No, The seed water potential if very sensitive. You have to have a water potential meter, type wescor equipment to get this measurement accurately

I'm not a water physics expert but it seems to me if you accurately know the RH and temperature that should be enough information.  But perhaps Marcelo is correct and the system is very sensitive to small variations in RH and T. 

You can measure seed water potential with a simple conversion once water partial pressure is known. You may follow ref 16 in this paper or check a table, All this assumes equilibrium in a closed system. http://www.plantphysiol.org/content/58/2/237.long and http://6e.plantphys.net/topic04.04.html

Dear Marcelo 

thanks for the answer.

Dear Paul 

thank for your answer.

Dear Guido Bongi

thanks for the answer and the paper you've attached.

Dear Aida, the seed is an organ composed of dozens, sometimes hundreds of biochemical compounds that decrease the osmotic potencial of the medium. Only by knowing the water potential and the relative humidity of the seed, I believe that it is not possible to calculate the osmotic potential of the seed. If we had been talking about a solution of  concentration and composition known, could use the equation of vant'off to find the osmotic potential of that solution since you also had the coefficient of dissociation of the molecule in question, but since the seed has many components , I believe that if I come to the osmotic potential of such a complete organ with only a mathematical calculation it would be overly contrived, but I also agree with Dr. Paul Milhan that, I am not a physicist, so may be mistaken.

Dear Colleague,

Water activity and water potential are measured by equilibrating a seed sample in a sealed chamber and measuring the relative humidity of the head space. The water activity is equal to the relative humidity (corrected for any temperature difference between the sensor and the sample), and the water potential is computed from eq. 1 using seed moisture content. (see in attached PDF document). Decagon’s AquaLab Series 3 water activity meter is ideally suited for making this measurement. It equilibrates approximately 5 grams of material in a sample chamber and determines the water activity from the dew point temperature of the air in equilibrium with the sample. The sample temperature is measured using an infrared thermometer, and used to correct the readings for temperature differences. The measurement takes about five minutes.

Hope this helps!

Dear Dr. Ronald Maldonado Rodriguez

thanks a lot for the answer. I found the attached pdf very helpfull.