I have decided to make a wall mount meter to calculate how much fuel oil my furnace burns thru out the day. The wall thermostat has low voltage ac that it switches, about 24ac to tell the furnace to turn on or off. I planning on wiring in a relay in parallel which will trigger my arduino. Then from their I'm going to time the furnace. Most furnaces have nozzles rated in gph so I'm planning on doing the math and having it displayed on my LCD. For example my nozzle is rated a .75gph so if the furnace runs for an hour I have used 3/4 gallon.
Any thoughts on this? How about using the really to trigger the arduino? Also is their a way to get real Time consumption updates? Versus waiting for the furnace to end it's cycle and then running the formula?
How about using the really to trigger the arduino?
Trigger doesn't mean anything. What exactly is the relay going to do? Are you expecting the relay to be activated when the furnace is turned on? Have you found a relay that can be activated by a 24V AC input?
I think it would be better to use a second relay that is exclusive to the Arduino. The rest is simple enough, you are just accumulating on-time per day and the simplest button-press sketch should suffice. You already know the real-time consumption. It is 0.75 gph.
You could get fancy by using the freetronics water flow meter project, but I can't see any point in doing that. You would be better off expanding what you already propose to establish date/time stamped yearly trends. This would involve a clock module.