Figure 9-4 Shut-off valve of the pressure sensor. A, The valve is open because the oxygen supply pressure is greater than the threshold value of 20 psig. B, The valve is closed because of inadequate oxygen pressure. (Adapted from Bowie E, Huffman LM: The Anesthesia Machine: Essentials for Understanding. Madison, WI, Ohmeda, a Division of BOC Health Care, 1985.)