The diffusion of gases, dry and moist, had been studied in 1874 by Louis Dufour, who observed a pressure difference across a porous wall separating moist air and a vessel containing either water or a desiccant. 70 years later, Heinrich Greinacher (1880-1974) claimed to have built the first Diffusion Hygrometer, proposed a theory and got a patent for it. Two different models were produced by firms of Lausanne, TESA and Rüeger. Eventually, they were a commercial failure. We found one exemplar of each maker in the collections of our Physics Museum and have tested them. A comparison of the performances with the then currently available hygrometers will be made.