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Thermal resistance of plywood

F

Fred Bartoli

Jan 1, 1970
0
John Larkin a écrit :
Where?

John

Thermal resistance is in K/W, while you're speaking of K/W/ohm.

I wonder how this should be called.
 
J

John Larkin

Jan 1, 1970
0
John Larkin a écrit :

Thermal resistance is in K/W, while you're speaking of K/W/ohm.

I wonder how this should be called.

Thermal resistance per ohm, just like I wrote it.

Most pure metals hit about the same value. Brass is about 2:1 higher,
so use brass screws to conduct electricity with minimum heat transfer.

John
 
J

John KD5YI

Jan 1, 1970
0
Jim Thompson said:
Several years ago, as an aside to another thermal question, I asked...

"Brings to mind a thing I've been pondering... what's the thermal
resistance of 3/4" plywood ?:)"

Never saw a response.


From http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html
about .13 W/mK is the thermal conductivity. So, about 7.7 mK/W resistivity.

More specifically, a closed plywood box, inside dimensions of 8" x 11"
x 18".

Outside ambient of around 77°.

How many watts of dissipation inside of box to raise interior air
temperature to 95°F? (I might use a fan to circulate the air.)


I estimate about 86 or so watts.

If I made one wall of the box 1/8" glass (8" x 18" side), how much
change?


Not much.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine Sometimes I even put it in the food


Cheers,
John
 
J

Jim Thompson

Jan 1, 1970
0
Jim Thompson said:
Several years ago, as an aside to another thermal question, I asked...

"Brings to mind a thing I've been pondering... what's the thermal
resistance of 3/4" plywood ?:)"

Never saw a response.


From http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html
about .13 W/mK is the thermal conductivity. So, about 7.7 mK/W resistivity.

More specifically, a closed plywood box, inside dimensions of 8" x 11"
x 18".

Outside ambient of around 77°.

How many watts of dissipation inside of box to raise interior air
temperature to 95°F? (I might use a fan to circulate the air.)


I estimate about 86 or so watts.

If I made one wall of the box 1/8" glass (8" x 18" side), how much
change?


Not much.

...Jim Thompson
[snip]


Cheers,
John

I swagged 63W... much more than I anticipated... another WAI
(wild-ass-idea) down the tubes :-(

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine Sometimes I even put it in the food
 
J

Jim Thompson

Jan 1, 1970
0
...

Actually, the required wattage is probably between 20 and 40 watts.
As I mentioned in a previous post, 40W maintains close to the
desired temperature in a larger box:

"Here are some measurements from a light-bulb-heated food dryer with its
vents closed: Turned on a 40W bulb at 3pm, 75F. At 4:20, with 90F near
the middle of the box, turned off 40W bulb, turned on 60W bulb. At 5:20,
with 108F near the middle, turned off 60W bulb, turned on 40W bulb. At
10pm, 94F near the middle. This box is about 10"x11"x28" inside, or
twice the volume of your 8"x11"x18" box. 50% of its surface is 3/4"
wood, 35% is 1/8" masonite, and the rest is window glass."

OK, useful numbers, and right on-point... dryer application. I don't
want the light, so I'll just use big wirewounds bolted to a finned
heatsink/source ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine Sometimes I even put it in the food
 
S

Spehro Pefhany

Jan 1, 1970
0
Thermal resistance per ohm, just like I wrote it.

Most pure metals hit about the same value.

At room temperature --- Wiedemann–Franz. The ratio is PTAT, right?
 
J

John KD5YI

Jan 1, 1970
0
z.b. said:
...

Actually, the required wattage is probably between 20 and 40 watts.
As I mentioned in a previous post, 40W maintains close to the
desired temperature in a larger box:

"Here are some measurements from a light-bulb-heated food dryer with its
vents closed: Turned on a 40W bulb at 3pm, 75F. At 4:20, with 90F near
the middle of the box, turned off 40W bulb, turned on 60W bulb. At 5:20,
with 108F near the middle, turned off 60W bulb, turned on 40W bulb. At
10pm, 94F near the middle. This box is about 10"x11"x28" inside, or
twice the volume of your 8"x11"x18" box. 50% of its surface is 3/4"
wood, 35% is 1/8" masonite, and the rest is window glass."


I stand corrected. I used the wrong coefficient for natural convection.

Thanks.

Cheers,
John
 
A

Archimedes' Lever

Jan 1, 1970
0
IIRC pure copper wins, but you have to use really really skinny screws.
Generally the winning scheme for running wires to some thermally
insulated thing is narrow trace Cu flex circuit, preferably with a
thermal ground at each cooling stage.

Cheers

Phil Hobbs

(Getting back to my parts lists now)

Use big nylon nuts and bolts. Or glass filled epoxy.
 
T

TheJoker

Jan 1, 1970
0
OK, useful numbers, and right on-point... dryer application. I don't
want the light, so I'll just use big wirewounds bolted to a finned
heatsink/source ;-)

...Jim Thompson
A rare moment of lucidity.
 
A

Archimedes' Lever

Jan 1, 1970
0
Which don't conduct electricity very well.

John


You do not have to rely on the fasteners to provide the conduction path
between two mating surfaces. In fact, you shouldn't.

I thought you were talking about thermal conduction anyway.
 
J

John Larkin

Jan 1, 1970
0
You do not have to rely on the fasteners to provide the conduction path
between two mating surfaces. In fact, you shouldn't.

I thought you were talking about thermal conduction anyway.

I was talking about the thermal conductivity of electrical conductors.
The same electrons that transport current also transport heat - pretty
much - so most metals are the same as regards how much current you can
carry for a given heat loss. This matters for stuff like getting power
into a crystal oven or a cryogenic gadget, where we really don't want
the heat loss through the leads.

People use stainless and manganin wire to run to really cold (liquid
helium) gadgets so as to not leak too much heat. Google "Garwin
thermal integral" for some messy math.

John
 
L

life imitates life

Jan 1, 1970
0
That's because you've got all the smart people plonked. There were at
least a half-dozen good responses, but apparently the sand got in
your eyes.
Thompson is such an asshole, he doesn't really deserve an answer - why
the hell should we do his googling for him?

Thanks,
Rich


Spot on.
 
J

John KD5YI

Jan 1, 1970
0
Rich Grise said:
That's because you've got all the smart people plonked. There were at
least a half-dozen good responses, but apparently the sand got in
your eyes.
Thompson is such an asshole, he doesn't really deserve an answer - why
the hell should we do his googling for him?

Thanks,
Rich


Well, first of all, it's not my job to try to decide who deserves an answer
and who does not.

Second, I find that I learn things by trying to help others. So, it's
somewhat of a selfish motive, I guess.

You're welcome,
John
 
J

Jim Thompson

Jan 1, 1970
0
Well, first of all, it's not my job to try to decide who deserves an answer
and who does not.

Second, I find that I learn things by trying to help others. So, it's
somewhat of a selfish motive, I guess.

You're welcome,
John

Aaaaah! Life is wonderful... I've been promoted to "asshole"! Are
there grades of "asshole", like "lieutenant asshole", or "rear admiral
asshole"... ?:)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

Obama: A reincarnation of Nixon, but without ANY scruples,
masquerading in politically-correct black-face.
 
K

krw

Jan 1, 1970
0
Aaaaah! Life is wonderful... I've been promoted to "asshole"! Are
there grades of "asshole", like "lieutenant asshole", or "rear admiral
asshole"... ?:)

Sure, many of us are privates, but there are also general assholes,
like Slowman and DimBulb. ...and of course, the leftist fairies'
assholes aren't private at all.
 
S

StickThatInYourPipeAndSmokeIt

Jan 1, 1970
0
Sure, many of us are privates, but there are also general assholes,
like Slowman and DimBulb. ...and of course, the leftist fairies'
assholes aren't private at all.


No I'm an Admiral.

You? You are like that of a sunspot on my dim bulb. That is the level
of your existence. A thermal void.
 
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