Figure 30-17 Volumeter. Flow can be described as volume over time. This design of flow meter allows gas to pass only in little aliquots, each of which turns a counter to measure the amount flowing past. When divided by time, this method results in flow measurement. (Adapted from Ehrenwerth J, Eisenkraft J: Anesthesia Equipment: Principles and Applications. St Louis, Mosby-Year Book, 1993.)