Hi guys -- love where my favorite thread gets going when I've turned my back (sorry, but speaking of high temperatures: we're just coming out of a heat wave over here where we hit record highs -- 44 C/112 F yesterday! Just the thought of standing over a hot stove undoes me ...)
VERY interesting discussion regarding the similarities/differences between cookers and canners. Here's my hubby's take on it all (asked not because he's into canning in any way other than eating the results, but because he's an engineer who regularly deals with large pressure vessels (hospital steam/pressure sterilizers) as part of his job, so one hopes he has the theory right!)
He seconds virtually everything Michael has said: pressure canning is about sterilization ... sterilization is relative to time and temperature ... and, in pressure vessels, temperature is relative to pressure. So whether the vessel in question is called a "cooker" or a "canner," it's all about what maximum pressure it's rated for in terms of whether it will TECHNICALLY do the job ... and thereafter it's more about its user-friendliness as to whether it makes sense to use it for the job (just re-phrasing Michael here!).
My pressure "cooker" for instance -- a very nice thing made in Germany by Fissler -- is rated to a maximum of 1.5 bar of pressure so it's obviously adequate (1 bar = 15 psi = 250F = 121C ... and the 121/250 is a known sterilization standard temp) ... BUT ... it doesn't have a rack on which to set the bottles ... nor does it have a handle to help remove those same bottles ... nor does it have the capacity to hold anything other than four half-liter bottles ... nor does it have a neat "15 psi" gauge to plunk on to know you're in safe territory.
Mind you, since I have said hubby and said hubby has cool tools hanging around like high-tech thermometers that use a little wire sensor, I could actually get around much of this and just gauge the internal temperature. However, presuming most people don't have access to such thermometers, they couldn't use the Fissler "cooker" (or similar, obviously) with such certainty and thus, just maybe, would be well advised NOT to use it for canning (it does have two heat/pressure settings on its dial-like gauge, the higher of which supposedly corresponds to 119C, just shy of the 121/15 psi which is recommended).
Meanwhile, honestly? Mostly I'm just glad that I have zippo interest in canning low-acid foods and thus all of this is moot!