# Compensating Development Timer

Developing traditional film and silver paper is very temperature
dependent.  The warmer the developer solution, the less time the film
or paper needed to be developed and vice-versa..  Historically, people
read the temperature and then manually corrected their development
time accordingly, using a correction table provided by the
manufacturer.

It turns out that these corrections for temperature are quite similar
across different manufacturers, although the corrections *are*
different for film and paper.

Many years ago, a company called "Zone VI" realized this and created
an analog timer that corrected for this effect.  You placed a
temperature probe into the developer and it corrected - via analog
adjustments - what a "virtual second" actually had to be.  The
photographer just looked up the normal development time for 68F
developer and the timer ran faster or slower based on the actual
temperature.  Better still, if the temperature of developer varied
*during* development, it corrected for that in realtime.  The Zone VI
Compensating Timer had settings for film, paper, and realtime.

The timer was a work of genius engineering and a really nice
addition to the serious photographer's wet dakroom.  I've depended
on one of these for years to make my darkroom work repeatable
with minimal thinking or measuring.  Mine is getting kind of old
now and I began to wonder what I would do if it broke.  The timer
did come with a "Lifetime Warranty",  Sadly Zone VI and
its founder, Fred Picker, are both now long gone making warranty
claims ... difficult.

While I could design an analog replacement or just figure out the
circuit of the Zone VI, it occurred to me that it would be easier to
just design a "work alike".  Thanks to the explosion of interest
in robotics and the Internet Of Things, there is an embarassment
of riches of computers, sensors, switches, temperature probes,
and so forth.  Not only can we build something like this ourselves,
doing so has several advantages over the old Zone VI timer:

    * It's digital, not analog, so we don't have mess with
      a bunch of precision parts and corrective feedback circuits.

    * It's software controlled so you can customize how this
      timer works to suit you.  Don't like my compensation factors?
      Want to adapt this for a different application?  Both are
      easily done with software changes.

    * It's cheap.  You can build one of these for well under $50.
      (The original Zone VI timer was around $200 if memory serves,
      and that was when money was still worth something. :)