Differential pressure transmitter

• US 3,999,435 A
• Filed: 01/09/1976
• Issued: 12/28/1976
• Est. Priority Date: 04/24/1975
• Status: Expired due to Term

• Current Assignee: Moore Products Co. (Siemens AG)
• Original Assignee: Fischer & Porter Comapny
• Inventors: Siegel, William A.
• Primary Examiner(s): Woodiel, Donald O.
• Application Number:
  US05/647,811
• Time in Patent Office: 354 Days
• Field of Search: 73/393, 73/406, 73/407 R, 73/205 R, 73/395
• US Class Current: 73/708
• CPC Class Codes: G01F 23/164 using a diaphragm, bellow as transmitting element
• CPC Class Codes: G01L 13/025 using diaphragms
• CPC Class Codes: G01L 19/04 Means for compensating for effects of changes of temperature , i.e. other than electric compensation
• CPC Class Codes: G01L 7/088 correcting or regulating means for flexible diaphragms

Abstract

A differential-pressure transmitter adapted to produce a signal proportional to a process variable such as flow rate in a line conducting fluid or the level of liquid in a tank, the process fluid being at a temperature markedly at variance with ambient temperature. The transmitter is constituted by high and low pressure sensors which are coupled to the force beam of a force balance transducer so that the beam is responsive to the difference between sensed high and low pressures derived from the process variable, the transducer being exposed to ambient temperature. Each sensor is constituted by a housing having a well therein covered by a sensing diaphragm and an insert seated in the well, the face of the insert being spaced from the diaphragm to define a fill chamber containing hydraulic fluid, the sensor diaphragm being exposed to the process liquid, whereby heat transfer takes place between the liquid and the hydraulic fluid. The insert is formed of a metal having a low coefficient of expansion relative to the material of the housing whereby variations in temperature produce changes in the dimensions of the housing relative to that of the insert and give rise to changes in the volume of the fill chamber to an extent compensating for the concurrent changes in the volume of hydraulic fluid as a result of said heat transfer, thereby rendering the sensor insensitive to temperature changes.

Claims

1. A transmitter for generating a signal proportional to a process variable in an application involving a fluid whose temperature is markedly at variance with ambient temperature, the fluid being contained or flowing through a structure having a wall, said transmitter comprising:

A. a force-balance meter having a pivoted force beam to produce an output signal depending on the force applied to said beam to deflect same, said meter being provided with a body having a high-pressure hydraulic fluid chamber covered by a force-measuring diaphragm mechanically linked within a low-pressure hydraulic fluid chamber to the lower end of the force beam;

B. a low-pressure and a high-pressure sensor, each having a housing attachable to said wall, said housing having a well therein covered by a sensing diaphragm whose face is exposed to the fluid through an opening in said wall, and an insert seated in said well, the face of said insert being spaced from the sensing diaphragm to define a fill chamber containing hydraulic fluid, the temperature of said hydraulic fluid being subject to the temperature of said process variable;

C. an extended capillary tube hydraulically coupling the fill chamber of each sensor to said meter, said high-pressure chamber being coupled to the capillary tube leading to the high-pressure sensor, said low-pressure chamber being coupled to the capillary tube leading to the low-pressure sensor, whereby the respective forces produced by the pressures of the process variable on the sensing diaphragms of the low and high pressure sensors are applied differentially to said beam to deflect same, said insert being formed of a metal having a coefficient of expansion that is low relative to the material of said housing, whereby variations in temperature produce changes in the dimension of the housing relative to that of the insert and result in changes in the volume of said fill chamber to an extent compensating for concurrent changes in the volume of the hydraulic fluid resulting from changes in the temperature thereof, thereby rendering the force applied to said beam by each sensor independent of said variations in temperature.

2. A transmitter as set forth in claim 1, wherein said insert is formed of an iron-nickel alloy.

3. A transmitter as set forth in claim 1, wherein said housing is formed of stainless steel.

4. A transmitter as set forth in claim 1, wherein said force balance transducer produces a pneumatic signal that is a function of the process variable.