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At 07:11 AM 3/17/00 -0800, Michael J. Black wrote: >Hans Petter Roverud wrote: > > As an aside there are current posts on the difference between heat > > conductance and heat capacity. In plain English, helium is a very > > poor insulator (inflation gas) but it's not colder to breathe a gas > > containing helium. Any gas you inhale will be heated to body > > temperature before you exhale it. > >Assuming you keep the inhaled gas in your lungs long enough to heat >it to body temp before exhaling. Which as you point out depends on >the heat capacity of the gas and its conductance. You will heat helium >quickly, because of high conductance, but you will heat nitrogen more >because of high capacity (but the nitrogen doesn't stay in long enough >to heat it to full capacity). I have heard this argument before, and >don't buy it. I submit that you lose heat from respired helium as >quickly as, or faster than, air. Now being the fine physiologist that >you are Hans, please tell us why we don't lose heat more quickly from >breathing helium mixes. MJB Because your first assumption holds -- inhaled gas will be heated to core temperature before it is exhaled. Whether it takes a split seconds or a few seconds does not change the total heat loss. However, it does change whether you feel the heat loss. Cold gas sapping heat from where you've got plenty of thermo-receptors will cause you to feel the chill. Peripheral regions, including your mouth, feel the heat loss while your lungs don't. This is well illustrated by sat divers going hypothermic with unheated gas and hot water suits. They may turn down the heat since "it's too hot" while their core temperature is plummeting. As long as the peripheral tissues are warm you feel warm, and vice versa, irrespective of core temperature. At one atmosphere respiratory heat loss is no big deal and we're not designed to monitor it properly. Instead, the body goes for monitoring skin temperature. That works fine on land. regards, Hans -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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