the delicate components of the pressure sensor from being directly exposed to the process fluid. 
   In some process environments, the process fluid may experience relatively high temperatures. However, transmitters typically have a maximum operating temperature of 250-300° F. Even in cases where the transmitter can withstand the high temperature, temperature extremes can still cause errors in pressure measurements. In processes which have temperatures which exceed the maximum temperature of the pressure transmitter, the transmitter itself must be positioned remotely from the process fluid and coupled to the process fluid using a long capillary tube. The capillary tube can run many feet and an isolation fluid is carried in tube. One end of the tube mounts to the process through an isolation diaphragm and the other end of the tube couples to the pressure transmitter. This long capillary tube and isolation diaphragm is generally referred to as a “remote seal.” 
   The use of a remote seal configuration increases the cost and complexity of the installation and reduces the accuracy of the pressure measurements. Further, the additional components provide another source of possible failure of the device. 
   SUMMARY 
   An isolation assembly for connection to a process transmitter and for mitigating high temperature effects of process fluid includes a process coupling face having an isolation diaphragm configured to contact process fluid. The isolation assembly further includes a transmitter coupling having a pressure coupling configured to couple to a pressure port of the process transmitter. An isolation fluid conduit extends between the process coupling face and the transmitter coupling and carries an isolation fluid which couples the isolation diaphragm to the pressure coupling. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded view of a prior art transmitter assembly. 
       FIG. 2A  is a side plan view of a pressure transmitter including an isolation assembly in accordance with the present invention. 
       FIG. 2B  is another side plan view of the assembly of  FIG. 2A . 
       FIG. 2C  is a bottom plan view of the isolation assembly of  FIGS. 2A and 2B . 
       FIG. 2D  is a top plan view of the isolation assembly of  FIGS. 2A and 2B . 
       FIG. 3  is a perspective view of a transmitter coupled to a compact orifice plate assembly through an isolation assembly. 
       FIG. 4  is a perspective view of a configuration using flexible conduits. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a partial exploded view of a prior art transmitter isolation assembly  10 . Assembly  10  includes diaphragms  12  and  14  (not shown) and couples to a differential pressure transmitter  20 . Flanges  22  and  24  bolt onto the diaphragms  12  and  14  using bolts  26  and nuts  28 . O-rings  30  are provided to seal the coupling. The flanges  22  and  24  couple to process piping through process connections  40  and  42 , respectively. Pressure transmitter  20  includes two isolation diaphragms (not shown) which generally lie in the same plane. Two conduits (not shown) carried within isolation assembly  10  couple each diaphragm  12  and  14  to one of the isolation diaphragms of the transmitter  20 . In the prior art configuration shown in  FIG. 1 , the two diaphragms  12  and  14  have their faces opposed to each other and lie in different parallel planes. In contrast, the two isolation diaphragms on the pressure transmitter  20  lie in the same plane. Therefore, process couplings which are configured to couple to the isolation diaphragm configuration of transmitter  20 , are incompatible with the opposed isolation diaphragm configuration provided by isolation assembly  10 . 
     FIG. 2A  is a first side plan view,  2 B is a second side plan view,  FIG. 2C  is a bottom plan view and  FIG. 2D  is a top plan view of a process transmitter isolation assembly  52  in accordance with the present invention. In  FIGS. 2A and 2B , the isolation assembly  52  is shown coupled to a differential pressure process transmitter  50 . As with the process transmitter shown in  FIG. 1 , transmitter  50  includes two isolation diaphragms (not shown) which lie in the same plane. Transmitter isolation assembly  52  couples to transmitter  50  through bolts  54  which extend through bolt holes  56 . As shown in  FIG. 2D , isolation assembly  52  includes a process coupling face  60  having a first pressure coupling  62  and a second pressure coupling  64  configured to couple to the isolation diaphragm  61  and  63  of the pressure transmitter. The pressure couplings  62  and  64  can comprise openings in the process coupling face configured to seal against the isolation diaphragms of the transmitter  50 . 
   Opposite the transmitter coupling face  60  is a process coupling face  70  illustrated in  FIG. 2C . The process coupling face  70  includes a first isolation diaphragm  72  and a second isolation diaphragm  74  configured to contact process fluid. Bolt holes  76  are provided for coupling the isolation diaphragm assembly  52  to a mounting assembly. 
   The process coupling face  70  is carried on a process mount  80  and the transmitter coupling face  60  is carried on a transmitter mount  82 . Coupling face  70  is configured to mount to a process coupling flange  71  shown in  FIG. 2A . It is appreciated that extension section  84  is configured to reduce transfer of heat or thermal energy from the process fluid to the transmitter. The specific dimension of extension section  84  depend on the particular parameters (such as temperature) of the process fluid to be measured. An extension section  84  extends between mounts  80  and  82 . Conduits  86  and  88  are shown in phantom. Conduit  86  extends between isolation diaphragm  72  and pressure coupling  62 . Conduit  88  extends between isolation diaphragm  74  and pressure coupling  64 . The conduits  86  and  88  are filled with a isolation fluid such as oil which is substantially incompressible. The fill fluid is introduced into conduits  86  and  88  after isolation assembly  52  is sealed to transmitter  50  through fluid fill ports  96  and  98  shown in  FIG. 2   a . When a process pressure is applied diaphragm  72  or  74 , it is transferred through the fill fluid within conduits  86  and  88  to the respective pressure coupling  62  and  64  and to the isolation diaphragms  61  and  63  of transmitter  50 . 
   In the configuration shown in  FIGS. 2A-2D , the isolation diaphragms  72  and  74  of the pressure transmitter isolation assembly  52  can be arranged to be in a similar configuration, such as lying in the same plane and having the same spacing and dimensions, as the isolation diaphragms of transmitter  50 . With such a configuration, hardware which is configured to mount directly to pressure transmitter  50 , such as a compact orifice plate assembly  118  as shown in  FIG. 3  or a flange  71  as shown in  FIGS. 2A and 2B , can also be used to mount to the process coupling face  70  of isolation assembly  52  without any modifications. The coupling of isolation assembly  52  to transmitter  50  can be through any appropriate technique such as welding or others. It is also appreciated that in one preferred embodiment, the isolation couplings  62 ,  64  lie in a plane parallel to isolation diaphragms  72 ,  74 . 
     FIG. 4  is a side view of another example embodiment of transmitter  50  coupled to flange  71  in a configuration in which flange  71  is spaced apart from the isolation diaphragms  61 - 63  of transmitter  50 . In the configuration of  FIG. 4 , a solid isolation assembly is not required. Instead, the isolation assembly can be formed by two flexible conduits  120  and  122  which extend between a transmitter flange  124  and process coupling flange  71 . In the configuration of  FIG. 4 , the conduits are illustrated as running separately. However, in a similar configuration, conduits  120  and  122  can be carried together. Additionally, the conduits  120  and  122  can be carried in a flexible support structure to provide additional strength. Such a configuration increases the number of mounting configurations because the transmitter  50  does not need to be physically located adjacent the process coupling flange  71 . Such a configuration can also provide increased thermal insulation. 
   The present invention also includes a method of temperature characterization of a pressure transmitter while it is coupled to an isolation assembly such as isolation assembly  52 . In accordance with the method, the assembled transmitter  50  and isolation assembly  52  is subjected to a characterization procedure which is typically used with transmitter  50  alone. In the characterization procedure, various pressures are applied to isolation diaphragm  62  and  64  while the assembled unit (transmitter  50  and isolation assembly  52 ) is exposed to different temperatures. The output of the device is monitored during this characterization process. A comparison can be performed between the actual output and the expected output while the transmitter is subjected to these different temperatures and pressures. A correction formula can be used within the transmitter to correct for the errors introduced due to temperature variations. For example, a polynomial equation can be used and the coefficients of the polynomial can be determined through the characterization process. 
   This method allows the entire assembled unit (transmitter  50  and isolation assembly  52 ) to be characterized across a temperature range. In contrast, in a typical remote seal type configuration, the assembled unit cannot be characterized because of size constraints in the remote seal/transmitter combination and the fact that the remote seals are often assembled and filled at a location other than the transmitter manufacturing factory. Thus, they do not lend themselves to be re-characterized after the remote seal has been installed. 
   During operation, transmitter circuitry  100  provides a pressure related output on two wire process control loop  102  as a function of pressure sensed by pressure sensor  104 , temperature sensed by temperature sensor  106  and compensation coefficients  108 . The compensation coefficient  109  can be determined using the method discussed above and stored in a memory  110 . This provides temperature compensation of the output as a function of the temperature of transmitter  50  and isolation assembly  52 . The compensation coefficients can be, for example, coefficients of a polynomial. 
   The isolation assembly of the present invention can be filled with any type of desired fill fluid. For example, when the assembly is used in a food processing environment, the fill fluid can be of a non-toxic material such as a vegetable oil. As the fill fluid in the isolation assembly does not contact the pressure sensor in transmitter  50 , the fill fluid in isolation assembly  50  is not restricted to having particular electrical characteristics. 
   Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.