Burning Desire For Efficiency

Editor’s Note: EarthTechling is proud to repost this article courtesy of Do The Math. Author credit goes to Tom Murphy.

Ever wonder how efficient it is to heat water? Of course you have! Ever measured it? Whoa, mister, now you’ve gone too far!

I recently devised a laser-phototransistor gauge to monitor my natural gas meter dial—like ya do. As a side benefit, I acquired good data on how much energy goes into various domestic uses of natural gas. Using this, I was able to figure out how much energy it takes to heat water on the stove, cook something in the oven, or heat water for a shower. Together with the knowledge of the heat capacity of water, I can compute heating efficiency from my measurements. What could be more fun? I’ll share the results here, some of which surprised me.

Heating Basics

The amount of energy it takes to heat water is so well-established, that it is the basis for several prominent units of energy. For instance, the calorie is the amount of energy it takes to heat one gram (1 mℓ) of water by 1°C. As a straightforward extension, 1 kcal = 4184 J (often Calorie with capital C) is how much energy it takes to heat one kilogram (or liter) of water by 1°C. Likewise, 1 Btu = 1055 J is the amount of energy it takes to heat one pound of water by 1°F.

So if I want to take 500 mℓ of water from 18°C to boiling, I need to expend 82×0.5 kcal to get the job done, or 171.6 kJ.

Measuring the Gas

My natural gas meter usually receives little attention from me—which is saying something for a person as measurement/data crazed as myself. The reason is that the meter does not provide sufficient information to track small expenditures without vigilant monitoring. As explained in the pilot lights post, there are dials that make revolutions once every half-cubic-foot and two-cubic-feet, then a jump to 1000 cubic feet. The jump is so large that one cannot walk up to the meter and know at a glance how many wraps the high-resolution dials have made since the last look.

Rather than parking myself outside for days on end to keep track of my gas gauge, I bought a cat-toy laser pointer and modified it to be powered by a constant 5-volt power supply. I aimed the laser at the ½-cf dial so that the black needle would interrupt the beam once per revolution. Then I set up a photo-transistor to “watch” the laser spot, and tuned the sensitivity so that the needle would make a robust change in the photocurrent. The laser and detector were shoved into a crudely drilled block of plywood to point at the same spot on the dial face, and then clamped to the meter. A dishtowel provided ambient light baffling, and a plastic shopping bag gave it some modest protection from rain and a certain trashy look. Thankfully, our gas meter information is now digitally transmitted. I can’t think what the meter reader’s reaction would be to walk up on this clap-trap arrangement (glowing red at night, no less).

Contraption attached to gas meter. A red glow can be seen from the laser. The green/black wires go to the phototrnasistor.

Close-up showing the laser spot illuminating the half-cubic-foot dial, ready to sense the passage of the dark needle. See also a picture of the same meter face-on in the pilot light post.

1 Comment

  • Reply June 11, 2012


    Why are you adjusting fossil fuels for electric and not gas?
    Gas is also processed useing fossil fuel.

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