Final answer:
The sight gauge on a hydraulic reservoir is used to check fluid levels and should be situated so the proper fluid level aligns with the top of the bull's-eye.
Step-by-step explanation:
When reading any instrument like a sight gauge, it is important to ensure that the instrument is level and that your eye is at the level of the top of the fluid. This is due to the meniscus, the curved surface of the liquid.
For accurate measurement, the reading should be taken at the bottom of the meniscus. The sight gauge is typically situated so the proper hydraulic fluid level is at the top of the bull's-eye when the system is at rest or in the recommended operating condition.
A manometer, which is a U-shaped tube partially filled with a fluid like mercury, is used to measure pressure. In the case of an open-tube manometer, the fluid depth on each side must be the same for the pressures to be balanced.