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Re: [Rollei] More OT Jobo advice

In a message dated 5/6/2003 12:58:05 PM Eastern Standard Time, dseifert   writes:

> Only repeating claim made by Jobo.
> David
> At 10:31 AM 5/6/2003 -0700, you wrote:
> >On Monday, May 5, 2003, at 05:25  PM, David Seifert wrote:
> >
> >>  Temperature control +/- .2 degree vs. .1 degree (matters for C-41 color)
> >
> >Just a comment-- I don't know of any commonly available thermometers that 
> >indicate 0.1 degree gradations.  My JOBO thermometer is calibrated in 0.2, 
> >and is at least 1.0 degree off from my Paterson, Kodak, or Frigidaire 
> >mercury thermometers.  So verifying the accuracy of temp control is an 
> >exercise in frustration.  C-41 development time is
> >commonly 3m 15sec, so rigid temp control doesn't have much meaning for 
> >single processing runs.  Although, it definitely is desireable on 
> >day-to-day or week-to-week turn-off/ turn-on repeatability.  I'm just a 
> >little skeptical that such tight tolerance is actually achieved in these 
> >machines.  And I'm also skeptical of the dire warnings about the need for 
> >such tight control.  Nice to have maybe, for critical commercial work, but 
> >for run of the mill amateur happiness, "no worries, mate".  I try to keep 
> >within 0.5 degree, so don't try to pin that 2 or 3 degree 
> slop on me :-)
> >
> >pk
> Best Regards,
> David Seifert
> dseifert  

As a bunch this technically disposed surely knows, repeatability and accuracy are two different matters entirely. Both can be important, but for most processes it's repeatability that you care most about. 

I'm not sure if the Jobo CPP2 is truly "accurate" because there's nothing easily at hand to verify it with, and as noted, mercury thermometers are hard to read to within .1 degree.  However, it sure seems to be repeatable. Mine is usually set at 24 degrees C and the reading will oscillate back and forth between 23.9 and 24.0 but not go outside this range.  Not much hysteresis in the sensor circuitry.

Someone mentioned the idea that the CPE was advantageous becaus it held LESS water.  It seems to me that from a thermal stability point of view this is wrong, and that the larger thermal inertia represented by the 4.5 gallon fill in the CPP2 makes it easier to maintain a fixed temperature.

G. King