This is a roller clamp for 3/8" outer diameter, 1/4" inner diameter flexible PVC tubing (e.g. a 20' roll from Home Depot is less than $5). I have included 2 rollers, a larger one and a smaller one. The large one is sufficient to completely stop flow through the tubing, and the small one allows for a small degree of leak when fully 'clamped.' I have not calibrated the flow rate through the clamp at various positions, so I cannot guarantee that it will suit your needs. The flow rate of a liquid or gas through a piece of tubing with this Thing attached can be controlled by rolling the dial up or down on the Thing. The degree to which the dial squeezes the tubing corresponds to a change in cross-sectional area of that segment of tubing, changing the rate at which fluid can easily flow through it. This clamp is very similar to one you might see attached to an IV in a hospital; a mental image of this sort of roller clamp was used to design this Thing. I use this clamp in a laboratory setting to slowly release a vacuum from a container attached to a piece of tubing.This Thing should be printed with support, such that the top of the clamp body (top of the first picture) is the face that is printed first on the print bed. The roller should also be printed with support, and on its side (i.e. on one of its 'axles'). Aside from removing the support, no other alterations should be needed. After printing, slide the roller into the body of the clamp, and pass the tubing through the body. The roller should now be locked into the body, but should not squeeze the tubing at its 'loosest' setting.