A measuring apparatus and method for use in measuring diffusible hydrogen concentrations in materials, structures, and other objects. In an embodiment of the invention for use in welding applications, the measuring apparatus (10) includes a sensor assembly (20) that, with an included sealing member (40), defines a sample area (17) on a weld bead (16) from which hydrogen evolves into a sample volume (18) defined by the sealing member (40), a sensor housing (34) and a sensor (22) of the sensor assembly (20). The hydrogen reacts with a sensing layer (28) and a reflector layer (30) positioned on the end of an optical fiber (24), all of which are included in the sensor assembly (20) and are sealably positioned within the sensor (22). The sensing layer (28) comprises a chemochromic material that undergoes changes in physical properties, such as optical transmission properties, when it reacts with hydrogen and these changes are measured by the measuring apparatus (10) to determine the amount of hydrogen evolving from the sample area (17). An optical fiber (46) is joined to the sensor optical fiber (24) to direct light (63) transmitted by a light source (62) in a hydrogen monitoring assembly (60) through the sensing layer (28) to strike the reflector layer (30) which reflects light (67) back through optical fiber (46) to a detector (68) in the hydrogen monitoring assembly (60). A signal analyzer (72) is included in the hydrogen monitoring assembly (60) and is calibrated and configured to measure the diffusible hydrogen concentration in the weld bead (16) based on measured changes in the optical transmission properties of the sensing layer (28).