Monitoring of hydrogen in argon cover gas is one of the effective methods used to detect an event of steam/water leakage into sodium in the steam generators of a fast reactor during its startup and low-power operation. A diffusion-based hydrogen meter employing a thermal conductivity detector (TCD) and argon as carrier gas is a simple, convenient, and inexpensive device for this purpose. This paper describes the fundamental criteria involved in designing such meters and experimental results obtained during the calibration of two such meters.