After releasing the hydrometer in the sample, how much time must be allotted for it to reach equilibrium when conducting an API gravity fuel oil test?

When conducting an API gravity fuel oil test using a hydrometer, it is important to allow sufficient time for the hydrometer to reach equilibrium within the sample. The correct time allotted for this process is 30 seconds. This duration ensures that the hydrometer accurately represents the specific gravity of the fuel oil sample, allowing for a reliable measurement of its API gravity.

This 30-second waiting period is essential because it allows the hydrometer to settle and adjust to the density of the oil, providing a stable and accurate reading of the fuel oil's specific gravity. If the hydrometer is read too soon, before it has settled, the result may be inaccurate, leading to potential errors in calculations or assessments.

In the context of this question, the other time options do not provide the necessary period for proper equilibrium. Waiting for either a shorter or longer duration could compromise the accuracy of the measurement. Thus, 30 seconds is the validated timeframe that ensures the accuracy and reliability of the test results.