What is the purpose of an atmospheric line on an indicator card from a double-acting ammonia compressor?

The purpose of an atmospheric line on an indicator card from a double-acting ammonia compressor serves as a pressure reference or standard. This line acts as the baseline from which the actual pressure of the compressor's cycles can be measured. By providing a reference point at atmospheric pressure, operators and engineers can accurately assess the pressure changes occurring during the compression process.

This reference is crucial for ensuring the safe and efficient operation of the compressor, as it allows for a clear understanding of how the pressure within the system fluctuates in relation to atmospheric conditions. Understanding this relationship helps in diagnosing performance issues and optimizing the operation of the refrigeration cycle.

When evaluating the other options, while piston stroke, volumetric efficiency, and compressor superheat are important metrics that can be derived from various other data and measurements, they do not fundamentally define the role of the atmospheric line in the context of the indicator card.