Only immersion thermal circulators specifically designed for culinary use should be used for food prep. This guide will explain which types are not suitable for culinary applications.
As my primary experience is as a chemist, and therefore experience is with laboratory brands of immersion circulators, this guide centers more on which units are laboratory units, that are usually encountered on the used market.
I will include comapanies I am experienced with that manufacture for both markets. These companies have applied years of experience in the laboratory market to the culinary market..
Immersion thermal circulators such as the Polyscience 7306C are built with food grade stainless steel parts. They should be used with clean water, it is necessary to change the water frequently to prevent corrosion or scale build-up.. Circulators built for laboratory use are designed to be used with distilled/deionized water. Of course this isn't always done, and many have corrosion and/or excessive scale build-up on the immersible parts. Another cause of corrosion is contamination with laboratory chemicals, or the wastes being analyzed.
I am a chemist with 10+ years experience in process, environmental, and hazardous waste laboratories. Over those years I have used immersion circulators in all of these settings. I also ran a microbiological testing lab for a few years. We used immersion circulators there also. I currently test, and rebuild laboratory equipment for Lehman Scientific.
There is no way to clean a used laboratory immersion circulator sufficiently, to be able to guarantee total decontamination. Chemical residues will lodge in screw threads, between parts, in the pump blades, and even be bonded to the metal parts themselves. I have even seen chemical residue inside the control box, probably from being in an over-filled bath. These residues may leach into your bath and contaminate all they come in contact with, potentially contaminating your sous vide, or surfaces in your kitchen. Also as in above, the parts in a laboratory immersion circulator will corrode if exposed to acidic conditions, or liquids containing salt. This will contaminate your bath, with metals, some of which may be toxic. Off flavors may even be introduced into your food.
Laboratory circulators may have the following names/part numbers. These should never be used for food prep.
The most common are:
Model 70, labeled as Polyscience*, Fisher Scientific, VWR, Cole-Parmer, etc.
Model 730, labeled as Polyscience*, Fisher Scientific, VWR, Cole-Parmer, etc.
Model 2150, Fisher, Isotemp, Polystat, etc.
Any immersion circulator labeled as Fisher, VWR, Cole-Parmer, Daigger, Haake, Lauda, Thermo, etc.
*See Polyscience below
There are many immersion circulators designed specifically for sous vide. They can be bought here on ebay, with prices ranging from $150 to over $1000. Like any item, the quality will vary, as companies rush an immersion circulator to market to get in on the it.
Polyscience* is one of the lab immersion circulator manufacturers that have adapted their expertise for the culinary market. Julabo is another example of a lab immersion circulator manufacturer using its expertise to enter the culinary market.
Be sure to ask fellow chefs about their experience with the low end units, vs. the more expensive ones. made by companies with decades of experience designing and manufacturing immersion circulators.
The model Polyscience 7306C retails for $900+ which can be a lot of money to a start-up. However scoring one of the laboratory ones on ebay for a hundred bucks or so is not a good deal as the unintended consequences of using one may be very costly.
If anyone has any question feel free to contact me.