(roofing) A device that contains a radioactive source to emit high velocity neutrons into a roof system. Reflecting neutrons are measured by a gauge that is used to detect hydrogen; the quantity of hydrogen detected may be linked to the pressure of water.

plural of nuclear hydrogen detection meter

an atomic clock which uses the 1.4 GHz transition in a hydrogen atom to measure time to within 0.45 nanoseconds over a span of 12 hours

plural of hydrogen maser atomic clock