49 CFR §230.51
Verified against eCFR.gov as of June 20, 2026View official text on eCFR.gov ↗
Every boiler shall be equipped with at least two water glasses. The lowest reading of the water glasses shall not be less than 3 inches above the highest part of the crown sheet. If gauge cocks are used, the reading of the lowest gauge cock shall not be less than 3 inches above the highest part of the crown sheet.