ALCOHOL TYPE MAGNETIC COMPASS LIQUID.

Alcohol-type compass fluid is a common liquid used to fill the hermetically sealed bowls of some magnetic compasses, particularly older models and certain modern marine compasses from specific manufacturers. The primary function of the liquid is to dampen the movement of the compass card, reducing oscillation and increasing stability and accuracy, especially on a moving vessel or aircraft. This fluid is typically an aqueous solution, meaning it is a mixture of an alcohol and distilled water. The alcohol used is commonly ethyl alcohol (ethanol), often at a concentration of about 43% to 64%, or sometimes isopropyl alcohol. The purpose of the alcohol is to significantly lower the freezing point of the mixture, allowing the compass to operate reliably in low-temperature environments (down to approximately $- 26^{\circ} C$ or lower for a $43%$ solution of ethyl alcohol and water), as required by maritime or aviation regulations. Some modern alcohol-free compass fluids use solutions of ethylene glycol (antifreeze) and water, or various mineral spirit or oil-based liquids like purified kerosene or specialty oils (e.g., Isopar L or M), which are more inert to plastic components and have replaced alcohol-water mixtures in many modern designs due to the potential for alcohol to damage certain plastics or the card itself. It is critical to use the exact type of fluid specified by the compass manufacturer, as mixing liquids or using the wrong type can severely damage the internal components of the compass.

Technical Specification

Property	Typical Value (Ethanol-Water Mixture)	Function
Composition	Ethanol and distilled water; often $40% - 55%$ alcohol concentration by volume.	Alcohol lowers the freezing point; water provides specific gravity and is non-corrosive. Some formulas use other alcohols (like Isopropyl) or additives.
Freezing Point	Around $- 3 0^{\circ} C$ (or $- 2 2^{\circ} F$ ) or lower.	Ensures the compass functions reliably in cold climates without the fluid solidifying. A $43%$ alcohol solution has a freezing point of about $- 2 6^{\circ} C$ ( $- 1 5^{\circ} F$ ).
Specific Gravity (Density)	Around $0.93$ (relative to water at $4^{\circ} C$ ).	Provides buoyancy to the compass card, reducing friction on the pivot and allowing the card to float freely. This value is slightly less than 1.0.
Coefficient of Expansion	Small coefficient of expansion.	Minimizes volume changes with temperature fluctuations, preventing excessive pressure buildup inside the compass bowl (though an expansion chamber is still typically used).
Chemical Compatibility	Non-corrosive and non-discoloring to the compass card and internal components.	Prevents damage and maintains the clarity of the compass card and liquid over time.
Viscosity	Low viscosity (for a fast-dampening, “dead-beat” movement).	Allows the compass card to quickly settle on a reading without excessive, prolonged oscillation (hunting).