The Shop > Electronics & IC Programing
Water Heater Monitoring
sparky961:
Greetings all, and Happy New Year. I spent two days of my New Year holidays replacing my recently deceased water heater, which included moving it to the other side of the house where it made more sense.
After all of this, I got to wondering how I could save some money in its operation (electric) and it seemed the first step would be to gather data on it's designed operation during normal household use over a period of time. For this, I need to set up a data logger.
Using what I had at hand, I've tried to set up a current transformer so I can indirectly measure it's power usage using my DVOM that can output serial data.
I thought I had it nailed, but right at the end I got confused. Perhaps someone can help me figure out this mystery?
1. Wind 100 turns magnet wire (perhaps 28ga or so?) onto small toroid
2. Insert 1 piece of 10ga solid copper wire with heavy insulation still attached
3. Characterize transformer using 60W and 100W incandescent light bulbs
[Result: 60W -> 4.95mA, 100W -> 8.2mA]
4. Do some math, resulting in an average of 12.2 W/mA
Here's where things get strange. Hooking my new current transformer up in series with one of the two wires connecting the water heater (240V single phase?) I took a reading from the meter... 102.8mA . Assuming this was the same as the light bulb test, that would give me 1254.2W, but this water heater has two 3800W elements that should be operating one after the other.
Who wants to play teacher and educate me on my mistake and how to get a correct reading? Or, maybe an easier (inexpensive) way?
vtsteam:
Well it doesn't explain the whole discrepancy, but your lightbulb is on 110V and your water heater is almost certainly on 220V, so you can at least double your estimated wattage.
awemawson:
Double the voltage and quadruple the power as wattage is 'I squared R' R stays the same and I is proportional to V so the wattage on 240 is four time that on 120
RussellT:
I think we need more information about how the water heater is wired before we can sort out that calculation. I don't understand what you mean by
--- Quote --- two 3800W elements that should be operating one after the other
--- End quote ---
However I wonder if you actually need to measure current. Water heaters are generally either on or off controlled by a thermostat. Wouldn't it be adequate to measure how much time it's on?
Russell
Spurry:
I understand the joys of making something yourself, but would a unit like an Efergy provide the answer to the "current" problem?
http://efergy.com/
Pete
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