Agitate › Time and temperature
The developer is never at twenty
Film Development Time and Temperature Calculator
Your published time assumes 20 degrees and your developer is at 21.4. Agitate converts that for black and white between 14 and 27 degrees, in Celsius or Fahrenheit, and refuses in three cases where a confident number would be worse than none.
The conversion
Development runs faster when it is warmer, so a temperature above the one the time was published at shortens the time. Agitate uses a factor of 1.1 per degree Celsius, applied to the difference between the published temperature and the one you measured. It is an empirical fit around room temperature, which is precisely why it comes with a band around it rather than being applied to anything you type in.
In practice you read the thermometer, the app converts, and the run starts from the converted time with the drain still called ten seconds before the end.
Three cases where the app refuses
Color film, always
C-41 and E-6 run in a narrow window near 38 degrees where a deviation shifts color balance rather than density, and no maker publishes a conversion rule of this form for either. A compensated color time would be ruining film in the name of precision, so there is not one.
Below 14 or above 27 degrees
Outside the band the factor was fitted for, the app warns instead of extrapolating. Bringing the developer back into the band is a real answer. A number produced ten degrees outside the fit is not.
A published time that is itself outside the band
Six of the bundled rows are published above 27 degrees, and converting one of those down to a normal room temperature is an extrapolation of up to ten degrees. The app refuses that direction too, which is the less obvious half of the rule.
That third case is not hypothetical. Six of the bundled rows are Foma times published at 30 degrees, one is ADOX CMS 20 II at 26, and one is Rollei RPX 400 at 25. Converting the Foma time of 6:00 at 30 degrees down to 20 degrees produces 15:34, which the arithmetic is perfectly happy with and which nobody should develop a film by.
Reading the thermometer part way through
This is the case that costs people rolls. You are four minutes into a thirteen minute development and the thermometer says the bath is not where you thought it was. Most timers make you abandon the run.
Agitate re-times the bath that is running. The part already developed is not undone, so what changes is the remainder: the fraction of development completed is held constant and the rest is stretched or shortened to the new total. What gets converted is the time the run is actually using, not the raw figure in the recipe, so a push, a pull and the surcharge for re-used developer all survive the correction, and two readings compose. Going back to the first temperature gives you back the first time.
If you simply started late, that is the other button: fifteen seconds on or off the running bath, without leaving the run.
Tolerance, which is a different question
Compensating is what you do in black and white. Holding the temperature is what you do in color. Agitate carries a published tolerance for each process and says so before you pour rather than after.
| Process | Tolerance | Conversion offered |
|---|---|---|
| Black and white | 0.5 °C | Yes, between 14 and 27 °C |
| C-41 | 0.15 °C | No |
| E-6 | 0.3 °C | No |
How far from the target temperature the app lets a bath sit before it calls it.
Most of the bundled black and white rows are published at 20 degrees, with a block at 24 and a scattering at 21, 22 and 23. Whichever one your row carries, the app converts from that number rather than assuming 20.
Convert from the temperature your row was published at
A factor of 1.1 per degree inside a band the app actually enforces, in both directions, and a re-time that does not throw away your push.
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