Twenty degrees, and the tap that won't give it to you
Nearly every black-and-white time you'll find is for developer at 20 °C (68 °F). Kodak's T-Max developer is the odd one out: its times on Kodak's summary sheet are for 24 °C. My cold tap runs well above 20 in summer, so I correct more often than I'd like.
The correction
Ilford prints a temperature chart in each film's datasheet. On log paper it's a set of straight lines, and their worked example reads: 4 minutes at 20 °C becomes 3 minutes at 23 °C, and 6 minutes at 16 °C. The Delta 400 sheet gives a second one: 6 minutes becomes 4½ at 23 and 9 at 16.
All four of those land on one simple rule: the time halves for every 7 °C warmer, or
new time = sheet time × 2(sheet temp − your temp) / 7
That's roughly 10% shorter per degree. It's my fit to their examples, not a formula Ilford publishes, and the chart only covers 14 to 24 °C. The calculator uses it and tells you when you're off the chart.
Keeping the bath where you want it
- Mix with a little hot water from a kettle, then check with a thermometer, not your hand.
- Stand the tank and the bottles in a tray of water at the target temperature for a few minutes before you start.
- Ilford asks for every solution (stop, fix, wash) to be within 5 °C of the developer.
Watch the short times
Warm developer shortens times fast. Kodak's sheet warns that development under 5 minutes may be uneven. If a correction pushes you under 5, cool the bath or use a more dilute developer instead of racing the clock.
- FP4 Plus technical information, Ilford (HARMAN technology), Nov 2018.
- Delta 400 Professional technical information, Ilford (HARMAN technology), Nov 2018.
- Processing Kodak Professional black-and-white films (ED-BWF), Kodak Alaris, Feb 2016.