It is specially treated to remove all water vapor so that it will not contribute to the signal. The other side contains the semiconductor gas. After bouncing around, the laser beam goes to a detector, which converts the laser power to an electronic signal. Because we modulate the laser wavelength 50,000 times per second, the laser power varies periodically when water vapor is present. All the components of the measurement system are made of electropolished stainless steel or other high vacuum-compatible materials, to minimize the stickiness.
Gas constantly flows through the measurement region to further minimize the effects of the surfaces.
The cell can be heated to help unstick any moisture present after the cell has been exposed to the atmosphere for maintenance. The instrumentation was brought to NIST for calibration and delivery. To produce these levels, air is blown through a long copper tube containing ice at a precisely defined temperature. By changing the temperature of the ice or the total air pressure, the water concentration can be controlled. The NIST moisture generator is calibrated from the thermodynamic properties of ice.