In various embodiments, rapid, sensitive detection of molecular hydrogen is achieved by chemically converting hydrogen to water vapor and then detecting the water vapor as a surrogate for the hydrogen. Detection may be enhanced by dampening variation in ambient water vapor and rapidly actively modulating a hydrogen-derived water vapor component. For example, the detector may receive sample gas that includes ambient water vapor and hydrogen, dry the sample gas to dampen variation in the ambient water vapor, divide the sample gas into a chemical conversion flow and a bypass flow, chemically convert hydrogen in the chemical conversion flow to water vapor, alternate between measuring water vapor in the converted chemical conversion flow or the bypass flow to produce a water vapor signal, separate the water vapor signal in the time domain to extract a hydrogen-derived water vapor signal, and output a hydrogen signal based on the hydrogen-derived water vapor signal.