What instrument is used to measure the specific gravity of the electrolyte in a lead-acid battery?

What instrument is used to measure the specific gravity of the electrolyte in a lead-acid battery?

• A. Voltmeter
• B. Ammeter
• C. Thermometer
• D. Hydrometer

Answer: (D)

Measuring the electrolyte’s density to gauge the battery’s state of charge relies on a device that directly reads fluid density. A hydrometer does this by floating in the electrolyte; the buoyancy of the float changes with the liquid’s density, and the scale on the device translates that buoyancy into a specific gravity reading. In a lead-acid battery, a higher specific gravity means a higher concentration of sulfuric acid and a higher state of charge, while a lower reading indicates discharge. Temperature can affect density, so readings are often temperature-compensated. The other instruments measure voltage, current, or temperature, not density, so they don’t directly measure specific gravity.

Measuring the electrolyte’s density to gauge the battery’s state of charge relies on a device that directly reads fluid density. A hydrometer does this by floating in the electrolyte; the buoyancy of the float changes with the liquid’s density, and the scale on the device translates that buoyancy into a specific gravity reading. In a lead-acid battery, a higher specific gravity means a higher concentration of sulfuric acid and a higher state of charge, while a lower reading indicates discharge. Temperature can affect density, so readings are often temperature-compensated. The other instruments measure voltage, current, or temperature, not density, so they don’t directly measure specific gravity.