The speed of reading depends on the length and diameter of the pressure passages inside the probe, the size of the pressure tubes to the manometer, and the displacement volume of the manometer. The time constant is very short for any of the standard tubes down to 1/8" diameter; however, it increases rapidly for smaller diameters. For this reason 1/16" OD is the smallest recommended size for ordinary use - this will take 15 to 60 seconds to reach equilibrium pressure with ordinary manometer hook-ups. These tubes have been made as small as 1/32" OD, but their time constant is as long as 15 minutes and they clog up very easily with fine dirt in the flow stream. If very small tubes are required, it is preferable to use separate total and static tubes rather than the combined total-static type. Where reinforcing stems are specified on small sizes, the inner tubes are enlarged at the same point to ensure minimum time constant.