| 摘要 |
1,067,779. Measuring pressure electrically. NORTH AMERICAN AVIATION Inc. Aug. 4, 1964 (Oct. 2, 1963], No. 31015/64. Heading G1N. Compressive stress produced in a rod 28, Fig. 1, forming part of a probe 2, by a fluid pressure applied to its end 38 is measured by semi-conductor strain gauges 22 mounted on a portion of the rod having a reduced cross-section, the gauges being arranged in a bridge so as to compensate for temperature dependent errors in the output signals of the gauges. For use in a rocket engine thrust chamber, the probe may be replaceable and be of a material which erodes at the same rate as does the chamber wall to ensure a flush surface at 4, thereby avoiding hot spots, etc. In one embodiment Fig. 3 (not shown) two p-type semi-conductor gauges form one pair of opposite bridge arms, and two n-type form the other pair, the two types giving opposite changes in resistance for a given strain change, and all the gauges having matched thermal resistance coefficients. Further, Fig. 4 (not shown) correction for resistance changes produced by differences in the thermal expansions of the p-type gauges and of the rod is made by having the two p-type gauges in one pair of arms, and two straininsensitive gauges in the other pair. The thermal coefficient of resistance of the insensitive gauges is chosen to produce a resistance change therein which is equal to the difference between the resistance changes produced in the p-type gauges by the said differential expansion and by their own thermal coefficient of resistance. A third embodiment Fig. 5 (not shown) has one p-type gauge in each arm, one pair being mounted parallel to the rod and the other pair being mounted transverse to it, so that the pairs are respectively axially compressed and extended, the gauges all having matched thermal resistive coefficients which therefore cancel, and also having strain-resistance coefficients of the same sign (all being p-type) so that errors due to the above stated difference in thermal expansions also cancel. The bridge supply is a constant current source so that by means of a resistor across it (R sb , Fig. 2, not shown) the sensitivity of the bridge may be adjusted. The transducer assembly may be cooled by ducts 40, 42, etc. |
