March 2012
A programmable thermostat switches a 24 VDC relay to control 240 VAC electric heaters. I would like to know when and for how long the heaters are on and off in a 24 hour time span, although 12 hours would be okay. Is there such a thing as the electronic equivalent of a chart recorder that can display via Windows XP, and be built for less than a small fortune in time and money?
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Assuming you're not interested in a permanent monitoring solution, but wish only to take occasional measurements, a fast and inexpensive solution is to use a multimeter that has an RS232 interface and a temperature probe. One such instrument is the UT-61C available for $60 from www. multimeterwarehouse.com. Attach the temperature probe to the cover of the baseboard heater (not the heater element itself), and record the temperature variations over time on your PC using the software included with the multimeter. When you're done, you have a multimeter (although not the highest quality) to use for other projects.
If you are interested in knowing how long the heaters were running for the least amount of time and money, I would suggest using a panel mounted hour meter. You can get the meters from Digi-Key, Allied Electronics, or any well stocked supplier. I would suggest getting a meter that responds to the 24 VDC relay coil voltage. The one advantage is that you can get a cumulative reading of the time the relay is energized without having to utilize a computer. While a computer program is nice, if you should take a power bump your computer may shut off and you lose the data you were trying to get. Since the meter is directly powered, if you lose power and then it is restored the meter will keep working.
If you are up to messing with a microcontroller, I have a solution based on using an Arduino and a Flash memory card to save the data. To better flesh out the project, I have created a web page with a description (I did bench test it) and schematics: https://sites.google.com/site/thermostattracker/home.
Basically, you use a small front end to transform the thermostat signal for the Arduino microcontroller input, run a program on the Arduino to collect the data, and store it on a Flash memory card that is plugged into a daughter card (referred to as a "shield") on the Arduino. Then, you can take the Flash memory card and plug it into a PC for post-collection data analysis (example, import to a spreadsheet). I'd estimate about $75 for the parts; almost all are available at a RadioShack. I also used the free Arduino simulator available at arduino.com.au to aid in the programming.
