If you can't raise numbers to powers in the PLC here is another solution, not as accurate but should be accurate enought for what you are doing.
a1 b1 a2 b2
0.004043651 -0.039353174 0.612121032 3.207748018
twb=(a1*tdb+b1)*rh+(a2*tdb+b2)
same range as...
How about this?
a1 b1 a2 b2
0.724582967 0.81706445 0.112208011 0.17578135
twb=(a1*tdb^b1)*rh^(a2*tdb^b2)
29.91 bar press
temp db 55 to 95, rh 30 to 100