Relative Humidity inside a thermally insulated chamber for a NACL solution

I ran an experiment, where I had to measure the relative humidity of a thermally insulated chamber containing NaCl solution. I have read from a few papers, that the relative humidity of a NaCl solution remains at 75% over a wide range of temperatures (20C to 30C).


But when I can change the temperature inside the chamber from 20C to 30C the relative humidity changes from 80% to 60%.


I am having trouble understanding what exactly is happening here. I thought it might be the pressure change caused by temperature change, so I created a small hole (needle width) to compensate for it but the result remains the same.


[![enter image description here](https://i.stack.imgur.com/yRSVb.png)](https://i.stack.imgur.com/yRSVb.png)


