This page is a follow-up and a continuation of my tempering tray article which can be read here.

As I said in the main article, it is possible to use a regular submersible aquarium heater to maintain temperatures in the range most commonly used for black and white processing.  Adding a submersible pump keeps the water mixed and the temperature consistent from area to area in the tempering tray.  However there were issues in maintaining temperature in a smaller volume tank.

This article primarily discusses a new heater arrangement that provides for even heating even in smaller volume trays and the possibility of maintaining temperatures sufficient for color processing.

There are now aquarium heaters on the market whose heating tubes are made of titanium rather than glass.  The obvious advantage of this is that the tubes are virtually unbreakable which is a good thing when dealing with anything that is submersed in water.  Another advantage is that at least some of these titanium heaters have an external temperature sensing probe which allows it to react faster than the thermostat inside the glass tube type heater.  Those with an external probe also have an external controller for setting the temperature.  You don't have to put your hand in the water to turn the temperature setting dial.

I purchased one of these heaters, sold as the "Pro Heat 150".  It is manufactured by the Won Company and is available for about $25.  It has a temperature controller external to the heater tube and an external temperature probe (meaning the temperature probe is not inside the heater tube).  The temperature controller unit has an integrated circuit which allows temperature to be maintained much more precisely than the bi-metal strip thermostats in traditional aquarium heaters.  The stated upper temperature range for this controller is 93 degrees F.

The controller part of the unit plugs directly into a three prong outlet.  If you have a power strip it will likely block one adjacent outlet.  It is about the size of a standard power "brick", around 2 1/2 inches by 3 1/2 inches by an inch or two thick.  It has a dial to set the temperature and a red LED to indicate when the heater is operating.  (You may want to tape over the LED if you handle film in the vicinity.  I haven't done any tests to see if it is bright enough to fog film.)

There are two cables coming from the controller, one is the power for the heater itself and the other is the temperature sensor probe.  The heater has suction cups at either end to hold is securely to the tempering tray.  The temperature probe is submersible of course and also has a suction cup which can be moved anywhere along the cable and will hold the temperature probe securely where ever you place it.  The cables are about five feet long allowing the controller to be placed well away from your tempering tray.

I have been testing this unit for a few days now and here are some of my findings.

1- The unit is capable of precisely maintaining temperature in the smaller tray that my traditional heater had problems with.  I believe this is due to the external temperature sensor reacting more quickly to water temperature changes than the internal sensor in the other heater.

2- The dial on which the temperature is set is not very accurate.  Setting 75 degrees F on the dial resulted in about 78 degree water.  However the setting seems to be very precise- it is extremely repeatable.  A separate thermometer is essential to get accurate water temperatures.

3- The upper temperature limit is substantially above 93 degrees F on my unit.  In testing it achieved temperatures of 108 degrees and maintained them.  I was able to dial the setting down and get a temperature of 100 degrees and have it maintained very accurately (less than 1 degree variation).  This means that it would be possible to use this unit to maintain the temperature necessary for color processing.

I don't know if this high temperature range is unique to my unit but I suspect that it isn't.  The circuitry is what controls the temperature and provided all the units use the same circuitry and temperature sensor it is very likely that they generally have the ability to heat to over 100 degrees.

Disclaimer!  Don't complain to me if you buy one of these heaters and find that it only heats to 93 degrees!

I put 60 degree tap water into a smaller tray (about 1 1/2 gallons) and allowed the heater to acclimatize for 10 minutes before turning it on.  It brought the temperature of the water up to 78 degrees in under 15 minutes.  A little testing is necessary to find the exact setting to maintain 75 degrees.  On my unit it is less than the 75 degree mark on the dial.  One could place chemical bottles in the tray at room temperature, turn on the unit and be ready to process in less than 30 minutes.

I will continue testing this setup, including testing in a larger tray (4 1/2 gallon capacity).  As it stands I believe this unit to be far superior to a standard glass-tube aquarium heater.

It has now been about three months since I began using the Pro Heat 150 to maintain the temperature in my tempering tray.  All of the above findings have held true over the months.  Temperature in both trays can be maintained to within 1/2 degree F.  The heater can also maintain 100 degrees F in both my large tray and my small tray also with 1/2 degree F precision.

I am very happy with this heater's performance in my darkroom.  Although I can't vouch for the performance of any sample other than the one I own at the price it would be worthwhile checking it out!