Why is my turbomolecular pump not reaching its rated ultimate pressure?

Work through it in this order — foreline pressure, then leak, then outgassing, then the gauge, then the pump itself. The order matters because the first three explain the large majority of cases and cost nothing to check, while the last one is expensive and is usually blamed too early.

First, foreline pressure. Measure it, do not assume it. If it sits above the turbo's critical backing pressure, compression collapses and the chamber will never get there no matter how healthy the turbo is. Causes: backing pump worn or undersized, its oil degraded, a throttled or leaking foreline, or a closed/partly closed foreline valve. This is the most common answer and the cheapest to fix.

Second, a real leak. A chamber settles at the pressure where the incoming leak rate equals the pump's throughput. If the pressure plateaus quickly and stays flat, suspect a leak; if it falls slowly and keeps creeping down, suspect outgassing instead. That distinction — plateau versus slow creep — is the fastest free diagnostic you have. Confirm with a helium leak test if the plateau is high.

Third, outgassing and water. A chamber that has been open to room air holds a water film on every internal surface that can take many hours to leave at room temperature. If the pressure keeps improving hour after hour, you are watching desorption, not a fault. Baking, or simply more patience, is the fix; a bigger pump is not.

Fourth, the gauge. An ionisation gauge with a contaminated or aged filament reads high, and a gauge mounted on a dead-end stub reads the stub, not the chamber. Before condemning the pump, cross-check against a second gauge or against the number this same system used to reach.

Only then the pump. Bearing wear shows up as noise, vibration, elevated current or a rotor that will not reach full speed. Contamination on the blades reduces compression. Both are real, both are repairable, and both are worth confirming before you order parts — send us the model, the foreline pressure you measured, the chamber pressure and how it behaves over time, and we will tell you which of these it looks like.