SUBSTRATE HEATING ENGINEERING
I know that using substrate heating is absolutely not necessary in the planted tank. My primary purpose for using it is to set up convection currents near the bottom of the tank to encourage increased water mixing without resorting to additional surface agitation.
Now, the secondary purpose is that I am a big ol’ geek that loves diy projects and making simple things far more complicated than they need to be. Additionally, mixing water and electricity adds that certain something that really makes this project attractive.
That said, let’s begin our engineering process. The most comprehensive site concerning diy substrate heating is this page
by Dan Resler and Uwe Behle. Their premise is to build a low-wattage system that can be used year round without radically affecting the temperature of the water; again, the purpose being to keep the substrate just slightly warmer than the temperature of the tank to induce micro currents to distribute nutrients and turn over the water column.
In their article they describe a system where the design calls for between 0.1 and 0.2 watts of power per liter. We will use .15 watts giving us about 30% leeway one way or the other for fudge factor.
First let’s convert to American measurements:
.15 Watts per liter = .57 watts per gallon
Now, what is our actual tank volume in gallons?
Our interior measurements come out to: 29.5” X 11.5” X 11.75” = 3986.2 cubic inches
3986.2 / 1728 (cubic inches in one cubic foot) = 2.3 cubic feet
2.3 cubic feet * 7.48 gallons per cubic foot = 17.25 gallons of water
17.25 gallons * .57 watts = 9.8 watts needed for our substrate heater.
Now let’s start working out electrical requirements. We want to use as little voltage as possible for safety reasons. However, we also have to consider the availability of high quality transformers and what voltages they are available in. Hammond power transformers are some of the best that are available. 6.3 volts is a very common and available voltage for these transformers, so that will be our starting point.
First we have to find out the amount of resistance that we need to develop 9.8 watts of heat. We do that with the following formula:
Resistance (ohms) = volts * volts / power in watts
6.3 * 6.3 / 9.8 = 4 ohms
We now have enough information to determine what size (in amps) transformer to use. Ohms law is states that Amps are equal to Volts/Ohms.
6.3 Volts / 4 Ohms = 1.57 Amps
If we go to Hammond’s enclosed power transformer page
we find they have a very nice 6.3 Volt 4 Amp transformer in the model 167N6. Drawing only 1.57 amps from a 4 amp transformer will keep coasting along without overheating and resulting in a much longer life and less heat (per pound of transformer) generated.
Now we need some wire to use as heating cable. We know we need 4 ohms of resistance. We need to have enough wire to stretch back and forth across the bottom of tank as many times as possible within reason. If we space our cables about 3/4 to 1 inch apart, that would give us about 37 feet of wire under our substrate.
If we divide 4 Ohms by our 37 feet we get:
4 Ohms / 37 feet = .108 Ohms per foot of wire.
There are many pages
on the web listing the resistance per foot of wire we find that 30 gauge wire at .103 Ohms per foot is just about perfect for our heating cable.
30 gauge Kynar insulated wire is readily found at our local Radio Shack with model number: 278-502
it is sold as Insulated Wrapping Wire in Red, White or Blue.
How close did we come to our goal of 9.8 Watts?
37 feet * .103 Ohms = 3.8 Ohms
6.3 Volts / 3.8 Ohms = 1.67 Amps
To determine wattage, we multiply Volts times Amps:
6.3 Volts * 1.67 Amps = 10.4 Watts
How many watts per gallon?
10.4 Watts / 17.25 Gallons = .6 Watts per gallon, well within our 30% tolerance.
So that’s the start. We’ll order a transformer, run to Radio Shack and pick up the wire and other odds and ends we’ll need. Not sure when we’ll get to this, but it gets filed in the planning notebook for the aquarium.