How does the inside diameter of a tube affect flow rate?
Occasionally, requests are made for specific flow rates in tubing produced by Vita Needle. The challenge with providing a tube that meets a specific flow requirement is that there are several variables that impact flow. The viscosity of the fluid itself (gases are also fluids), the length of the tube, the ID surface finish, the pressure applied and perhaps most obviously the inside diameter of the tube impact the flow rate. The variables that have the greatest impact on flow from a tube manufacturing standpoint are the inside diameter and the finish. At smaller diameters, maintaining a tightly tolerance flow rate can be problematic because small changes in tube diameter (within manufacturing tolerances) can impact flow. Poiseuille’s law, described below, shows that if the radius of a tube is halved, flow resistance increases 16 fold! Small changes can have a significant impact, to the point where high pressures are required to permit gas flow at the smallest hypodermic gauge sizes over long lengths of tubing.
Further, measuring ID by flow instead of using plug gauges (the industry standard) or vision systems, is problematic for many of the reasons described above: simply too many variables that result in dubious measurements at best. Therefore, measuring ID by flow is a method normally avoided except for highly specialized applications where it is critical to function and there are no alternate means to specify inside diameter product requirements.
Δp = pressure difference between the two ends
μ = dynamic viscosity
L = length of pipe
π = pi
R = pipe radius