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2-stop tubing is used for standard-sized peristaltic pumps. The two colored markers denote the size of the inner diameter. The markers also hold the tubing in place, stopping it from slipping while the pump is moving.
3-stop tubing is used for pumps that have heads that are smaller than the standard-sized. The colored markers denote the inner diameter size. The markers also hold the tubing in place, stopping it from slipping while the pump is moving.