This, one of the most important part of the gas meters, is made in thermosetting material. The dimensions of these parts have to remain constant over time despite temperature change or humidity changes or the contact with hydrocarbon in form of gas.
It is perfectly reasonable that a measure as to be the same despite the outside temperature or humidity of the gas. In addition, the part is rarely be affected by the impurities such as water, zinc, brick rubbish which are sometimes in the gas.
It has been used for various volume of gas meters.