Temperature drop in Jobo Tank
I just finished temperature drop measurements with 38°C water in a 1520 Jobo tank, here is the test setup and the results:
- Jobo 1520 Tank with both film spools but no film
- Tank preheated by filling it with 38°C water twice and leaving it in the tank for 30 seconds both times (which is incidentally the method PhotoEngineer recommends for preheating)
- No efforts made to keep the tank warm, I just put it on my washing machine during the experiment
- The thermometer is a color thermometer with 0.2°C scale resolution
- The thermometer would stay in the film tank, which means the lid was open throughout the test.
- After 25 seconds I cycled the tank for 5 seconds to simulate agitation - note I couldn't tilt the tank because there was no lid on it.
- This 25 + 5 second step was repeated 20 times for a total of 10 minutes.
- Room temperature was 28°C
Here's the results:
Here's the interpretation of the results:
- Initially the temperature seems to rise slightly. I blame this on the fact, that the thermometer takes some time to reach its correct temperature. So we can safely assume the temperature during the first minute was about 38.2°C
- During the first 6 minutes temperature drops very slowly. This may be the case because the tank is still at 38°C from the preheating. This is the most significant time frame since even E6 developer takes 6:45 at most (unless one pushes slide film)
- After 6 minutes the outside of the tank has cooled down to room temperature so the water inside cools faster. The total temperature drop is still small, only 1°C after 10 minutes
- The temperature drop measured is slightly higher than what I expect during real film development, since the tank lid remained off during the measurement.
- The expected temperature drop will be larger and start earlier if room temperature is lower, which I expect to happen from fall till spring. The difference should not be all that great IMHO.
- For this experiment the tank was completely filled with dev. Therefore the results may not apply to rotary processors, where less dev is used and the temperature may drop faster without extra heating.
Given these results I am confident that for hand development I do not need any tank warmers or similar rigs to keep the temperature well controlled inside my dev tank.
Good news. I suspect that the temp change would have been zero for typical B&W dev used at 24C. At the temp you need for colour neg developer and time of 3 mins 15 secs there is no temp drop at all which is great news if C41 is ever your desire.
The problem if there ever is one might come in the winter unless you can maintain a room temp of 28C. I don't do E6 so I have no idea how crucial temp change is but it might be worth re-checking in say Dec/Jan. Might be risky to assume that the drop won't be all that great.
Assuming the room temperature ever gets as low as 18°C, you can expect about twice the temperature drop, which is still only about 1°C after 6:30 (first E6 dev time after a few uses). After about 1 year I can say I never saw any problems with this type of processing, just wanted to measure whether I was lucky or blind or whether the temperature drop is really low enough to be insignificant.
I tried this experiment today using a Jobo 2830 print drum (my normal drum for 2 sheets of 8x10) on a Unicolor motor base. Target temperature of 38°C (100°F), room temp of 24°C (75°). Two pre-soaks of 30 sec. each at 38°C using 400ml of water for each, then a 3:15 minute cycle of 200ml of water at 38°C to simulate C-41 developer. The temperature of the "developer" after the time expired was 34°C (94°F). This is too much drift to use C-41 without a water bath in my opinion.
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I tried again, this time filling a tray with hot water for a few minutes to heat the tray, then replacing that water with some at 38°C, then repeating the above procedure with the tray acting as a water bath, thus replacing the motor base, and agitating by hand. After 3:15 minutes, the water bath and developer were again at 34°C. I am trying again with a standard electric heating pad under the tray and will post the results once I am done.
Due to unsatisfactory results with C41 process and my CPE2 I ran a simpler test today with CPE2 and 2521 drum with standard reel. BTW, I have an aquarium pump circulating the water and everything set up like it should.
Room temperature was about 22 degrees Celsius.
Prewarmed the drum for more than 5 minutes, with the water bath at 38 degrees. Poured in 500ml water which had a temperature of 37.8 degrees. After the 3min 15 sec "development" time the temperature in the drum was 37.0 degrees. This is well below the temperature tolerance of the development, which is 38 +/- 0.5 degrees celsius.
Then I adjusted the water bath to 39 degrees. Poured 500ml water with the temp of 38.5 degrees into the prewarmed drum. After 3min15sec development time the water in the tank had a temperature of 37.8 degrees.
The result showed that under these conditions the temperature in the tank goes down well below the temperature of the water bath. I wonder if a really constant temperature is even achievable with Jobo CPE2/CPP2/CPA2 processors? Probably not, as most of the drums surface is exposed to room temperature air all the time. Perhaps I should use the CPE2 in my sauna....
There is an experiment posted elsewhere here on APUG that shows the useful effect of a good prewet in maintaining the temperature of tanks.
I have found that the Jobo controller and the sump of water in my Jobo, combined with a prewet, keeps my temperature at 100F regardless of the time and regardless of the ambient temperature.
I have two Jobo units and have tested this with both and with the huge variety of tanks that I have.
If you adjust the water level in the Jobo processor correctly, the rotating drum carries a continuous or nearly continuous layer of tempered water clinging to the outside of the drum.
Assuming the water bath temperature is stabilized, the heat from the continuously replenished 38C water layer is absorbed by the walls of the drum preventing any significant temperature change to the tank, reels, film, and chemical solution inside.
You might need to fine-tune the position of the adjustable water level drain to ensure proper water depth to carry the required amount of water on the outside of the rotating drum.
I adjust the water level only slightly less than that which would make the drum float upward off of the rollers to maximize the water layer adhering to the outside of the drum. In this way I obtain the most stable temperature for the process.
Even if your process cools more than you want, so long as the difference is minor, you can compensate adequately by simply increasing the time until the density of the negatives are correct.
For the small temperature drift cited in post #7, increasing the time from 3:15 to 3:30-3:40 ought to solve the problem.
If one wants to develop C-41 or E-6 and doesn't have a rotative processor, I suppose he should rather employ a warm water bath where to place the tank during development time. With a minimal hassle, he would greatly improve its operations IMO.
In the described experiments the tank is in contact with room air. This causes two problems:
- Room air is at a low temperature and chills the tank;
- Room air is at a different temperature in different seasons or times of day, and doesn't allow repeatability.
If the tank is immersed in a warm bath (fill the sink with hot water) the "slope" of the temperature graph should be greatly reduced and one might have a system which is an acceptable surrogate for a rotary processor: the tank will be chilled more slowly, and room temperature will have less of an effect (its effect being "indirect" on the tank) so that after a few experiments the development time (which might be slightly longer than the standard 3:15, e.g. 3:30) will give more repeatable results between seasons.
Also, the temperature in the sink can be somehow controlled by just opening the hot water tap.
That would probably be a reasonable compromise: no thermostatic control, no pump, no motor, but much better than just letting the tank chill in room air.
PS Actual developing time for Fujichrome using rotary processors is 7:30 for first use, I use 8:10 for second use.
Kodak advices 7:00 for first use in document Z119-10.