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re: i wonder why .....
21 apr 1999
elaine wrote:

>if i were you, i'd pull out both the heat and the ac.  we don't use either
>here. people adjust.

very frugal, elaine...

joe foor wrote:
  
> humidity below 55% is low enough to cause my nose to bleed, after a few
> hours.

some alien star trek swamp species had that problem. they walked around
the ship with little bubbly swamp things like harmonicas under their noses,
and were smarter than wesley crusher. maybe your humidity meter reads high.

>the humidity comfort zone starts around 70% and goes up.

according to ashrae, "comfort" begins at less than 30% and ends at 60%.
i think people and things tend to collect mildew and mold above that.

>>whatever this ashrae may be, it isn't very reliable when dealing with
>>human variety.

the american society of heating, refrigeration and air-conditioning engineers
is a large old non-profit organization without many axes to grind, except
they seem to enjoy research, a victimless crime, for the most part, and some
of their volunteers get paid by their universities and agencies and other
employers (some of whom just happen to manufacture air conditioners) for
that research. if anything, they seem too concerned about human comfort,
with elaborate studies of what constitutes "comfort," based on measuring
the percentage of dissatisfied people who vote, in real experiments, and
so on. ashrae standard 55 (iso standard 7730) codifies "comfort," and they 
predict human comfort based on air and radiant temperatures, radiant source
asymmetry, humidity, air velocity, clothing and human activities. their
handbook of fundamentals (hof) may have more than you ever wanted to know 
about "comfort." 

the ashrae comfort zone is a 4-sided figure drawn in a box with a linear
horizontal axis of "operative temperature" (ot, something like room
temperature) vs humidity ratio (water vapor weight/dry air weight.)

roughly speaking, ashrae humans are comfy between 68 and 81 f and 25 to 60%
relative humidity (rh.) at their lower left hand corner, ot = 68 f and
rh = 30%. then we travel along to the right on a line of constant humidity
ratio (0.046, or a 37 f dew point) and increasing temp and decreasing rh
to an ot of 81 f and an "effective temperature" et* = 79 f. then we rise up
along the constant et* = 79 line to 60% rh with ot = 78 f, then down to
the left along the drooping constant 60% rh curve to ot = 68 f, then back
down the et* = 68 f line to 30% rh.

the ashrae hof elaborates: 

  the effective temperature et* is probably the most common environmental
  index and has the widest range of application. it combines temperature
  and humidity into a single index, so two environments with the same et*
  should evoke the same thermal response even though they have different
  temperatures and humidities, but they must have the same air velocities.
  the original effective temperature was developed by houghton and yaglou
  (1923). gagge et al. (1971) defined a new effective temperature using a
  rational approach. defined mathematically in equation (33),

           et* = t0 + wimlr(pa-0.5pet*,s)               (33),

[where ot t0 is the average of mean radiant and ambient air temps, weighted
by their respective heat transfer coefficients, w is "skin wettedness"
("with no regulatory sweating, skin wettedness due to diffusion is
approximately 0.06 for normal conditions. for large values of emax or long
exposures to low humidities, the value may drop to as low as 0.02, since
dehydration of the outer skin layers alters its diffusive characteristics.
with regulatory sweating, the 0.06 value applies only to the portion of skin
not covered with sweat (1-wrsw)..."), im is the moisture permeability index,
the ratio of the actual evaporative heat flow capability between the skin
and the environment to the sensible heat flow capability as compared to
your lewis ratio, lr = he/hc [see chapter 5], the ratio of evaporative to
conductive heat transfer coefficients, approximately 205f/psi at typical
indoor conditions, pa is the ambient partial vapor pressure of water in air 
in psi, and pet*,s is the saturated vapor pressure at et* in psi.]

  this is the temperature of an environment at 50% rh that results in
  the same total heat loss from the skin esk as in the actual environment.
  since the index is defined in terms of operative temperature t0, it
  combines the effect of three parameters, tr-, ta and pa--into a single
  index. skin wettedness and the permeability index im must be specified
  and are constant for a given et* line for a particular situation. the
  two-node model is used to determine skin wettedness in the zone of
  evaporative regulation...

  since et* depends on clothing and activity [not to mention culture
  and economics], it is not possible to generate a universal et* chart.
  calculation of et* can also be tedious, requiring the solution of
  multiple coupled equations to determine skin wettedness. these equations
  can be solved with the appropriate computer routines (gagge et al. 1971),
  but not always with hand calculations...

saying that ashrae doesn't know human comfort is like saying that frank
purdue doesn't know chickens. (ashrae also knows about chicken and cow
comfort, but they just measure weight gain and egg and milk production
and animal reproduction, since chickens and cows can't vote.)

put 100 people in a room and change the temperature and humidity and ask
if any are dissatisfied, and some will likely say yes, but many humans
get used to living outside standard ashrae comfort zones. inuits chop holes
in their igloos for more ventilation when the indoor air temperature rises
to 45 f, and people in tropical countries wear coats when temperatures drop
below 80 f. my grandmother used to tell us about a local 2-sigma kentucky
character named "frozen usher," who only drove his horse-drawn wagon to
town on the hottest day of the year, dressed in a warm coat and hat and
gloves and many warm layers of clothing... 

then again, the local lutheran church was built in the 1700s, and it's
never had a heating system, just uninsulated stone walls and body heat
from people who brought warm bricks from home and spread straw on the
floor to help keep their feet warm. the old building is still used in
the summer and for christmas eve services.

then again, each december 5,000 fur and technoclad guests per year begin 
to arrive from all over the world at "the world's coolest" ice hotel in
jukkasjarvi, sweden (population 400), 125 miles north of the arctic circle,
where reindeer outnumber humans. the walls and beds and chairs and sculptures
and chandeliers of the hotel are built each winter by 30 artisans from
22,000 tons of snow and ice harvested from the frozen torne river, and
it melts completely by spring. 

this unheated hotel was founded in 1991. recent sponsors include volvo and
absolut vodka. the 13,000 square foot structure has a chapel, an art gallery,
a cinema with an ice screen, and 45 bedrooms with fireplaces, headboards and
night tables all crafted from ice. ice beds are covered with thin mattresses
and lavishly strewn with reindeer pelts (according to shelly branch, writing
in the march 1, 1999 issue of fortune magazine.) 

  back in the absolut ice bar for a nightcap, it's 25 f and the music--
  greek--is blaring. one group kicks their legs a la rockettes ("this
  is life!") while puffing on cartier cigarettes. a japanese duo regards
  the spectacle silently. an ungloved bartender serves thimble-sized
  drinks [in ice glasses] for $11... 

  claiming an ice booth, i contemplate this surreal scene... and whether
  the group from astra corp... signed up for reindeer herding--an odd but
  apparently popular diversion. not all visitors, i know, are intrepid 
  enough to actually sleep here. some execs and celebs have been known to
  go soft by staying in the cop-out cottages--heated two-bedroom chalets
  that dot the property.

  such will not be our fate. our mummy-shaped fjallraven sleeping bags can
  sustain body heat down to -35 c, we've been told. at the moment of truth,
  we shuck our space suits and dive in. my [swedish] friend reminds me that
  less is more, so we strip and zip up so only our noses are exposed...
  incredibly, we manage eight hours of frigid slumber before a staffer
  serving warm lingonberry juice wakes us up.

  perhaps it's nordic bravado that prompts me to join the morning dogsled
  ride. seated at the helm of the bumpy craft (my lucky lot as the lightest),
  i tell myself that the reindeer sightings--rudolph!--are worth the cold's
  sting. but after 20 minutes as a human windshield, i've had my fill. it's
  bad enough that i can't feel my toes and that my hair has frozen to my
  fur-lined hat. but who knew that alaskan huskies can poop while they run?
  i escape their kicktrail--aimed smack at my head--by refusing to sit first
  on the way back...

  the best western ice hotel (011-46-980-668-00) is open december through
  april. doubles are $165 to $250. scandinavian airlines flies to kiruna,
  a 15 minute drive from the hotel.

nick




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