View Full Version : PMK Development Time-Temp Curve
Reading the thread on compensating timers has raised (for me) the question of temperature compensation for PMK. The instruction sheet gives the time reduction for increased temperatures above 68F or 70F, but says nothing about lower temperatures. While summer is coming, my NJ basement darkroom still hovers around 62F (I was just down there printing), raising the obvious question of whether anyone knows PMK compensation factors for temperatures below 68F. I tray develop, and start my processing at 70F, but obviously until summer I have temperature drift downwards. My ZoneVI compensating timer (yup, I'm one of those, having attended one of Fred's courses a very long time ago, and ended up with a very ZoneVI-equipped darkroom!) works well for prints, but I've had some less happy results with PMK in trays. So anyone with curve data (or their experience at specific temperature points) would be great!
Why not merely warm your developing chemicals to 68-70 degrees?
The components in developers don't work as well in colder temperatures.
If they did work, development times would become extremely long.
My temperature conversion chart only goes down to 18°C (66°F) as well. Many developing agents lose activity when it gets colder than that, so it's probably not a good idea to develop at lower temperatures.
I use a Zone VI compensating timer, with the probe in the water bath for tray developing 4x5 in PMK with great success. I do start with fairly warm developer (21°C) in winter, however, to make sure the temp does not drop too low, which may affect the developer activity. If your developer ends up below 18°C, then this may be the cause of your inconsistencies.
If you want to not use your compensating timer, the thing to do when the ambient temperature is consistently low, is simply start with a bit warmer developer. As mentioned on the thread you referred to (by me :-), the effect of a different ambient temperature on a developer, especially when tempered in a large-volume water bath, is not that great.
Take your developer's temperature at the end of the developing time to see if the temp dropped a lot. If you temper with a water bath, I'll wager it will be less than a degree C, which, over a 15-minute developing time is about a 10% or less difference. This is within parameters for many. However, if you need more accuratcy, there are a couple of strategies to get closer than this.
The first is to just aim for repeatability instead of consistent temperature. For example, if my darkroom were always on the cool side, I'd just start with 21°C developer and develop, using a fairly large amount of the same temperature water as a water bath for the tank or tray. The temperature drop for any given developing time would be fairly constant as long as the ambient temperature were the same or similar. If you establish your developing times under those conditions, then there is no problem at all. Any temperature drop will be compensated for by your calibration to those conditions. The only concern would be if you find that your developer temp has dropped below about 18°C by the end of developing time. Then you'd have to start warmer or control ambient temp more.
Remember also, if a temperature rise or drop does occur, the median temperature is, for all practical purposes, the one you can consider the overall developing temperature. Therefore, another approach is to measure developer temperature shortly before the end of development time and find an adjusted time based on the mean temperature. If, for example, you started at 20°C, and just before the end of developing your developer measured 18°C, then you would just use the 19°C time from your conversion chart. For a developing time of 15 minutes, that would mean 3.5 more minutes in the developer. If you use a steel tank or tray in a tempering bath, you can simply measure the temperature of the water bath. (This can be a bit tricky when working in total darkness; one would need a thermometer with a luminous dial, but ensure that it doesn't fog film...)
In the case of higher temperature, one can simply develop at ambient temperature (using solutions at the same temp, of course) and then use a temperature conversion chart to find the resulting shorter developing time.
Sorry this has got so long, but the bottom line is: If you always develop in cooler than normal conditions, just establish a repeatable routine and test to that. If you don't, you need to control the temperature environment by regulating ambient temperature or adjusting developing time in one of the ways mentioned above.
One more note: the temperature probe on my Zone VI time went bad once; times were all over the place. You may want to check and see if yours is functioning properly.
Hope this helps,
Can you use a space heater to bring up the room to 68-70 deg.
All, and especially Doremus for his detailed suggestions, thanks! I think my first approach will be, as Doremus suggested, to use my ZoneVI (its on its second probe, which seems to be working properly, the first one died) and simply start my developer warmer than I have been, so that any downward drift still remains in the active temperature zone. And "mother nature" seems to have heard me, today here in New Jersey it was 20F warmer than yesterday, perhaps we are finally getting seasonal weather, which will pretty much eliminate my basement problem for a while. (Bob: my wife also suggested the space heater for the colder months. I was resisting because my darkroom is not sealed off, but rather just one corner of the large unfinished basement area, so I thought the space heater would primarily be dissipating its warmth over too large a space. But I will keep the suggestion in mind.) Again, thanks to all who responded.
I found a magic combination for the winter in California. I run 75-degree F tempering water running slowly. The ambient temperature, stainless steel sink and trays all seem to absorb 7 degrees F, leaving me at a steady state of 68-degrees F.
I know in the summer, my tapwater runs 70-degrees F. So then I put a coiled hose in an ice-chest and run the water through slowly. I can get it to come out 68-degrees F for about 8 hours on a bag of ice.
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