The Care and Keeping of -80C Freezers
How many times have you gone to the Minus 80 freezer and been unable to find a sample you are sure you stored in there? You know it’s there, but the frost is so deep you could ski on it. How many times have you stood at the Minus 80 with the door open for several minutes while pondering the secrets of the Universe?
You should not have to do either one of these in the lab. The former is completely preventable, and the latter contributes to the former. Ultra-Low Temp Freezers (ULT’s) operate under the same principles as your home freezer, though the components of a lab unit are typically higher quality, and they may have multiple types of refrigerants.
The best way to keep your freezer from developing a blanket of snow or having icicles for door gaskets is to clean it regularly. No matter how quickly you get in and out of the freezer, some warm air will enter it while you are storing or retrieving samples. You may have noticed that a freezer is difficult to open just after you closed it. That’s because the warm, humid air is being cooled quickly and as it cools its volume decreases. This created a vacuum temporarily until the air temp is stable inside. The air that gets inside usually has moisture in it, and the cold door gaskets will freeze moisture from the surrounding air while open, all contributing to frost and ice build-up.
Every now and then, depending on your environs, you need to clean out your ULT’s. The same goes for your Minus 20 freezers. I have found the best way to go about this is:
1. Announce to everyone that you are going to clean the freezer and explain that this includes defrosting and removing all samples from it (you need a standby freezer). Set a date and get concurrence from all at least 48 hours before this date so that you don’t ruin a million-dollar experiment.
2. On the day of the previously announced cleaning, Inspect the freezer to ENSURE it is empty before you start. It seems there is always one person who didn’t see the notification or just forgot. It happens, if you have to move the schedule one day to the right, so be it.
3. Now that you’ve verified you are ready to start, turn off the unit, unplug it, and tape the cord up off the floor so it doesn’t get soaked or become a trip hazard.
4. Place lots of absorbent pads on the floor around the freezer to soak up any water that accumulates during the ice melting phase. I like the blue and white mats by Pig.
5. If you want to speed things up, and I do, you can remove the thick layer of frost by hand (wear cryo gloves) and put it in a mop bucket or similar waterproof container. Take this opportunity to look for small items that may have been overlooked by the team emptying the freezer. Trust me, they’ll thank you for it.
6. For cleaning the gaskets and interior of ice, DO NOT EVER use an ice pick, screwdriver, or another sharp instrument. YOU can ruin the unit and/or hurt yourself. Imagine puncturing a refrigerant line and getting sprayed in the eyes with something as cold as liquid nitrogen. Yeah, that would be bad. No sharp objects, enough said.
7. Once the freezer is defrosted, you must clean it and dry it thoroughly, moisture is the enemy.
8. A 10% bleach solution is ideal for cleaning a lab freezer. I use an absorbent paper towel or suitable substitute and dry everything afterward.
9. To dispose of the bucket of frost and ice, let it melt, and add enough bleach to make a 10% solution. Let it sit for 30 minutes and you can pour it down the drain. This step assumes you are using your freezer for biologics of course.
10. The last step, now that your freezer is clean and dry, is to vacuum the condenser coils and make sure the air filters are clean. Most units have a washable filter in the front of the units that you can access easily. Consult the owner’s manual for coil cleaning, it’s not difficult, and remember to always use a vacuum with a HEPA filter in the lab.
Remember to take care of your equipment and it will take care of you. A freezer can last you 20 years if you take care of it properly, and that’s money that can be spent elsewhere in your lab.