Backflow Prevention Practice Exam

A differential pressure gauge reading of 0.2 psid is represented on a water column sight tube by _____?

1. 1/16 in. (0.16 cm)

2. 1/2 in. (1.3 cm)

3. 2 in. (5.1 cm)

4. 5.5 in. (14 cm)

The correct answer is: 5.5 in. (14 cm)

The correct answer of 5.5 inches (14 cm) corresponds to a differential pressure gauge reading of 0.2 psid because of the established relationship between pressure and the height of a liquid column. In this case, the pressure is being measured in pounds per square inch differential (psid), and the water column's height can be calculated using the formula for hydrostatic pressure, which states that pressure is equal to the height of the liquid column times the density of the liquid and the acceleration due to gravity. For water, the specific weight is approximately 62.4 lb/ft³. To convert psid to inches of water column, one would typically use the conversion factor that 1 psid roughly equals 28.9 inches of water column. Thus, a reading of 0.2 psid would translate to approximately 5.78 inches of water column (0.2 psi * 28.9 inches/psi), which closely aligns with the 5.5-inch option provided. This calculation reinforces why a reading of 5.5 inches is the accurate representation for the given pressure in the context of the backflow prevention system monitoring and is fundamental in ensuring that sufficient differential pressure exists to prevent backflow.