Tuesday, February 19, 2013

Measuring Temperature and Comfort Levels

Humans are terrible thermometers.

You see, the way thermometers (things that measure temperature) work is they sit in something or are surrounded by something at the temperature they want to measure. As they sit, they exchange heat through conduction or convection or radiation. After some time, the thermometer is in thermal equilibrium with its surroundings. It becomes the same temperature as the surroundings!

People don't do that. In fact, they hate that. They die when they do that.

We're finicky. We want to be kept at precisely 98.6 degrees Fahrenheit on the inside. But we don't like 98.6 degree weather. Have you ever wondered why?

We are heat sources. Every cell in our bodies consumes sugar and oxygen and gives a little burst of warmth when it combusts. And that warmth builds up. We keep exactly enough to maintain our core temp (98.6 degrees) and dump the rest into the outside world. Heat only travels from hot areas to cold areas, so we actually prefer a cooler environment (say, 70 degrees).

Heat sources make terrible thermometers. Instead of settling on a constant temperature (like most things), we like to settle on heat rates: the amount of heat energy we produce.

The FDA has recommended food intake of about 2,000 kilocalories per day for an average human. That converts to about 100 watts (or 100 Joules per second). Granted, not all of that is converted into heat, but it's a good round number (and I eat way more than 2,000 kcal per day), so let's go with it.

This means that we feel most comfortable not in a particular temperature range, but in a particular heat transfer rate range. Around 100 watts.

What's the difference? Heat transfer rates depend on temperature difference (from your body to the air outside), but that's not all. Heat transfer rates also depend on wind speed, humidity, thermal conductivity of insulation (everything from parkas to blubber), insulation thickness, and surface area. If you're in view of something very hot (like the sun or a fire) or very cold (like a cloudless night sky), the color and finish of your surface will play a big role as well.

Thinking about your optimal heat transfer rate instead of your optimal outside temperature may help you make better comfort decisions.

Here are some examples:

  1. You know those terrible gloves that don't work? They may actually be increasing your surface area enough to counteract the minimal insulation they provide.
  2. I remember discovering as a child that blankets don't make "things" warm. They only work on people and animals (well, really only mammals and birds), because we are heat sources. Insulation just slows down heat flow, it doesn't generate any heat.
  3. Turn on your fan, even if it is blowing 85 degree air at you. Higher air speed increases the heat transfer rate.


Unknown said...

Knowing that we give off about 100 W makes me think of the lightbulbs in my house which then makes me think, "oh man, it'd be really cool if instead of giving off energy in the form of heat our bodies gave off light."

Thaddeus said...

Our bodies actually do emit light! it's just mostly in the infrared part of the spectrum. You would emit visible light if you closer to 3600°F like a lightbulb filament.