A metal rod should be used to probe the filter on a grid system to locate and record the depth to gravel. At a minimum, how often should this be done?

• A. Every 6 months
• B. Once per year
• C. Every 2 years
• D. Every 3 years

Answer: (B)

Regular checks of the filter gravel depth using a probing rod are done to confirm the media bed stays within its design depth and to detect any settling or compaction over time. Probing on a grid ensures you get a representative picture of the entire bed rather than a single spot. This should be done at least once per year because gradual changes can accumulate and impact filtration performance, headloss, or bed distribution if left unchecked. Keeping a record over time helps you spot trends and plan maintenance, with more frequent checks warranted after events like backwashing or if the system shows signs of instability.