Patent GB-961852-A (GB)
Title: Strain transducer
IPC: G01L9/00
Filed: 1961-03-23
Abstract: 961,852. Measuring electrically. ROSEMOUNT ENGINEERING CO. March 23, 1961 [April 14, 1960(2)], No. 10733/61. Heading G1N. [Also in Division H1] A pressure transducer comprises a container to be subjected to pressure together with means for measuring the electrical characteristics of a portion of the wall of the container to determine the stress on the wall and hence the pressure. The characteristic determined may be the change of resistance between spaced terminals on the wall of the container or the voltage developed between a first pair of terminals mounted transversely to the flow of current between a second pair of terminals. The container, or a portion of its wall, may consist of a semi-conductor material, manganin, or a nickel-iron alloy, or a container of a pure semiconductor may have a doped area to which terminals are applied. Fig. 2 shows a container 20 to which pressure is applied through pipe 13, the change in resistance being measured between terminals 24, 25. Alternatively the container may be sealed and mounted inside a second container to which the pressure is applied (Fig. 3, not shown). To compensate for temperature changes two containers may be connected in a bridge circuit (Fig. 4), container 20 being pressurised while container 4 is vented to the ambient atmosphere. Fig. 6 (not shown) is similar to Fig. 4 except that the containers are capillary tubes placed end-to-end, or in a modification (Fig. 7 not shown) coiled together. Alternatively the tubes may be concentric (Fig. 8, not shown) and may then be machined from a cylinder of semi-conductor material (Fig. 9). Further alternatives are similar to the arrangement shown in Fig. 10 in which a hemispherical cup is closed by a disc to form the container. Pressure may be applied through a pipe 214, or the container may be sealed and mounted in a second container. The change in resistance of either the hemisphere or the disc may be measured. A differential arrangement (Fig. 14, not shown) has two concentric hemispherical cups between which pressure is applied. In the arrangement of Fig. 12 (not shown) two discs are sealed on either side of an insulating ring and the resulting container is enclosed in a second container. The change in resistance between the centre and periphery of each disc is measured. Alternatives using semiconductor discs as part of the wall of the container are described, Fig. 15 showing an embodiment in which the edge of the disc is clamped, leading to a reversal of stress across the radius of the disc when the container is pressurised. Three electrodes are attached to the disc and are connected to differentially unbalance a bridge circuit. The disc may be provided with free edge suspension by mounting it between O-rings (Fig. 17, not shown), or providing it with circumferential grooves, in which case only two electrodes are provided as the stress does not change sign along a radius. Measurements along different radii may however be of opposite sign. Fig. 25 shows an arrangement in which a doped region 427 has electrodes 428,429 through which a current is passed and electrodes 431,432 across which a voltage is developed when the disc is stressed. Embodiments utilizing this principle in the tubular and hemispherical types of gauge are also described (Figs. 22 and 26, not shown). The pressure may be indicated either by measuring the output current from a bridge circuit containing the transducer, or by using a manually rebalanced bridge having a calibrated variable resistor.