The oil of a diffusion pump cannot be exposed to the atmosphere when hot. These oils are the most chemical and heat resistant type of diffusion pump oil. One issue was pumping speed&colon; alternatives such as the diffusion pump are much faster. The oil diffusion pump is operated with an oil of low vapor pressure. The diffusion pump is widely used in both industrial and research applications. The two main types of molecular pumps are the diffusion pump and the turbomolecular pump. In modern textbooks, the diffusion pump is categorized as a momentum transfer pump.