What is the most common type of true direct level measurement indicator?

The most common type of true direct level measurement indicator is a sight glass. A sight glass provides a visual indication of the level of liquid within a tank or vessel by allowing operators to see the liquid directly. This type of measurement is straightforward and does not rely on complex electronics or calculations, making it easy to read and interpret.

Sight glasses are effective in many applications, especially in situations where a clear view of the liquid level is essential for safe operations. They also help in quickly diagnosing any issues with the level without needing external power sources or calibration, thereby enhancing operational safety and reliability.

Other options, while they are used for level measurement, either involve more complicated sensor technology or do not provide a direct visual indication of liquid level.