Dear Maryam Safaei, To stand a chance of reaching such dry levels, the glass jar obviously should be somehow sealable from uncontrolled ambient air inflow. In the past I have succesfully applied continuously flushing with a small but steady flow of dry nitrogen. Another trick that works well (but requires some "through the wall" tinkering) is having a peltier element surface at freezing temperature on the inside of the reservoir. But both of these solutions are costly. The trick with salts should work well, however. You could use CaCl, and a good tip is to first dry the salt for several hours in a small oven at 110 degrees Celsius, before placing it in the glass jar and seal it. Clearly it also depends on WHAT stuff you are trying to keep so dry. If it contains water that can evaporate, then your salt absorber will get saturated quickly. When I worked on extreme vacuum, it could take days before my turbo pumps had cleared the water monolayer from ambient air that stuck to the chamber wall when vented (even just a few seconds). Be also sure to use a measurement device that is indeed accurate at such low values! It is a difficult task. Even most climate chambers are not capable to reach such low Rh values (except the ones located in countries that witness arctic winters).

Sealing the glass jars, especially those with metal closure is challenging somewhat. Make sure the jars all have gas kits and are sealed properly. The combination of the vacuum chamber and dried salt is an effective way to stop leaking and make low RH. Put the dried salt in Jars, seal properly, and then keep your samples inside the vacuum chamber.

Dear Maryam,

I suggest to use calcium chloride powder. This substance is used as a super water absorber (RT= 0%) in water vapor permeability test.

You can read this method in the following article:

DOI

https://doi.org/10.1007/s11947-020-02561-3

Silica gel can impose the relative humidity below 10%.