What could be a reason for an increase in vacuum inside suction piping from 0.6 inches to 1.0 inches after a few months?

An increase in vacuum inside suction piping from 0.6 inches to 1.0 inches after a few months can be attributed to the condition of the surrounding soil, specifically if it has become wet and is restricting airflow. When soil moisture levels increase, it can lead to a reduction in the air permeability of the soil matrix. This wet soil can create conditions where the vacuum system struggles to draw air through the soil, thereby causing an increase in measured vacuum within the system. The interaction between the moisture content and the porosity of the soil ultimately affects the airflow dynamics, resulting in the observed increase in vacuum.

The other reasons proposed, while plausible in terms of issues that may arise in a suction system, do not directly correlate to the specific scenario of an increase in vacuum over time in the way that changes in soil moisture do. For instance, improper installation would typically lead to inconsistency rather than a steady increase in vacuum. A malfunctioning suction fan would likely result in erratic vacuum readings or a decrease rather than a steady rise. A decrease in external pressure generally refers to atmospheric conditions that would not typically affect the established vacuum levels within a properly functioning system.