When is a hydrometer most accurate and easiest to read?

A hydrometer measures the specific gravity of liquids, which can vary significantly with temperature. When coolant is near operating temperature, it is generally at the ideal density for measurement, allowing for the most accurate readings. The properties of the liquid are influenced by its temperature; near the operating temperature, the coolant is in a stable state, minimizing fluctuations in density that could lead to inaccurate readings.

At low temperatures, the liquid may become denser, and the hydrometer could indicate a misleading result. When the coolant is fully cooled, its density might continue to change as it stabilizes, which could also affect accuracy. If the coolant is boiling, the vapor and fluctuations in density could lead to erroneous readings, as the liquid state of the coolant is compromised and bubbles can skew the hydrometer's measurement. Thus, measuring at operating temperature maximizes reliability and accuracy.