Joule-Thomson Coefficient pT

Hey guys,

I have question to Joule-Thomson Coefficient!
J-T Coefficient tells us the Temperature change of the gas by expansion prom pressure1 to pressure2.
As you can see at this table the J-T Coefficient depends on initial pressure and temperature.



When I have a closed tank, filled with Argon at 200 atm at 200 degC and I open the valve and let the gas expand, the pressure in the tank will drop with the time right.
Does it means that my J-T Coefficient will change (get better) by the time???!!!

Thanks for help!