Question

is gas measureble?if yes,,how?

Answer 1

yes, if you fix a constant pressure and temperature.

Answer 2

The typical things we look at in a gas are the volume, temperature, and pressure. I think what Rodrigo is saying is that if you want to see how one changes, in his case perhaps volume, it is best to fix the other two so you only change one variable at a time.Here is a good explanation of how the different variables interrelate:

https://en.wikipedia.org/wiki/Gas_laws