The manifold gauge set has an inches-of-mercury [vacuum] scale on which gauge?

The low-side gauge on a manifold gauge set is equipped with an inches-of-mercury (inHg) scale for measuring vacuum pressure. This gauge is crucial for assessing the system when performing tasks like recovering refrigerant or evacuating the system before charging it.

When technicians connect the low-side gauge to the system, they often need to create a vacuum to remove moisture and non-condensables. The inHg scale helps directly indicate how deep the vacuum is, thus ensuring effective evacuation. By contrast, while the high-side gauge typically measures pressures in the system when it is charged with refrigerant, it does not feature a vacuum scale. Auxiliary gauges, which may serve various functions, are not specifically designed for reading vacuum pressure either.

In summary, the low-side gauge stands out as the correct answer because it directly recognizes vacuum levels in inches of mercury, essential for proper A/C and heating system service.