What instrument is used to measure the depth to gravel in a filter bed?

• A. Metal rod
• B. Tape measure
• C. Sonar depth finder
• D. Ruler

Answer: (A)

Measuring how deep the gravel sits in a filter bed is done with a rigid sounding rod. You push the metal rod straight into the bed until it contacts the gravel layer, then read the depth from the marks on the rod at the surface. The metal rod is ideal because it’s durable, long enough to reach the gravel, and provides a direct tactile indication of when you’ve hit the gravel. A tape measure is less practical to insert and can bend or snag in loose media; a sonar depth finder is meant for open water, not internal bed measurements; a ruler is far too short to reach the gravel depth.