What is the first sign that the drum of ammonia refrigerant is nearly empty?

The first sign that the drum of ammonia refrigerant is nearly empty is often indicated by a hissing or whistling noise from the drum. This noise occurs because as the refrigerant level decreases, the pressure inside the drum changes, leading to the escape of gas vapor that can create these audible sounds. Recognizing the hissing or whistling helps technicians and operators identify that the refrigerant supply is low and may require immediate attention to prevent operational issues.

The other signs listed, while they may indicate other conditions or potential issues, are not as direct indicators of a nearly empty drum as the hissing sound. Frost may suggest a cold surface due to low temperatures or leakage, fog might indicate condensation or temperature differences, and a gurgling noise could point to liquid refrigerant moving through a system but does not specifically signify that the drum is close to being empty.