Figure 30-19 Pitot tube. As flows increase, wall pressure decreases as a result of the Bernoulli principle. The Pitot tube measures the difference in pressure from the middle of the flow to the wall and converts this difference to a flow measurement (see Appendix 5 ). (Adapted from Ehrenwerth J, Eisenkraft J: Anesthesia Equipment: Principles and Applications. St Louis, Mosby-Year Book, 1993.