For determining the dew point comprising a glass tube 150 х 25mm with polished silver thimble closed bottom of the tube and the upper end by a rubber bung. The bung has 2 glass air tubes with right angled bends. One tube ending just below the bung, the other reaching nearly to the bottom of the thimble and a hole for thermometer which reads 10 to 50°Cx 0.5°C.