Backup Systems for ULT Freezers: Your Reliable Safeguard.

By Miriam Carrillo, 07 June 2024

Think about the contents of your ultra-low temperature freezer (or, if you don’t have one yet, the future contents). All the specimen collection and all the experiments conducted. Now think about how terrible it would be to lose them all. If you are operating a ULT freezer without a backup system, you’re rolling the dice on those samples. You are trusting that your ULT freezer will last indefinitely and your facility will always have continuous power. However, blackouts happen, and no ULT freezer will last forever.

That’s where backup systems come into play. In this post, let's explore the types of backup systems available in the market, how they function, their key differences, and when users should consider acquiring one.

When should you consider getting a Backup System?

If you rely on ULT freezers for storing critical and irreplaceable samples, you should consider implementing a backup system. Scenarios to consider include:

  • Frequent Power Outages: Facilities in areas prone to power disruptions must safeguard against potential temperature fluctuations.
  • Critical Sample Integrity: Clinical samples, biological specimens, and other one-of-a-kind samples necessitate continuous temperature control.
  • Research Continuity: Ensuring ongoing research projects are not disrupted by unforeseen system failures.

One thing that should be noted is that back-up systems almost universally must be factory-installed at the time the ULT freezer is fabricated. Retrofitting a ULT freezer in your lab is almost never possible. Therefore, if you think you will need a backup system in the future, you must order it at the time your freezer is ordered. 

Let's take a closer look at the key characteristics and differences between the options available in the market so you can make an informed decision.

Types of backup systems.

CO2 Back-Up Systems: Capable of maintaining temperatures between -50°C and -70°C, liquid CO2 backup systems offer a reliable solution for non-critical ultra-low temperature requirements. These systems provide a reliable source of CO2 in emergency situations, such as power outages or electronic system failures, that would otherwise cause the temperature inside the freezer to rise and potentially damage stored samples. The CO2 backup system utilizes liquid CO2 and requires a siphon tube to draw the liquid from the bottom of the tank. Liquid CO2 can remain pressurized until needed, allowing a tank connected to the backup system to stay full for six months, a year, or longer if the system hasn't been activated. A standard 50 lb cylinder can maintain a freezer that is 3/4 full to fully loaded at -60°C for approximately 8 hours, depending on the freezer's size, ambient temperatures, and door seal. Tanks need to be replaced every 8 hours during use, or multiple tanks can be manifolded together to extend the operating time.

LN2 Back-Up Systems: When you need to go colder, LN2 backup systems can maintain temperatures as low as -85°C. This makes them ideal for applications requiring more stringent temperature control. LN2 cannot remain pressurized and therefore continuously boils off, whether it is being used or not. Consequently, a tank of LN2 connected to a backup system will not retain any LN2 after 6-8 weeks. However, a fresh 160-liter cylinder of LN2 can maintain a freezer that is 3/4 full to fully loaded at -60°C for 45 to 50 hours, depending on the freezer's size, ambient temperatures, and the door seal. If you opt for an LN2 backup system, you need to be aware that the LN2 tank will need to be replaced approximately every 3-4 weeks if kept connected continuously. The most effective approach for an LN2 backup system is to link the freezer to a remote alarm system and establish a protocol where a designated person connects a fresh LN2 tank and opens the valve upon receiving an over-temperature alarm notification. This way, the backup system can be activated quickly and efficiently, minimizing the risk of material loss and ensuring the integrity of stored samples.

Eppendorf CO2 and LN2 Back-up System

Backup Compressor: A backup compressor system maintains ultra-low temperatures even if one of the compressors fails. Unlike traditional cascade refrigeration systems that depend on a single or multi-compressor arrangement, the backup compressor system features two independent one-compressor, auto-cascade cooling circuits. Each circuit operates independently, offering enhanced redundancy and reliability. In the event of a compressor or circuit failure, the other circuit can continue to operate uninterrupted, ensuring the integrity of the samples is maintained. At its core, the technological innovation of the backup compressor system is rooted in redundancy. Featuring independent refrigeration circuits that can each sustain ultra-low temperatures, this system minimizes the risk of complete system failure. This design boosts the reliability and durability of the freezer, offering crucial peace of mind for laboratories and facilities that store highly sensitive samples.

The temperature curve of -90°C normal setting.1

Key Differences to consider when acquiring a back up system

Temperature Range: LN2 can achieve much lower temperatures (between -80°C and -85°C) compared to CO2 (between -50°C and -70°C).

Storage and Handling: CO2 is typically easier to store and handle, due to lower pressure requirements and reduced frost buildup. 

Cost and Maintenance: LN2 systems can be more expensive to install and maintain, but they offer unparalleled cooling capabilities for demanding applications.

Emergency support: Please note that while CO2 and LN2 backup systems will be beneficial during a power outage, a backup compressor will not be effective. In the event of a power outage, both compressors will cease operation. Consequently, this approach safeguards against freezer failure, but not against the facility’s electrical failure.

Choosing between CO2, LN2 or backup compressor is totally up to you. Each offers unique advantages and serves specific needs, making the choice highly dependent on the particular requirements of the facility and the nature of the stored materials.

Another important consideration is managing the factors within your control. Regular maintenance is crucial to avoid unnecessary strain on the compressor and can significantly extend the freezer's lifespan. Tasks such as cleaning the filter and condenser, wiping seals, de-icing valves, defrosting, and replacing or repairing damaged components will reduce the compressor’s workload and help ensure long-term performance. Proper training is essential for anyone performing maintenance—they should be thoroughly trained and familiar with the manufacturer’s guidelines. While a typical maintenance schedule is provided here as a resource (download the article), users should always refer to the maintenance and calibration manuals for specific activities.

One of the key advantages of enhancing efficiency is the peace of mind that comes with better sample protection. By reducing strain on the compressor, the freezer's lifespan is extended, thereby lowering the risk of failure. Additionally, in case of a power outage, many of the features that boost energy efficiency will also slow down the warm-up time.

Lastly, it is essential to properly train laboratory staff on how to handle LN2 or CO2 tanks safely and effectively. Proper handling procedures can prevent accidents or injuries while also ensuring that backup systems are always ready for use when needed.

CO2 and LN2 Precautions2

The following precautions should be observed when using liquid CO2 and LN2 backup systems:

WARNING: If a CO2 or LN2 cylinder falls and its valve is knocked off, it becomes a dangerous, unguided missile. Always transport cylinders using a hand-truck or cart with secure chains. After connecting cylinders to equipment, ensure they are securely attached to a solid, stationary object like a building column with chains.

WARNING: Although CO2 and LN2 liquids are non-toxic, they are extremely cold and can cause severe burns to unprotected skin. Always wear protective eyewear and clothing when handling cylinders or working on piping systems connected to an active liquid refrigerant source.

WARNING: The gases produced by evaporating CO2 or LN2 are non-toxic but can displace oxygen in confined spaces, leading to asphyxiation. Never store cylinders in subsurface or enclosed areas.


  2. Manual for VWR ULT Upright Freezers (FDA Approved).