When you have precious samples to protect, sometimes an Ultra-Low Temperature (ULT) freezer is an absolute requirement. Whether it’s the first time you’re buying one or you have to replace an old unit, there are several important factors to consider. These units are understandably far pricier than regular freezers, and you need to know you’re getting the very best freezer for your needs.
We already discussed what people generally look for in a freezer, but here we’ll look closely at the factors you need to consider when purchasing a ULT freezer.
Of course, one of the most important features is how much a freezer can hold. All of our ULT freezers come in a range of sizes.
For example, So-Low Ultra-low Chest Freezers come in 10 different sizes 2–27 cu ft.
You also need to decide if you want a chest freezer or an upright freezer. An upright freezer is more convenient in terms of accessibility, so may be preferred if you need to frequently access samples. However, upright freezers are a little less efficient as during door openings, the cold air is more likely to escape, due to the tendency of colder air to flow downwards. As such, if accessibility is less of an issue, such as with long-term storage, you may want to go with a chest freezer.
The choice may also depend on the space you’re working with as the footprint of a chest freezer will be larger than that of an upright freezer with the same capacity. When considering how much space you have available to house the freezer, you need to account for ample room for ventilation. A suggested minimum clearance is 5” at the back and sides and 8” on top of freezer.
While the size and shape is important, so is the way in which samples may be stored. For example, having adjustable (or at least removable) shelves is a plus as it gives you flexibility in what size samples you can store and helps you utilize space better. It’s also important to note what size racks the unit can hold and how readily available these are. For example, So-Low offers a range of racks for upright and chest ULT freezers.
When it comes to cooldown time, there are two main parameters to consider. It’s helpful to know the pulldown time, which is how long the freezer takes to get to its setpoint temperature after installation or setup. Secondly, you’ll want to consider how quickly the freezer will return to its setpoint temperature after a door opening. This is especially important if you anticipate frequent door openings.
Another important factor is how long the freezer will stay within a given temperature range in case of a power failure. After all, you need to know how long you have before your samples are compromised. Some manufacturers will be able to send you graphs showing such temperature data.
Speaking of power failures, it’s prudent to look at whether or not backup systems are available for a given unit. For example, there’s an automatic backup system available for So-Low freezers that uses CO2 or LN2 to lower the temperature of the freezer if it reaches a specific temperature.
American BioTech (ABS) Supply Ultra Low Temperature Freezers come with a battery backup for electronics (such as alarms and controls) during power outages.
Temperature Stability or Differential
Most ULT freezers have a set temperature differential which indicates how far the actual temperature might deviate from the setpoint temperature. The reason for this is to give the compressor a break between cycles. This decreases wear on the compressor, extending the lifespan of the freezer.
You may want to know how much the temperature might vary from the setpoint at various points across the chamber. Knowing the minimum and maximum temperatures can help you decide if the freezer is even suitable for your application, and if so, what an appropriate setpoint temperature is to ensure the integrity of your samples (by guaranteeing they’ll be kept at a particular minimum temperature).
For example, if the maximum temperature is 2°C above the setpoint and you need your samples to be at a maximum of -70°C, then the setpoint temperature should be at least as low -72°C.
Some units enable you to set the temperature stability yourself. For example, ABS Ultra Low Temperature Freezers are typically set to +/-5°C (the range can be set by the factory or the user). The manufacturer does recommend not setting the differential any tighter than +/-3°C as the rapid cycling will increase wear on the compressors and decrease compressor life.
Temperature deviations can spell disaster for your samples, so you want to know if anything has gone awry. Having sufficient warning systems in place can give you chance to address the issue before it’s too late.
Some units come with visible and audible alarms that signal things like temperature fluctuation or power failure. For example, freezers in the So-Low ULT range, such as the So-Low Ultra Low Mini-Freezers (below left), have audible and visual alarms that signal when there is an electrical or mechanical failure.
The alarm on the So-Low Platinum Series Ultra Low Freezer (above right) also has an over and under-temperature setting.
According to a 2014 US Department of Energy report, at the time of the study, a 25 cu ft. -80oC freezer used around 20 kWh per day, which is about the same amount of energy as a small house. However, that was five years ago and advances in compressor design, better insulation, and improved cabinet organization have enabled manufacturers to improve the efficiency of ULT freezers.
That being said, energy costs remain high, so it’s important to look at the financial impact of running a ULT freezer, especially if you plan on purchasing multiple units. According to a 2016 report by the University of California Riverside Office of Sustainability, an average new unit consumes 17 kWh per day. In contrast, an energy-efficient unit consumes on average 8.86 kWh per day. This ties in with Energy Star’s listings, with the most efficient ULT freezer on the list consuming around 7.52 kWh per day.
Aside from looking at energy consumption, it’s a good idea to look out for key features that will improve the efficiency of a unit, such as double-walled doors and additional insulation in critical areas.
A ULT freezer is a powerful machine so it’s worth checking the required voltage and wiring conditions for a given unit. The larger an ultra-low freezer is, the more likely that it will only be available in higher voltages.
There are several voltages available for the So-Low Ultra Low Upright Freezers.
There may also be potential issues with voltage fluctuations, especially if your lab is in an older building. If the voltage is too high, it could cause failure of components due to overheating. Too low a voltage increases the amperage which can again lead to overheating. In either case, the lifespan of the machine will be reduced.
Some manufacturers will provide a recommended maximum voltage fluctuation, for example, 10% of the nominal voltage. Certain units come with features that overcome voltage issues such as a voltage stabilizer for fluctuating voltages or a voltage booster for low line voltages. The latter is a feature of the ABS Ultra Low Temperature Freezers
As with any expensive piece of equipment, you’ll likely want a guarantee that you’re not going to have to spend money on repairs, at least in the short term. Most manufacturers offer at least a one-year warranty as standard, and some have option to pay for an extended warranty.
In So-Low’s case, the initial 12-month warranty comes with your purchase and includes parts and labor (including compressor issues). An optional extended warranty can be applied to the compressor for up to four years (five years in total from the purchase date), although this doesn’t cover labor costs.
With American BioTech Supply, units typically come with a two-year parts and labor warranty and a three-year compressor parts warranty.