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.

 

[Scott J.]

@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.

I agree that hysteresis isn't the only source of temperature instability in these processors, but it is still a source. Used, similar-condition CPA2s and CPP2s seem to sell for around the same price these days, so if I'm going to invest in one, I'd rather invest in the model that performs better in that regard. I don't know if there's a difference between them, hence the request for help.

 

[Scott J.]

I don't think there's anything different with the sensor.

Thanks, that's exactly what I was looking to find out. And thanks to the others for sharing their positive experiences with the CPA2. That's a big help knowing that, at least with regard to that one characteristic, the two machines can be treated as performing the same.

 

[koraks]

I agree that hysteresis isn't the only source of temperature instability in these processors

My main point is that arguably, hysteresis as such is not a source of temperature instability to begin with, if we limit the requirement for temperature stability to hitting the target temperature on average for the duration of the development time. Very simply put, it just doesn't matter whether the hysteresis is 0.1C or 0.5C or 0.001C. What does matter is whether there's drift, and this is not necessarily dependent on or associated with hysteresis.

 

[mshchem]

I want one of you research types to put a low mass TC and recorder/transmitter inside the tank to measure the actual solution temperature that the film sees. It could be an instrument package attached to a strip of polyester 35mm film base that threads onto a spiral.
There's the assignment, my work here is done!

Eventually this communicates directly with the processor, wirelessly

 

[koraks]

I think that's already been done. There's at least one commercial product out there (not a Jobo) that relies on a probe that sticks into the tank and measures solution temperature.

 

[mshchem]

I think that's already been done. There's at least one commercial product out there (not a Jobo) that relies on a probe that sticks into the tank and measures solution temperature.

Yes, but it doesn't control the temperature. The nonsense that drifting temperature and time compensation is equal to a proper processor is well, nonsense. Black and white printing, compensating timer for a range of say 17°C to 25°C sure, absolutely, but not for proper film development. If you don't intend to make proper color prints from negatives, or project transparencies I guess it doesn't matter

There's a right way to do things. I give credit to the OP for fretting about these things

 

[Scott J.]

I wanted report back with a little data that addresses the question in my original post. I found a CPA2 in basically unused condition for what I considered to be a good price. It's an earlier serial number (first upgrade only) but my research on the issue of serial numbers and Expert drums seems to indicate that an earlier unit with at least the first of the three motor and circuit board upgrades is typically fine as long as you don't drive the motor too hard by filling the drum to its volumetric limit (e.g., >1,000 mL). Time will tell, but it's an extremely clean unit and I anticipate I'll at least get my money's worth out of it.

I set up an experiment to measure the variation/hysteresis in water temperature in the upper trough under real-use conditions (e.g., with semi-filled 1-liter bottles in the lower trough, with a drum rotating on the lift, etc.). On my unit, I had to set the analog dial to 36 deg. C to achieve an upper trough water temperature of 37.8 deg. C (to mimic C-41 development), but the inaccuracy of the dial doesn't bother me and is a known shortcoming of the CPA2. I recorded the temperature change over the course of 10 minutes and noted when the heater turned on/off. In short, I was very pleasantly surprised by the results, which are plotted below:

The temperature stayed between 37.6 and 37.9 degrees C for the entire 10-minute test, which gives a hysteresis value of +/- 0.15 deg. C. Keeping in mind what Koraks mentioned previously, I also calculated a time-averaged (as opposed to an arithmetic average) temperature, and got 37.78 deg. C, which is negligible error for C-41 development. This should all be taken with a grain of salt, of course, considering that the trough temperature may not be reflective of the temperature inside the drum, but these results do, at least for me, settle the question of whether the CPA2's thermostat maintains an adequately narrow hysteresis band (it does).

I'd like to repeat this test with the addition of measuring the internal drum temperature, but I need to track down a long, flexible PT100 sensor that can be snaked down through the lift and into a 2500 drum (might have one at work). Expert drums don't use a center core, so there's no easy way to insert a temperature probe into an Expert drum, hence the need to perform the experiment using a 2500 drum. Barring a flexible PT100 sensor, I might settle for using a flexible type-K thermocouple that's been calibrated to the PT100 sensor in the upper trough. I'll report back when I can.

 

[DREW WILEY]

Seems to be the same heating mechanism as in the Jobo tempering box, which is quite reliable in holding temperature within reasonable limits provided you start out with water just slightly below your necessary working temperature. In other words, it's neither a water heater per se, nor a highly precise thermoregulator, which would cost over a thousand dollars just by itself if that were that the case. The nice thing is, that if you accidentally turn the Jobo on dry, a sensor will act like a little circuit breaker which needs to be reset, to prevent the unit from overheating.

My Jobo tempering box keeps the water jacket within plus/minus 2F, while my true industrial grade thermoregulator can do it inside plus/minus 0.1 deg F; but that's overkill for typical development needs, and uses a lot of wattage, so I rarely bother to set it up anymore.

A basic aquarium pump can be used to keep the water circulating if you see the need, but itself adds some temperature.

But what is even more important is the consistency of temperature INSIDE the drum itself when development is occurring. That depends on several factors: ambient air temp itself, how effectively you preheat the inside of the drum using an effective water pre-rinse, the volume of your development solution, the length of development, and how well your drum either insulates itself from external temperature or else transmits the water bath temp through the drum walls. Being thin plastic, Jobo drums are somewhat of a compromise in that last respect. So you can't outright ignore ambient air temp if its uncomfortably outside the range from your desired solution working temp.