What does occlusion mean?
Occlusion is the squeezing of peristaltic pump tubing between the pump head rollers and the tubing bed.

What's the difference between fixed- and adjustable-occlusion pump heads?
Adjustable-occlusion pump heads let you increase occlusion to maximize performance and obtain better suction lift, or reduce occlusion to extend tubing life. Fixed-occlusion pump heads provide high flow rates at economical prices and are factory calibrated for average occlusion.

When would I want lower occlusion?
When you want to extend tubing life, or when pumping fluids with particulates.

When would I want higher occlusion?
When you need to maximize suction lift, especially when pumping viscous fluids.