Hello from San Diego:
> If there is a portable IT product which bottom surface can become hot
> (around 49 C) during use, would you recommend to put a warning label onto
> the product or would you consider a warning in the operator's manual as
> sufficient ?
Here's an experiment:
Place a number of materials, e.g., plastic, metal, metal foil,
in a saucepan add water, and heat to boiling. When the water
boils, all of the materials are at 100 C.
Using tongs, remove each material, one at a time, shake the
water from it, and touch it -- CAREFULLY!
You will find that you can easily hold a plastic part at 100 C.
You will also find that you can barely touch a thick metal part
at 100 C. But, you will find that you can easily grasp a thin
metal foil.
When it comes to the safety of hot parts (i.e., protection
against a thermal injury), temperature is only one criterion,
and cannot be considered alone (unless you want to be ultra-
conservative).
The other criteria that must be considered are:
Thermal conductivity (or conversely thermal resistance).
Thermal capacity (stored thermal energy)
Plastics have very low thermal conductivity (conversely, very
high thermal resistance). This means the transfer of thermal
energy from a plastic object to the skin is very slow. The
slow energy transfer means that the surface temperature of
the plastic will drop very quickly to near skin temperature,
followed by a slow skin/plastic surface temperature rise.
The skin will never reach the temperature of the plastic
object due to the temperature drop internal to the plastic.
Consequently, plastic at 100 C cannot cause a thermal injury
(burn).
Plastics also have low thermal capacity. They cool quickly.
Metals have very high thermal conductivity (conversely very
low thermal resistance). This means the transfer of thermal
energy from a metal object to the skin is very fast. The
fast energy transfer means that the surface temperature of
the metal will drop very little when touched, and quickly
raises the temperature of the skin.
Metals have very high thermal capacity. They cool slowly.
If the metal is thick, a lot of energy will be quickly
transferred to the skin, the skin temperature will rise,
and a thermal injury (burn) is likely to occur.
If the metal is thin (e.g., a foil), there will be little
stored thermal energy, and the metal temperature will quickly
approach that of the skin. A thermal injury (burn) is not
likely. Only a small amount of thermal energy is transferred
to the skin.
The clue to a thermal injury (burn) is rate of energy
transfer to bring the skin temperature high enough to
destroy the cells before the energy stored in the object
is dissipated such that the object temperature drops to
less than skin injury temperature.
This evaluation is straight-forward thermodynamics. The
equations are slightly more complicated than Ohm's law,
but not more complicated than ac circuit theory.
Our safety standards take the overly-conservative position
by specifying maximum temperture, and ignore the other two
parameters.
This would be okay if the standards would also give us the
option of performing a thermal evaluation. Or, set
temperature limits based on material and mass.
Best regards,
Rich
-------------------------------------------------------------
Richard Nute Product Safety Engineer
Hewlett-Packard Company Product Regulations Group
All-In-One Division Tel : +1 619 655 3329
16399 West Bernardo Drive FAX : +1 619 655 4979
San Diego, California 92127 e-mail: ri...@sdd.hp.com
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