MDPE is Medium Density Polyethylene. Used for underground gas lines. IE firepit or to an outdoor BBQ. Is most common. MDPE gas pipe CANNOT be used inside of any building. Outside use ONLY!

26mm

It is not recommended to use Yellow MDPE Gas Pipe for internal use. This type of pipe is specifically designed for outdoor gas distribution and should not be used for indoor plumbing or heating systems. It is important to use the appropriate type of pipe for each specific application to ensure safety and compliance with building codes.

approximately 50mm internal diameter but you'll need to verify this with manufactures if you need it to be precise. For modelling purposes this will give you a rough idea.

Polyethylene is a pure substance.

Polyethylene is a semicrystalline homogeneous polymer with a molecular weigh profile based upon the production chemistry. Common forms include HDPE, MDPE, LLDPE, LDPE, UHMWPE, etc.