| 摘要 |
1484364 Measuring pressure electrically SIEMENS AG 17 Oct 1974 [17 Oct 1973] 45108/74 Heading G1N A transducer responsive to pressure in a first liquid comprises a measuring cell and a connecting tube having one end opening into the cell and the other end open for immersion in the first liquid; means for introducing a second liquid into the whole of the connecting tube and part of the cell so as to trap gas in the cell; and means for generating an electrical signal representing the volume of trapped gas. A cylinder 1 communicates with one end of a connecting tube 9, e.g. a catheter, which, with the end of a filling tube 2, defines a measuring cell 4 within the cylinder 1. A capillary 5, extending from the filling tube 2 along most of the axis of the cell 4, allows the connecting tube 9 and part of the cell 4 to be occupied by liquid, e.g. physiological saline solution, admitted via a valve 10. When the free end of the connecting tube 9 is immersed in a specimen liquid, e.g. by insertion in a blood vessel, hydrostatic pressure compresses the gas 8 trapped in the cell 4 and increases the conductance between two platinum wires 3 extending along the radial gap between the cylinder 1 and the tubes 2 and 9 and sealed into the assembly with epoxy resin 6. A 25 KHz bridge 11-15 supplying 100 mV provides measurement and a display on a control signal. Non-linearity inherent in the response of the cell and bridge may be corrected by analogue or digital means. Compensation for thermal variations in the trapped air volume 8 is conveniently inherent in the thermal variation in the electrical conductivity of the saline 7. For safety the transducer is connected to an isolating amplifier 14. In another embodiment the wires 3 are embedded in an insulative cylinder shown in broken lines at 16, the liquid 7 then preferably having a high dielectric constant and low conductivity. The dielectric constant of the liquid 7 may be chosen to compensate for thermal expansion of the air volume 8. The embodiment has an outer diameter of 1À3 mm and the length of the capillary 5 is 1À9 mm. This arrangement is found to have, in response to fluctuating pressure, a resonant frequency of 95 Hz and a damping factor of 0À5. |
