Which of the following is an example of a direct measurement indicator?

In the context of direct measurement indicators, the sight glass is a clear tube that allows an operator to visually observe the level of liquid within a tank or vessel. This method provides immediate and direct visual confirmation of the liquid level, which is a fundamental characteristic of direct measurements.

Direct measurement indicators, such as sight glasses, are designed to reflect the actual condition or status of a process variable without the need for calibration or interpretation. This makes them very user-friendly, allowing operators to make quick assessments of process conditions. While other options like pressure gauges, bulb thermometers, and flowmeters also provide measurements, they often involve indirect processes or require interpretation of data to understand the primary measurement, making them not as straightforward as a sight glass.