Figure 9-30 Machine outlet check valve. The check valve is located downstream from the vaporizers and upstream from the oxygen flush valve. It is open (left) in the absence of backpressure. Gas flow from the manifold moves the rubber flapper valve off its seat and allows gas to proceed freely to the common outlet. The valve closes (right) when backpressure is exerted on it. (Adapted from Bowie E, Huffman LM: The Anesthesia Machine: Essentials for Understanding. Madison, WI, Ohmeda, a Division of BOC Health Care, 1985.)