Airlock reverse flow

One of the major concern of any Meadmaker is the oxidation and contamination of their mead during the aging process.

The permanence of airlocks during the fermentation on secondary can bring some problems when fermentation stops and there is the possibility of large room temperature fluctuations.

The transition between hot and cold days, due to the difference in temperature, can cause a reverse in the gas flow in the airlock if the fermentor have a large empty space.

The process is explained by one of the ideal gas laws, the law of Charles and Gay-Lussac.

In 1787, the French physicist Jacques Alexandre Charles Caesar, studied the variations in volume of some gases samples and air caused by temperature variations.
Later, in 1802, the French chemist and physicist Joseph Louis Gay-Lussac, showed that other gases have the same behavior as the gases studied by Charles, that is, a gas sample is heated, keeping the pressure constant, it suffers one volume increase proportional to the increase in temperature.
Gay-Lussac also found that starting from the same value of initial volume, the volume increase was the same for all gases to the same temperature increase.

If the pressure is constant, this relation is expressed mathematically by the equation:

Applying this equation to the airlock operation, we verify than, when the fermentation and CO2 production ended, an increase in temperature causes an increase in the volume of gas inside the fermentor, forcing its way out through the airlock. When the temperature drops, the volume of gas inside the fermentor decreases and forces the incoming of air from the outside into the fermentor.

This can be very disastrous in the case of using a 3 parts airlock instead of a S airlock, because in these cases, the liquid inside the airlock can be pulled inside your fermentor.