A vacuum hose is rated on two things that never appear on a pressure table: how much external pressure it will take before it collapses, and how much gas leaks into it per second. A hose rated 38 bar internal may collapse under one bar external, because a braid takes tension and nothing else. The braid that makes a hose strong does nothing at all when the load is pushing inwards.
What this service does to a hose
Collapse is a stability problem, not a strength one. Under external pressure a thin-walled tube fails by buckling, and buckling depends on wall thickness, diameter and the stiffness of the geometry — not on the tensile strength the braid provides. An annular corrugated hose is actually quite good here, because the corrugations act as circumferential stiffening rings. But the rating has to be established for the construction and stated, and the larger the bore the harder it gets. Ask us for the vacuum rating explicitly; do not read it off the pressure column.
Leak rate is the other rating. Below a few millibar it stops mattering whether the assembly holds and starts mattering how fast it fills. That is a measured leak rate, usually with helium, and it is a different test from the hydrostatic test we run as standard. If your system has an ultimate pressure specification, the leak rate is what determines whether you reach it.
Outgassing and virtual leaks. Below about 10⁻³ mbar the gas load stops coming from outside and starts coming from the inside surface — adsorbed water, hydrocarbons from handling, oils from manufacturing. A corrugated bore has several times the internal surface area of a smooth tube of the same length, which is why hoses have a reputation in high-vacuum work that they do not deserve on rough vacuum. A trapped volume — a blind space behind a ferrule, a gap under a liner — empties so slowly it behaves exactly like a leak, which is why these are called virtual leaks.
Where this actually sits for us. Rough and medium vacuum — process vessels, filters, transfer lines, vacuum drying, suction on a pump — is ordinary work. High and ultra-high vacuum is a specialist field with its own fittings, cleaning and bakeout requirements, and we will say plainly whether a given specification is inside or outside what we can certify.
What we would fit
| Element | What we would supply | Why |
|---|---|---|
| Hose | 316L annular corrugated — the corrugation is what resists collapse | Convolutions act as stiffening rings; a smooth thin tube would buckle first |
| Braid | Fitted where the line also sees positive pressure or mechanical load | It does nothing for collapse, so on pure vacuum it is there for handling |
| Ends | Welded or metal-face seals; no tape, no thread sealant | Threads leak inward and sealant outgasses |
| Bore | Sized generously — conductance falls very fast as bore drops | At low pressure a narrow hose throttles the pump more than the pump ratings suggest |
| Cleaning | Degreased and capped; specified cleaning where the ultimate pressure is tight | Below 10⁻³ mbar the gas load is the surface, not a leak |
What we would not fit, and what happens
A hose selected from the pressure column. Internal rating says nothing about collapse.
A PTFE-lined hose on medium or high vacuum. The gap between liner and metal is a trapped volume, and a trapped volume is a virtual leak.
Interlock hose. The seam is deliberately not gas-tight.
An undersized bore. The pump will never reach its rated speed and the hose will be blamed for the vessel taking an hour to pump down.
What we need to size it
| We need | Because |
|---|---|
| Vacuum level required, and the ultimate pressure of the system | It decides whether this is routine work or specialist |
| Whether the line also sees positive pressure | It changes whether a braid is needed at all |
| Bore and the pumping speed you need to preserve | Conductance falls sharply with bore at low pressure |
| Any leak rate specification and the test standard | It is a different test from the hydro and may be outside our scope |
| Whether bakeout is required, and to what temperature | Bakeout is a high-vacuum requirement and changes everything |
| End connections — flanged, clamped or a vacuum fitting family | Vacuum fitting families are specific and must be stated |