Yes, I suppose that you can use specific salt solutions for producing different RH. Such as those mentioned in the attached paper (p. 2).

"These were sodium chloride (75%), calcium nitrate (50%), magnesium chloride (32%), and lithium chloride (11%). The lowest relative humidity of 1% was created using concentrated sulphuric acid. The relative humidities were established based on the method of Winston and Bates [8] using specific standard salt solutions."

One thing I am concerned is that:  if a RH we really want (such as 25%), how could we create this RH? 

Nowadays, some growth chambers have equipped with RH functions. However, the advantage of it is that you can only use one 1 RH condition for one treatment (experiment) every time.

if such growth chamber is not available, and the experiment is involving different RH as treatments ? 

Thanks for that response

It seems to be a little bit complicated. You might also need to control the temperature to control the RH. See this statement from the website attached.

"Relative humidity is actually relative to the temperature. The higher the temperature, the more moisture the air can hold at a given temperature. Relative humidity is the ratio of the amount of moisture in the air vs. how much it can hold. And herein lies the best reason to have temperature control: If the temperature changes, the relative humidity changes. So a stable temperature helps ensure a stable humidity level."

http://www.espec.com/na/chamber_faq/answer/humidity_control

that a very helpful note too. here is what I've found after searching many sources: im gonna use different kinds of saturated salt solutions which is being used for RH control in sealed containers. The main point is to have continuous monitoring and control of the RH levels throughout the incubation period. 

Before doing this I though is one easy and basic test to evaluate the effect of different levels of known RH on the treatment used, but I found out that in order to have sound results its alot complicated to do... glory for everyone who stay nights and days trying to make his data as sound as possible, and shame on those who predict more than actual doing!

Thanks so much for your response Prof Yuan

Yes, I suppose that you can use specific salt solutions for producing different RH. Such as those mentioned in the attached paper (p. 2).

"These were sodium chloride (75%), calcium nitrate (50%), magnesium chloride (32%), and lithium chloride (11%). The lowest relative humidity of 1% was created using concentrated sulphuric acid. The relative humidities were established based on the method of Winston and Bates [8] using specific standard salt solutions."

One thing I am concerned is that:  if a RH we really want (such as 25%), how could we create this RH? 

I'm speechless, Thank you so much for the valuable information. I appreciate that you have offered my concern some of your valuable time. I promise to do the same when other ask me for help :) 

for that experiment, I guess its now being more clear on my mind about the set up of conditions and I'm currently looking for a RH measurement tool to monitor throughout the experiment. 

Thank you so much