ZoneVI Developing Timer Repair or Adjust?

(Apologies to anyone who already saw my similar post on the LFPF site.) My ZoneVI Compensating Development Timer is in desperate need of repair/recalibration/adjustment, and I wondered if anyone knows if this is even possible?

After seeing that recently developed film was significantly under-developed, I decided to check the compensating aspects of the timer. I placed the ZoneVI probe in a beaker of water measured at 68F using a Kodak thermometer, and compared the ZoneVI times with a stop watch. My assumption is that at 68F, the compensated time and real time should be the same, or very close. Instead I found that while the ZoneVI "real time" setting agrees with my stop watch, the "film" and "paper" settings are about 50% off, so that when the ZoneVI timer says, for example, that 4:30 has elapsed, the stop-watch elapsed time is only 3:00; if anything the error is slightly worse on the "paper" setting, almost a 60% difference.

My sense is that either there is a problem with the circuitry in the timer, or something has failed in the temperature probe. Any suggestions would be greatly appreciated. (Currently I am using the timer in Real Time mode, but of course that loses the whole point of having a compensating timer.)

Replacement of the temperature probe would probably be easier and cheaper.

The timer needs to be internally recalibrated. Doing so by using known 68F/20C water is the correct approach. You can do this with only a small screwdriver and an accurate external clock for comparison.

Give the following 6-year-old thread a close read. Note my own post contributions at #4, #8, and #12. This is the procedure I still use periodically to adjust my own Zone VI developing timer. Works like a charm.

(In fact, I performed the described recalibration again just prior to making my print for the current Blind Print Exchange round.)

(there was a url link here which no longer exists)

Ken

Ken: Thanks, your instructions were exactly what I needed. However, while adjusting the left-hand potentiometer improved things a lot, even with the pot turned as far counter-clockwise as it will go, the timer still reads 3 seconds per minute faster than it should (i.e. with the probe in 68F water according to a Kodak lab thermometer, the timer says 1 minute when my watch has only reached 57 seconds, at 2 minutes on the timer my watch is only at 1:54, etc., a consistent 5% error). Any further thoughts? The real time setting is spot on.

Adjusting those two controls is all I ever had to do. My correct set points always fall within the range of the potentiometers, and usually within the middle one-third of those ranges. But my first thought is that an error of only 5% error is actually pretty good. It may very well be within the normal error spread for that overall system.

Consider that a 5% error in reading a thermometer is only 1/20th of a degree. Most eyes are likely not even going to be able to accurately judge that. And based on my experience with this device a 3 second error in 60 resolves down to an almost imperceptible turn of the pot thread.

But just to speculate on other possible sources of error, I might consider...

Thermometer/water bath not fully stabilized?
Thermometer probe not fully immersed?
Thermometer probe defective (two sources for the original error)
Timer electronics aged and out of spec?
Timer electronics not warmed up?
Potentiometer contacts worn or corroded?

I suppose it would also be possible to give up one calibration in favor of the other. Meaning, leave your compensation pot set to where it currently is, then with the timer mode set to film or paper compensation, adjust your real-time pot until the timer generates the correct elapsed time on your watch for 68F/20C.

You would be giving up an accurate real time operating mode in return for gaining a usable compensated time operating mode.

Ken

Ken (or others): Further testing results in a specific question about the ZoneVI firmware curves.

I re-tested using a second temperature probe (I have both the original "U-shaped" clip-on probe, and the newer stainless steel rod probe; I've been using and did my original tests with the SS probe, so I retested with the U-shaped probe). Results were consistent, so it is not a defective probe, and the results are repeatable. But while the timer on "Film" at 68F measures 5% fast (i.e. when the timer says 4:00 minutes, my stopwatch says 3:49), on "Paper" the timer is very close to spot on (i.e. when the timer says 4:00 minutes, my stopwatch says 4:02, so on "Paper" the timer is 0.8% slow, a negligible error factor).

The fact that the differences for "Film" and "Paper" are technically in opposite directions, but in real terms only "Film" seems to have a problem (now that I recalibrated; see earlier post), do you have any idea if this is in fact "normal" and Mr. Picker programmed in shorter film development times versus "real time?" When you re-calibrated your ZoneVI, were you spot-on for both Film and Paper at 68F?