Troubleshooting Your Freeze Dryer & Vacuum Pump

Compared to other machines typically found in laboratories, freeze dryers are rather simple. However, troubleshooting them can be time-consuming and monotonous. The key to successfully diagnosing a freeze dryer issue is testing, and retesting, different components in a specific order to narrow down the problem's origin. Stepping through the process can quickly become inefficient if one does not approach it with deliberation and forethought.

Our technical staff here at NLS have spent a lot of time testing and fixing freeze dryers, so we've refined our troubleshooting procedures to be as efficient as possible. We'd like to share two such processes in our troubleshooting guide regarding two common issues: insufficient vacuums and weak refrigeration.

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Problem: The freeze dryer has an insufficient vacuum

As stated, diagnosing freeze dryer malfunctions can be time-consuming. This is especially the case with weak vacuums. A faulty vacuum pull can be caused by the smallest compromise in the setup, from a microscopic hole in the vacuum tube to a faulty manifold valve. As such, locating the breach boils down to one thing: isolating components.

Isolating the Vacuum Pump

When your freeze dryer isn't pulling a vacuum sufficient for your application, you should first determine if the source of the problem is freeze dryer itself, or the vacuum pump.

Isolating the pump is simply a matter of:

(1) changing the vacuum pump oil and

(2) capping the vacuum inlet in the freeze dryer's chamber. 

We recommend changing the oil three times, back to back, to ensure that all foreign moisture is removed before pulling a vacuum. Alternatively, you can buy flushing oil concocted specifically to rid the oil well of contaminants.

After depositing fresh oil and capping the vacuum inlet, activate the pump and monitor its strength. If the pump is strong, then the compromise originates with the freeze dryer.

However, if you've followed all of the above steps and you still have a weak vacuum, then either the tube or the pump itself is the source of the problem. Swap the tube and run another vacuum pull. If it's strong, then you simply need a new tube.

But if the vacuum pump is still struggling at this point, it's safe to conclude that the pump is the problem. You'll either need to rebuild your pump or purchase another one. 

Isolating the Freeze Dryer

So you've determined that neither the vacuum pump nor its tube are the source of the leak. This can only mean that the leak originates in the freeze dryer's chamber or its manifold. Now what?

You must continue whittling down to the source by isolating the chamber. You can accomplish this by plugging the port that leads into the manifold, then pulling a vacuum. If the vacuum is weak, then you'll likely have to replace the door gasket. If it's strong, then that leaves just the manifold in the list of potential culprits.

Should the leak come from the manifold, you'll then need to test the valves -- or, more specifically, their plastic selector knobs. Those plastic parts corrode much more easily than the rubber valves themselves, so in most cases the knobs are all that will need replaced.

To test the valves, remove each one, cap the outlets, and pull a vacuum. If the target vacuum is reached, then one or more of the selector knobs is faulty and you'll need to order new ones. Here's a guide on how to quickly and easily locate faulty valves with nothing but foaming hand soap.

While unlikely, it's also possible that one of the manifold tubes could be the source of the leak. Occasionally, the welding in a manifold can have tiny pinholes which allow pressure to escape. This can only be ameliorated by re-welding the joints or by purchasing a new manifold entirely.

Problem: The freeze dryer does not reach the proper temperature

Sometimes freeze dryers have no problems holding a strong vacuum but don't drop the chamber's temperature far enough. Luckily, tracing the origin of such an issue is considerably simpler than tracing that of a weak vacuum:

If the problems occurs after a defrost, your defrost valve is stuck. Otherwise, there is a problem with the freeze dryer's refrigeration system. Unfortunately, refrigeration issues can be very difficult to resolve so you are likely best off hiring a qualified technician to repair it for you.

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Still stumped by your vacuum system? Feel free to contact us at and we'll run your problem by our freeze dryer specialist.

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