When developing film there are three main variables that must be controlled to achieve consistent results; time, temperature and agitation.  Of those three probably the most difficult to control is temperature.  Fortunately black and white developing chemicals will work over a wide temperature range and it is possible to apply a temperature chart so that time can be adjusted depending upon temperature.

Many photographers simply use their chemical solutions at room temperature and adjust the time according to temperature charts to achieve consistent results.  It is easier however if you can count on your temperatures to always be the same so that you don't have to vary your times.  This can lead to greater consistency.  To that end I built a tempering tray for my chemicals.

A tempering tray is simply a water bath that is maintained at a constant temperature and bottles of chemical solutions are placed in this tray.  The chemicals are all brought to the same controlled temperature and that temperature is the same from session to session meaning the photographer doesn't have to be concerned about variation.

To build a tempering tray one needs only a few things.  The first is the tray itself.  It must be waterproof and large enough to hold all of the chemical bottles that one will use in a session.  It should also have a lid.  The chemical bottles could be simply floated in the tray but if they leak at all water will enter the bottles and dilute the solutions.  Having a lid allows the bottles to be held upright.  It also slows evaporation from the tray and helps to insulate the tray aiding in maintaining temperature.

The next requirement is a heater of some sort.  I suppose in a very hot environment a cooling system might be required and that would be more complicated.  For my purposes however my darkroom air temperature is maintained lower than the 75 degrees Fahrenheit I use for my black and white film processing.

The last requirement is one I discovered after building a tray; a pump of some type to circulate the water.  Without circulation the water temperature will vary from one area of the tray to another and that difference in my experience can be up to four degrees.  That variation is too much for consistent film processing.

The tray I decided to use is a storage bin designed for storing sweaters or other items.  It is plastic, watertight and comes with a snap on lid.  They come in many sizes and I was able to find one that was the size I needed.  A tray like this can be had for about $4 at a department store.  This one measures about 19 inches by 14 inches by 6 inches deep.

For a heater I decided to use an inexpensive aquarium heater.  The key here is that the heater must be completely submersible.  The tempering tray isn't deep enough for the type of heater that hangs over the side of an aquarium and if such a heater is used it will crack because only part of it is submerged.  I chose a relatively powerful heater for the small tray.

Aquarium heaters are generally about 50 watts per ten gallons of water.  My tray holds about 3 1/2 to 4 gallons with the bottles installed so 50 watts would be sufficient.  I chose a 150 watt heater so the desired temperature could be attained more quickly.  I also thought that having more power available might make it easier to control the temperature more accurately.  [See update note at the end of this article for "late breaking" heater news.]

The heater is installed close to one long side of the tray.  Be careful in your placement of the heater, not because of even heating but so you don't hit it with the chemical bottles when they are installed.  I chose to put my heater in the "back" of the tempering tray.  The aquarium heater I chose cost about $18 dollars at a local pet store.

The next item needed is a pump to circulate the water.  My first attempt at making a tempering tray resulted in wide fluctuations in water temperature.  The temperature not only varied from area to area in the tray it also varied in each area over time due to the convection currents set up by the heater.  The temperature wasn't being maintained accurately enough to be useful. 

My first thought was that the thermostat on the heater wasn't consistent enough or had too wide a range between on and off.  It then occurred to me that with the variation of temperature in the tray the heater thermostat wasn't getting a consistent reading.  It might help to circulate the water.  Back to the pet store for a pump.  I could have chosen a "power head" type pump that works with under gravel filters but they seemed a bit too powerful and I didn't want water splashing out of the tray.  I found a small submersible pump designed to create waterfalls in terrariums or to run small indoor fountains.  The cost was under $18.

The next task was to cut holes in the lid to hold my chemical bottles.  At the time I was using a developing tank that held nearly a liter of chemicals so the bottles I was using were 2 liter bottles.  I wanted to be able to have a complete set of chemical solutions tempered and ready to go so I arranged the bottles on the lid of the tray to get them all to fit.  It is important when arranging the bottles that you don't put them over either the pump or the heater.  Keep enough distance between bottles to allow for water circulation also.

I traced around them with a "Sharpie" marker and then used a razor knife to cut out the openings.  I wanted the openings to be relatively snug so that bottles that were nearly empty wouldn't float up and tip over.

Cut the holes for the bottles close to the lines at first and then enlarge them as necessary.  Note the holes leave space near the back for the heater and pump.  Notice the hole in the corner for the power cords.

Next set the thermostat on the heater for your desired temperature.  It is likely that the thermostat won't be completely accurate and that you will have to adjust the setting once or twice before it stabilizes at the temperature you need.  Don't depend on the heater's setting, use a darkroom thermometer to read the temperature.

Install the heater near the "back" wall of the tray.

Next put the pump in the tray.  Position the pump so the output will flow over the length of the heater.  The circulation of the water is the key to maintaining an exact temperature.

Now run the power cords for the heater and the pump through the hole in the corner of the lid and put the lid on tightly.  Put the tray in position in your darkroom.

Now put the bottles in the holes you have cut for them.  Leave one bottle out and fill the tray with water through that hole.  My tray holds about 3 1/2 gallons.  It is important not to fill the tray too full or it will overflow when you put the bottles in place.  That's why I like to have most of the bottles in when filling the tray.  Also if you fill the bottles with water you can measure their temperatures to make sure the heating is even.

I put a darkroom dial type thermometer in the small hole in the front of the tray.  I read the temperature from that thermometer rather than depending on the heater to be accurate.  I have found that my thermometer reads 75 degrees when the heater is set for 73.

Let the water stand in the tray for at least 30 minutes before plugging in the heater and pump.  The heater must acclimatize to the temperature of the water before you plug it in.  If it isn't allowed time do to that it won't maintain the temperature accurately.

Once the water heater has been submersed for at least 30 minutes plug in the heater and the pump.  Check temperature every 15 minutes.  Within an hour the temperature should be stabilized.  It might not be at the temperature you set on the thermostat, but it should be stable.  If the temperature doesn't stabilize at the setting you desire you will have to adjust the heater setting.

Unplug the heater and pump before putting your hand in the water!  Turn the dial on the heater either up or down a degree at a time.  Plug it back in and start checking every 15 minutes again.  Repeat that process until the temperature stabilizes at your desired setting.  Make a note of the actual setting on the heater when your desired temperature is maintained.

You now have a tempering tray you can use to maintain your chemicals at the desired temperature and to bring your chemicals to the desired temperature quickly.  One less variable to worry about in developing your film!

Above I stated that I thought a more powerful heater might make temperature control easier and more accurate.  I have since done some testing and have found that isn't necessarily the case.

It seems I chose the right combination of heater and water volume quite by accident.  I decided to move my heater and pump to a much smaller tray for testing and was quite surprised by what happened.  My main tray holds not quite four gallons of water (about 15 liters).  The aquarium heater and pump combination allows me to maintain 75 degrees F (24C) to within 1 degree F accuracy.

I moved the pump and heater to a tray that holds a little over a gallon of water (about 4 liters).  I followed my own advice and put in the pump and heater then filled the tray with  water slightly cooler than I wanted to attain in the end.  I let the heater sit unplugged to let its thermostat acclimate to the water temperature for about an hour.  I then plugged in the pump and heater.

To my surprise the temperature of the water in the tray quickly topped the 80 degree F mark!  I had not moved the thermostat setting from the previous tray.  I left everything plugged in and checked in an hour later and was pleased to see the temperature at 75 degrees once again.  However checking it 30 minutes later I saw the temp again at near 80 degrees.  I was hoping it was simply "bouncing" until everything settled down but that was not the case.  Over 24 hours later the temperature fluctuations were still happening.

It seems that the heater warms up the smaller quantity of water very quickly, much more quickly than the internal thermostat can react to.  The water cools much more slowly than it heats up and so the "dip" on the low side was within a degree of the right setting but the "bounce" on the high side was far to extreme.  This much variation would make consistent development impossible.

Moving the heater back to the larger tray resulted in the steady temperature holding I had before.  From a couple of days of testing I would conclude that for a tray with a capacity of about a gallon of water a much smaller heater should be used, perhaps as small as 50 watts.  It is important to note that these results and recommendations are based on using the heater's internal thermostat.

I am testing a different heater/controller setup.  The information about that heater is here.