EMO vaporizer and bellows compensation device

What is the purpose of the bellows compensation device for the EMO vaporizer? According to my physics book the EMO has a bimetallic strip as the temperature control valve. Is this used for the addition of a gas carrier like oxygen if it is hooked up to it?
Anyone else us the Davis book for the physics portion of the program? I find thid book pretty poor in explaining alot of things, but luckily our instructor is good at explaining the material in the book. The book is Basic Physics and Measurements in Anesthesia by Paul Davis.

thanks for the feedback