[m1234]

hi all,
system @ T=30 celcius, P=10 bar and dew point of 5 celsius, goes to state 2 where
T=30 celcius, P=1 bar what will be the new RH% at this pressure?
??

[UG]

Are you using a psychrometric chart(s) to answer the question?

[m1234]

i can use psychrometry charts (since i have the chart for 10 bar).
i can be satisfied with calculation but i don't know how to solve it.
the real problem with the carts is that i'm not sure i can take data from a chart for 10 bar and use it in the other chart (of 1 bar).

[UG]

Well you know at the dew point temperature the air is saturated. The dry bulb and wet bulb temperatures are going to be the same (i.e. 5^oC), so you can find this point on the psychrometric chart. You can then use this point to find the absolute humidity of the air at the dew point (which will be the same as the absolute humidity of the air at 30^oC). Then go to your 1 atm chart and find the point at 30^oC which has the absolute humidity found above, and read off the relative humidity.