Question 23 DDE01 - Designated Duty Engineer - Unlimited HP

The Correct Answer is D. ### Why Option D (A water manometer) is Correct The restriction across a dry-type air filter on a diesel engine is a measurement of the differential pressure (pressure drop) between the air upstream (clean side) and the air downstream (engine side, which is typically under slight vacuum relative to the surrounding atmosphere). This pressure drop is usually very small, often measured in inches of water ($"H_2O$). A **water manometer** is an instrument designed to measure very low pressures or low differential pressures with high accuracy. The fluid (water) has a low density, which means a small pressure change results in a large displacement of the water column, making the measurement highly visible and precise. For typical engine air filter restriction measurements (e.g., maximum allowable restriction is often 15 to 25 $"H_2O$), the water manometer provides the necessary sensitivity and accuracy. ### Why the Other Options are Incorrect **A) A bourdon tube compound gauge:** A compound gauge measures both pressure and vacuum (usually in psi, bar, or inches of mercury, $"Hg$). While highly rugged and suitable for higher pressures (like oil pressure or air compressor output), it lacks the sensitivity required to accurately measure the very small pressure differential across an air filter, which is best expressed in inches of water. Its accuracy range is typically too broad for this specific, low-value measurement. **B) A bourdon tube vacuum gauge:** A vacuum gauge measures pressure below atmospheric pressure (vacuum). While the intake manifold side of the filter is under vacuum, measuring the restriction requires a differential pressure measurement (intake air pressure minus manifold pressure). Furthermore, like the compound gauge, a typical bourdon tube vacuum gauge is calibrated in units like $"Hg$ or kPa, and its inherent scale resolution is too coarse to accurately measure the minute pressure drops specified in $"H_2O$. **C) A mercury manometer:** A mercury manometer is extremely accurate for differential pressure, but mercury (with 13.6 times the density of water) is used for measuring significantly higher pressures (e.g., barometric pressure or high manifold vacuum) because it requires less column height. Using a mercury manometer to measure a restriction of only 15 $"H_2O$ would result in a column displacement of less than 1.1 $"Hg$ ($15/13.6 \approx 1.1$), making the reading tiny, hard to read accurately, and sacrificing the necessary precision for such a low-pressure application. Furthermore, mercury presents toxicity concerns in a shipboard environment.