Figure F4. Schematic representation of the basic principles for the Sacks-Evertson strainmeter. The strainmeter is connected to the rock by grout so that the resilient body of the instrument is compressed and can then accurately follow subsequent changes in deformation of the surrounding rock. In normal operation the valve (V) is closed. As the sensing volume changes due to earth deformation, the top of the bellows (B) moves up or down. Since the ratio of sensing volume to bellows area is very large, the instrument employs high hydraulic amplification, which gives it extreme sensitivity. The core of a differential transformer (DT) is connected to the top of the bellows with the body (coils) of the DT fixed relative to the top of the sensing volume. A valve can be opened and closed by the control electronics so that the sensing system is maintained near the neutral position. When the valve is opened, oil can flow between the sensing volume and the reservoir (R). An expansion space above the reservoir is provided so that the overall pressure in the oil does not change significantly as the instrument is deformed.