Jobo CPA2 vs. CPP2: Are their temperature hysteresis bands really the same width?

[Scott J.]

I'm planning to upgrade from my CPE2+ to either a CPA2 or CPP2 so I can use the Expert drums, but as I primarily use the Jobo for E6 and C41 processing, temperature stability is something I'd like to maximize, where possible. My CPE2+, to which I've added an aquarium pump for water circulation, does pretty good in that regard—I'd estimate it exhibits about +/- 0.5 deg. C of hysteresis when processing, meaning: if the dial setting produces an average water temperature of 38 deg. C, the heating element will turn on if the water drops below 37.5 and turn off if it exceeds 38.5. I use a digital reference thermometer with a pt100 probe to confirm temperature accuracy within +/- 0.1 deg. C before starting a processing run, so the accuracy of the dials on the unit isn't really a concern for me (i.e., if the Jobo dials have to be set to 37 degrees to produce a water temperature that I know from external measurement is 38 degrees, that little hiccup isn't a big deal to me). Rather, my interest is in how the CPA2 and CPP2 compare in terms of the narrowness of their respective hysteresis bands.

I've dug through about 20 years of forum posts on here and a couple other sites, and although the general consensus seems to be that the temperature control system on the CPP2 is more advanced, it's not always clear which aspect of the system is being praised (e.g., "user convenience," "dial accuracy," "temperature stability," etc.). Interestingly, the Jobo FAQ on the Catlabs site seems to indicate that the CPA2 and CPP2 perform identically in terms of hysteresis. Specifically, of the CPA2 (and CPE2, for that matter), it says: "Though the [analog] dial itself is not very precise, the thermostat is accurate up to 0.1C. The system is designed to be used with a trusted thermometer, such as the Jobo #3321 process thermometer... The temp is set against the thermometer reading, and the dial is adjusted as needed. Once the desired temperature is attained, the machine will hold it within 0.1C."

And of the CPP2, it says essentially the same thing: "This system is also accurate to 0.1C. Because the [LED] readout displays the actual temperature, there is no need for an auxiliary thermometer."

The +/- 0.1 deg. C hysteresis claim contradicts my experience with my CPE2+, so I'm wondering what users of the CPA2 and/or CPP2 would say about their machines. How narrow are their respective hysteresis bands (particularly in the region of 38 deg. C), and do you think one model is noticeably better than the other in that regard? I realize there are other considerations to make when choosing one of these units (e.g., need for a cold-water solenoid, motor upgrade type, etc.), but I'm specifically hoping for feedback regarding the issue of temperature stability. Thanks in advance for sharing your insight.

 

[wiltw]

I have successfully used a CPA2 to process Cibachrome color prints (a.ka. Ilfochrome), and printed one print for a group Xmas exchange, producing almost 3 dozen prints having absolutely identical results for the print exchange...so the stability of the temperature control was not ever questioned. During use, I never noticed any hysteresis, and one would not think that there would be identical looking results across dozens of prints of the same image. (The CPA2 can process E-6 and C-41 color film also. )
I just looked at Jobo original documentation and they state that the differences are that the CPP2 has a 3-dial electronic temperture selection thermostat and a +-0.1degreeC 'temperature accuracy' (Jobo's exact words) and an LED readout of temperature, while the CPA2 tolerance is +-0.2degreeC tolerance and the temp conrol is a single dial it comes with a conventional thermometer.

 

[Sirius Glass]

I have nothing but praise for the CPA2 processor.

 

[mshchem]

I don't think there's anything different with the sensor. I would look for low mileage, less to go wrong. The advantage goes to strongest motor etc. Be careful of the "birds foot" switch that reverses the drum, I fought that on a very cheap, very used CPP2.

 

[Robert Ley]

I used a CPA-2 for many years for C-41 and B&W film. For C-41 it was able to hold temps within about 1/2 degree C and it worked great for 120 and 4x5 with the 2905 reels and an expert drum. I second the suggestion of getting one with a later serial number as it will have the stronger motor. Not sure of the cutoff for the motor but you should be able to find out on the internet. Mine even though it was an earlier unit would handle the expert drum just fine.

 

[koraks]

@Scott J. the question is how relevant it is to focus on hysteresis. Arguably, it doesn't really matter as long as it's within a reasonable bandwidth and the average doesn't drift too far off the target. What you don't want at least with E6 is a systematic drift upward or downward during the time of the first developer so that the average temperature deviates too far from the target. But even if the hysteresis turns out to be 0.5C or so, as long as the average is fairly stable, you should be fine.

I would in general also not overcomplicate this. Yes, you can focus on hysteresis of the PID or thermostat. But if you're going to aim for +/-0.1C on that parameter, you'll then have to contend with local fluctuations inside the tank. I bet that's not a homogeneous temperature either in practice, and in reality there's no way to even find out what the distribution is. Focusing on one parameter just because you can control it doesn't mean the rest of the system will be stable. Of course, that doesn't mean that any kind of control is useless, but I do think there's very limited utility in trying to "fix" the (unbroken) CPA2 temp control system.