Calibrating a Zone VI compensating developer timer

[Payral]

Something similar: https://rhdesigns.co.uk/product/process-master-iim/
May be you can ask there.
I use a Zone VI compensating developer timer from around 1977 and it still works like a charm.

[neil poulsen]

My zone vi compensating developer timer is running too fast at higher temperatures. It works well enough from about 66F to 70F. Inside the unit there are two adjustment screws labeled PCAL and RTCAL. Does anyone know what these do, and the method for calibrating?

This is really interesting. I've used my timer for at least a couple of decades or more, and I didn't realize that these adjustments were available. (Yippeee! )

It looks like the two adjustments are a standard intercept/slope calibration. I'm not sure what the "P" in PCAL stands for, but the "RT" in RTCAL likely stands for "rate", which is another word for slope.

During the 90's, I did a lot of experimentation with my compensating timer, collecting data, etc. I determined that the base temp was 68 degrees, because it was at this temperature that time on the timer was the same as wall-clock time. I also was able to experimentally determine the exponential/logarithmic equation used to determine the time duration at a given temperature. It's really quite a simple matter. I can't remember the exact procedure that I used, but a similar procedure that would do the same would be the following.

Pick several temperatures, let's say 66, 67, 68, 69, 70. 71, 72, 73, 74, 75 and use a stop watch to time how long it takes for three minutes to elapse on the timer at each temperature. I set up a water bath, and for each data point, bring the water-bath temperature up to each of the above temperatures. At each temperature, one can measure elapsed timer-time on the stop watch (as described above) by putting the thermocouple into the water bath. (The water bath temperature changes very little during three minutes.)

This gives ten datapoints in the form of ten ordered pairs, (temp, time), with temp as the independent variable and time as the dependent variable. I have statistical software that I can use the model the data and come up with the formula that gives "time" as a function of "temp". But just about any scientific calculator that includes statistical capability could do the same. (i.e. a TI 84 Plus) It's a very common family of curves. It was clear that I had the correct formula, because it almost perfectly fit the ten datapoints.

What's interesting is that, with the two PCAL RTCAL adjustments, one could potentially calibrate the timer to their own film-developer combination. (Richard points out that it depends more on the developer than on the film used.) It requires a densitometer, and one could expose several sheets of film at exposed at Zone VI. Etc. Etc. (During a workshop, John Sexton commented that Zone VI is most sensitive exposure to changes in development and development temperature.)

I could speculate on a procedure. But it gets a bit complicated. Let me know if anyone's interested.

But, it's not really needed. In using my compensating timer, I do all my Zone System calibrations and my developing at 70 degrees, and I don't let my temperature vary from this by much more than a degree or so. I use my timer to compensate only for these small differences from 70 degrees in temperature. The factory calibrations are sufficient to provide compensation in this small range.

For example, I would not let my developing temperature vary by as much as say 4 degrees, and then expect my timer to compensate for that huge difference.

[Keith Pitman]

From my notes accumulated over the years, the Zone VI Compensating Developing Timer has four potentiometers (POTS). Two are visible on the face plate and are labeled for adjusting display brightness and sound volume. Two others can be found by removing the faceplate. Of these two internal POTs, the right hand POT adjusts the "real time" timer and the left hand POT adjusts the compensation factor. For both POTs, a clockwise turn shortens the timing, and counter-clockwise lengthens the timing. It does not that much of a turn to make a significant adjustment.

I have adjusted the real time timing on my timer a few times as it seem this drifts over time. I have never adjusted the compensation POT. It seems adjusting the compensation POT would take some experimentation with developer temperatures versus developing times. I have not perceived that there is a significant variation in my timer.

[Edward Pierce]

So the "RTCAL" on the right is real time calibration, and "PCAL" on the left is the compensation factor, thank you.

I actually have two of these timers. Timer B is my original one, which went rogue so I sent it to Calumet. They replaced a resistor. But it still wasn't right so I acquired another used one, Timer A, which I use every day. Timer A has the pencil-style sensor while Timer B has the c-clamp style. I ran a few comparison tests last night:

ALL TESTS 5:00 (300 seconds) MEASURED WITH IPHONE STOPWATCH:

68F:
Timer A 4:57 (297s) (OK)
Timer B 3:21 (201s)

70F:
Timer A 5:41 (341s) (factor = 0.88)
Timer B 3:50 (230s)

72F:
Timer A 6:30 (390s) (factor = 0.77)
Timer B 4:09 (249s)

I then switched the sensors at 72F:
Timer A with sensor B 6:21 (381s)
Timer B with sensor A 5:55 (355s)

So I would say that the sensor makes a big difference.

Looking at a few data sheets I can see that every film/developer combination seems to have it's own compensation factor. For example, for Kodak TXP320 in trays in D76 1:1, the factor is 0.94 at 70F and 0.86 at 72F. I have an Ilford chart showing 0.9 at 70F and 0.8 at 72F.

I think the only way to really know what's going on is to test with exposed film and a densitometer as Richard suggested. And, keeping my developer at 68F will help.