Three requirements, three vessels — not one unit with options
Ambient air is the simplest case: heating and control only, with no sealing or gas handling.
A controlled atmosphere needs gas inlet and exhaust, flow control and safety interlocks. Whether the chamber runs at positive pressure or slight vacuum, and whether the gas is combustible, changes the scale of both structure and safety design.
Vacuum needs a sealed shell, a pump set and vacuum instrumentation. A vacuum vessel is designed for external pressure, and its sealing class, flange forms and viewport construction are not the same as an ambient furnace.
Reactor-type work adds pressure rating on top. The price difference between these cases comes mostly from structure rather than controls, which is why settling the atmosphere cannot be skipped.
Ultimate vacuum and working vacuum are two different numbers
An ultimate vacuum figure is easy to make look good, but it is normally taken with an empty, cold, purged chamber — an equipment capability number, not what you will see in production.
What governs the process is working vacuum: the level that can be held after loading and during the ramp. The workload outgasses; oils and volatiles are released; all of it pulls the vacuum down.
So the question to ask is what the vacuum level is after loading and during the ramp, not the ultimate figure alone. A proposal that quotes only the ultimate value and avoids the working value will be disputed at acceptance.
Three common reasons vacuum falls short
First, high outgassing from chamber materials — insufficient bake-out, or a material simply unsuited to vacuum service.
Second, contaminated sealing surfaces. Dust, grease and scratches all create leak paths. Seals are a maintenance item, not a one-time installation.
Third, oils or volatiles on the workload itself. This is the cause most often blamed on the equipment when it is really a pre-treatment issue.
Work the list backwards: confirm the workload and its pre-treatment first, then inspect sealing surfaces, and only then suspect chamber materials. The ultimate vacuum of an empty, cold, purged chamber is the equipment's true figure.