Inches of Mercury

Inches of mercury is a pressure measurement that indicates how much lower the pressure inside the vacuum system is compared to atmospheric pressure. Standard atmospheric pressure at sea level is 29.92 inHg. A hydrovac blower rated at 27 inHg can create a pressure differential of 27 inches of mercury, which represents approximately 90 percent of a perfect vacuum.

Positive displacement blowers typically achieve higher vacuum levels (15 to 27 inHg) compared to centrifugal blowers (up to about 16 inHg). This higher vacuum capability is what gives PD blowers their advantage in deep dig and remote dig applications, where the vacuum system must overcome significant friction losses and the weight of material in the hose.

When comparing hydrovac trucks, it is important to evaluate both CFM (airflow volume) and inches of mercury (vacuum depth) together. A truck with high CFM but low vacuum depth will perform well on shallow, open excavations but struggle with deep digs. A truck with high vacuum depth but low CFM will maintain suction at depth but move material more slowly. The ideal combination depends on the type of work the truck will perform.