Would infrared or laser be of any help? In a lab setting.
This graph from the Wikipedia article on Electromagnetic absorption by water shows the absorption of infrared light as a function of wavelength - the green line is the water vapour:
The absorption is greatest in the far infrared, though I suspect equipment that works in this region of the spectrum might be expensive. There is an absorption peak at 950nm, and a quick Google suggests LEDs with a wavelength of 950nm are widely available so it's possible you'd get a useful measurement at this wavelength.