Re: compensating timer.
On this subject, if you are using a naked sensor then I would
recommend using a microcontroller with an on-board A/D. I use
the Atmel AVR line for product development of this sort. The AVR
is the chip found on the Arduino boards.
If you are using an RTD/thermistor/thermocouple/paperclip (iron
makes a not-bad temperature sensor) then don't bother with
anything but linear amplification of the sensor signal. Do all
the 'signal conditioning' for turning the signal into temperature
in software. The standard compiler math library for the
ubiquitous gcc compiler has all the floating point
log/exponentiation function you could ever need. Analog
circuitry for doing math is inaccurate and drift prone unless you
are willing to spend a small fortune on it and pot it in a
temperature controlled block. Software is free and doesn't
drift, age or get noisy.
However, for a cheap temperature sensor look at the Maxim DS18B20
http://datasheets.maximintegrated.com/en/ds/DS18B20.pdf- it spits
out the temperature directly in degrees, saving you the drudgery
of NBS thermocouple/RTD tables and polynomial curve fitting.
There's an outfit in China packaging the chip in a hermetic
stainless tube with a 3' cable for $1.78 - see ebay
http://www.ebay.com/itm/like/391264264310?lpid=82&chn=ps&ul_noapp=true .
There are snippets of Arduino interfacing code available on the
'net.
Philosophically, I find the 'compensating timer' approach to be a
kludge. Kodak publishes time/temperature data for all the
film/developer combinations. I keep all the film & developer
datasheets in the darkroom in a three ring binder. No big deal
to look it up before processing.
As to paper processing, the time/temperature relationship isn't
critical. Unless you have a voltage regulator on the enlarger
you will see far greater variation in print density from the
kitchen refrigerator cycling on and off. God knows I have done
enough work on paper characterization to know that normal
variation in darkroom temperature has close to nil effect if you
develop paper to completion. If you are pulling the paper early
then you will see some temperature sensitivity - but you are
better off just going down a contrast grade and developing to
completion.
Nicholas O. Lindan
Darkroom Automation
Cleveland Engineering Design, LLC
Cleveland, Ohio 44121
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