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  • Karel Schmiedberger

Which units to use when freeze-drying food and why Torr is the best choice

Evangelista Torricelli

Those who read our blog regularly, or who have not been afraid to call me and ask me everything they are interested in regarding freeze-drying, may have come across our unconventional determination of pressure, or vacuum. So today we'll take a brief look at the options on offer and why everyone should use Torr.

Bar (formerly atmosphere) or pascal as units is probably familiar to anyone who has ever inflated a tire on a bicycle or car. However, vacuum is essential in lyophilisation. The ordinary European would probably be the first to think of pascals or bar, as these are SI units. I have even once been asked what we have in these American units, and he was referring to the Torr unit.

Below, we'll look at all 3 units and say the same values for each

1. Atmospheric pressure

2. The pressure at which AMARU begins lyophilization

3. the minimum vacuum reachable in the AMARU chamber


The bar is a simple unit, popular for its illustrative nature. Everyone knows that 1 bar is equivalent to atmospheric pressure. After all, that's why the term "atmosphere" used to be used. In the SI system, it is a secondary unit that will probably be removed from the system altogether at some point, as its inclusion is marked as temporary.

1 bar

1,333 mbar

0,133 mbar


Pascal is a derived unit of the SI system. It also can be commonly found on compressors at gas stations, but is probably better known in meteorology due to the derived unit Hectopascal (hPa). Pascals and their multiples are sometimes required for laboratory lyophilisers

101.325 Pa

133,322 Pa

13,332 Pa


The torr is a unit of hydrostatic pressure, named after the Italian scientist (so much for the alleged "American unit") Evangelista Torricelli. Older generations may know it as the "millimetre of mercury column" with the designation mm Hg. As with bars, we use the unit Torr, and just as we use its thousandth militorr (mTorr).

760 Torr

1000 mTorr

100 mTorr

And now to the reasons why we recommend using Torry for working with the freeze dryer. Many laboratories do so because this unit is simply operated by scientists around the world and is used to express vacuum in various scientific publications.

However, for the processor of freeze-dried foods there is a much simpler reason, which is probably evident from the three values above, and that is readability and memorability. It is simply easier to remember a limit of 100 mTorr than, say, 13,332 Pa or 0,133 mbar.


A little tip at the end - if you're floundering when converting units (not just pressure), try Aevumsoft's Unit Converter.

We have been using it ourselves for years and are extremely happy with it.



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