Relative humidity above various mixtures of table salt and water I heard that if I saturate some water with NaCl and put it in a closed box, the relative humidity near the water surface should be around 75 % at room temperature.

I am using this to calibrate some electronic sensors measuring relative humidity.

If a saturated solution gives 75 % RH, how can i know which RH more diluted mixtures should result in?

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My setup is a small 1l plastic container where in I put a mixture of table salt (iodized) and tap water (about 150 ml). The sensor is hanging a few cm above the water surface and measures for a day or so.

The solutions I use are:


Fully saturated water (Plenty of undissolved salt crystals)
75 % saturated water (3 parts saturated, 1 part pure water)
66 % saturated water (2 parts saturated, 1 part pure water)
50 % saturated water (1 part saturated, 1 part pure water)


I am also monitoring the temperature in the container, and it is usually swinging a few degrees below 20 °C.
Your information on relative humidity (RH) above saturated $\ce{NaCl}$ is valid within about 0.5% over the range of 0 °C to 80 °C. For other standard humidity solutions, such as $\ce{MgCl2}$ or $\ce{K2CO3}$ the U. S. National Institute of Standards &amp; Technology provides a convenient reference.

However, though it is possible to adjust RH by varying concentration, there are, "special  problems...  because  their  concentrations  must  be  measured  and controlled." If you need only a few accurate RH points, it would be simpler to use saturated solutions of some alternate salts.

BTW, it may take some time for RH to equilibriate because of stratification. It would help to keep the solution on a slow magnetic stirrer, moving not only the liquid but causing eddies in the air above it.