I want to discuss a special form of gas flow measurement - water vapor - otherwise known as steam flow. Traditionally, steam has been measured with orifice plates. The steam was typically saturated or had a known superheat, so the density could be obtained from the steam tables. However, steam naturally cools as it goes down the pipe, so the pressure and temperature at the point of use is generally not the same as when it left the boiler; thus, density errors are introduced and the mass flow of the steam is no longer correct. Obviously, T and P compensation can be added, but that causes the same problems we discussed earlier. We looked at this problem in detail, and developed our Innova-Mass® 240 & 241 Mass Vortex Multivariable flow meters in response. Vortex flow meters use the fact that the number of vortices shed by a bluff body per unit time is a function of the fluid velocity. The typical analogy is a flag moving with the wind. The flag flaps faster as wind speed increases. The actual flapping is caused by vortices shed from the flagpole (the “bluff body”).  If we can count the number of flaps per unit time, we can determine the wind speed. But, how do you turn that into a flowmeter? The answer, my friend, is "blowing in the wind." Find out in my next post on steam flow!