This topic has come up enough that it deserves promotion from a Newsletter to a permanent blog post...
People have asked about using a water bath and the SP-445.
Bear in mind that the SP-445 is molded out of rather thick plastic. This is a much better insulator than a highly conductive steel tank or even the comparatively thin walls of most other plastic tanks. (Some of which are surprisingly thin.)
But how much better? Here's an exciting chart that describes the thermodynamic characteristics of the SP-445:
The temperature in the SP-445 will shift by less than 3 degrees over a fifteen minute period (note this is at a room temperature of 76F.)
That doesn't sound like much. But just how accurately do you need to hold the temperature? We discussed this with chemists, emulsion engineers, college profs and other guys who have been doing this forever. The answer might be a little surprising and some will consider it heresy:
Only the average temperature during development really matters.
Okay, now calm down! First, some disclaimers: we know it's a non-linear process and that most of the development occurs in the first minute or so. Yes, you've been taught to panic if the temperature ever drifted by more than half a degree. And there are exotic processes that require high precision. Very long (or very short) processing times complicate things. But for 98% of photographers, using the average temperature works just fine (and is a lot less messy than a water jacket.)
Therefore, you don't need to hold your processing temperature absolutely precisely. Just adjust your processing time based on the average temperature and you'll be fine. In fact, all the major film/chemistry manufacturers have published the data tables for this very purpose.
You can download our version here: Time vs Temperature Table
So if you knew the average temperature of the solution, you could look up the correct time and not have to worry about maintaining a precise temperature. One minor problem, you can't calculate the average temperature until after you're done processing!
Now if you're processing at the same target temperature for the same length of time, you can determine the average temperature pretty easily. Just fill the tank with water at the desired temperature (no film, of course) and measure the temperature every minute or so and calculate the average. If the temperature in your work space varies wildly, you'd have to do this for several different ambient temperatures.
Fortunately, there is a better answer. It's easy to measure the actual temperature, calculate the error, and use the above mentioned tables to estimate the time adjustment required. By measuring and adjusting continuously, you can get really, really close to the desired time. Okay, it's not all that easy for you to do, but a computer can do it and still go to sleep for 95% of the time.
In fact, we're not the first ones to think of this. Back in the 80's, Zone VI introduced the Compensating Development Timer. It continuously measured the temperature of the solution and adjusted the time base of its clock: speeding up if things got too warm, slowing it down if things cooled off.
Wow. Seems like a great idea. Must be, people are still dropping $300 to grab a used one.
Using it is pretty simple: you drop the sensor in the solution and develop normally. If you're target time is 7:00, just wait for the display to read 7:00. Now in real time, it might be eight minutes or six minutes. The Compensating Timer determined the correction factor and did the work for you.
Since the original design has been out of production for a decade or so, we're design a modern version.
As for the "form factor", we've considered all kinds of options: build it into the lid, wireless sensors that interface to your smartphone, submersible modules and those aren't even the crazy ideas! Final answer: built it into the cap. It will probably look something like this:
The biggest question: how much will it cost? Probably around $50. We hope to be in production by the end of 2018. We're still in the design phase, so send your comments to: email@example.com
UPDATE 2019: After more testing, we've concluded that:
1. The market probably won't justify the NRE for this project.
2. Figuring out the average temperature is pretty easy and works extremely well. You can then adjust either the starting temperature (to hit the desired average temperature) or adjust the time (per the chart referenced above.)
3. The real secret is consistency! Figure out what works and keep doing it.