For years, we’ve all heard the mantra of gpm (gallons per minute) vs. Btus (British thermal units). We’ve heard and seen this so many times that it must be true, right? Although this axiom has served us well, and I couldn’t agree more with the tenet that sufficient water puts out fire, the truth is that it’s not entirely accurate.
So why am I throwing stones at this long-held adage? Because it’s inaccurate and the math is wrong. When we compare gpm to Btus, we’re comparing apples to oranges. A simple change in our terminology can both enhance our understanding of fire behavior and reinforce the importance of speed on the fireground.
To clarify, Btu measures the energy produced by the fire, whereas gpm refers to our fire flow, or the power absorbed by the water. Power (P) equals the amount of heat energy (E) per unit time (t) that the water can absorb, or P = E/t. As we can see from the equation above, power does not equal energy-they’re not the same thing. Although they’re related, they’re still two completely different physical quantities. Energy measures the total quantity of work done-not how fast it gets done. Consider the difference between running a mile and walking a mile. It takes about the same amount of energy to move your body the 5,280 feet, but it requires greater power when you run. This is the reason you’re more tired and sweat more when you run.
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