Calibration vacuum on the tube
Tube calibration draws the extrudate against a sizing sleeve under vacuum in a cooling bath, and that vacuum touches the surface that becomes a patient-contact device. It has to be oil-free and steady, because the vacuum level helps set the tube's final dimensions. A dry YBDVP source holds calibration vacuum constant through the bath without any oil path to the product, which is exactly what a device line needs from grip to gauge.