Archived from groups: alt.comp.hardware.homebuilt (More info?)
"David Maynard" switches to MicroATX:
>Timothy Daniels wrote:
>>>>I gave you the NLX and ATX specs, which
>>>>delineate maximum hole size for EMI shielding,
>>>>and an entire article on using waveguide technology,
>>>>for fan vents, to improve EMI shielding.
> [...]
>> C'mon, Davey. Post the link. Let's see the ATX specs
>> which tell about EMI and the maximum size for vent holes.
>> C'mon, c'mon. I wanna learrrrrn. <hee, hee>
>
> Okay, laughing boy, for the sake of variety,
> let's look at the microATX thermal design
> suggestions from formfactor.org
Before we dance off to MicroATX,
we should note that the ATX specs
(http://www.formfactors.org/developer/specs/atx2_2.pdf)
make no mention of size of vent holes - versus EMI
or air flow or anything else.
Here is all the ATX specs say about cooling:
"Adequate venting should be provided in the system
to allow for unimpeded and well-directed airflow to
cool key components such as the processor. One
recommendation that is implicit in the ATX specification
is the placement of the power supply. The power supply
should be placed in close proximity to the processor if
the power supply is expected to cool the processor
properly (but be sure to observe the component height
keepouts over the PC board). Chassis venting should
be placed strategically to allow for proper cooling of
other components such as peripherals and add-in cards.
A system fan should be considered to allow for proper
cooling of all system components."
It says nothing about EMI from vents, nothing about size
of vents, nothing about quality of air flow except that it
should be "unimpeded" and "well-directed" to cool things
like the processor, and that the chassis venting should be
"placed strategically" to cool non-motherboard cards and
hard drives.
Now then, the MicroATX specs:
>http://www.formfactors.org/developer/specs/microatx_thermal_dg01.pdf
>
>"2.8.1 Chassis and Bezel Venting
> Proper venting is a key element in any good thermal design.
> A balanced vent configuration is a critical factor in this design.
> Implementing an insufficient amount does not allow enough
> air into the system for adequate cooling...
>
> Key considerations:...
>
> Front chassis and bezel venting -
> The bezel vent area should be as large as possible because
> it serves as the main air inlet for the system. Ensure the plastic
> bezel vent pattern allows air to enter freely so it does not
> overly restrict airflow into the system."
>
> Note there is no mention to include 'right angle bends'
> in the 'fancy 3-dimensional molded tight tolerance plastic fascia'
> to accommodate 'Tims turbulence theory'. No, it says 'allows
> air to enter FREELY." In fact, you won't find one word about
> 'ensuring turbulence' anywhere, much less 'vent hole turbulence'.
> Everything is eliminate resistance, eliminate resistance, and
> eliminate resistance.
Read it again. It says "allows air to enter freely so it does not
OVERLY RESTRICT airflow into the system". It does not
define "overly", neither does it recommend against turbulence.
That MicroATX says nothing about turbulence of the air
flowing into the case only means that turbulence is hard
to define. How can anyone define how much swirl, what
diameter of vortices, their relative orientation, speed of
rotation, etc.? That is why it takes repeated a cut-'n-try
experimentation by the labs of the name brand vendors to
get it right and why the sellers of empty cases cannot compete.
To expect that such a low level of specification should be
contained in the MicroATX specs is to expect that all PCs
be built exactly the same. Obviously, there is no intention
of doing that. And there is no mention of "eliminate resistance"
of air flow at the entrance to the case. The free air flow
described is that which is WITHIN THE CASE, between
the components to be cooled. The greater efficiency in cooling
afforded by turbulent air is not all addressed by the MicroATX
spec because it is a technique used by the name brand vendors
to give better cooling with less cost and less intake noise.
> For EMI:
> "(NOTE: To eliminate possible electromagnetic compliance issues,
> neither the maximum vertical nor maximum horizontal dimensions
> of ventilation apertures, I/O ports, and open areas along chassis
> seams less than 1/20th of a wavelength of the highest harmonic
> frequency of interest.)"
>
> Well, shazzam, an EMI caution.
Well, shazzam, an EMI caution for MicroATX form factor.
> Well, let's see what the microATX document specific for EMI says.
> http://www.formfactors.org/developer%5Cspecs%5Cmatxemc.pdf
>
> "2.5.2 Apertures
> Keep maximum linear dimensions of ventilation apertures,
> I/O ports, and open areas along chassis seams less than 1/20th
> of a wavelength ( l ) of the highest harmonic frequency ( f ) of
> concern (1/20th rule, see also Figure 2). Absorption and shield
> thickness may contain low frequency magnetic fields, but high
> frequency electric field radiation out of slots becomes the next
> concern. Apertures (or slots) can be viewed as half-wave
> dipole antennas and are thus able to radiate maximum energy
> at dimensions of 1/2 a wavelength. In fact, slots longer than
> 1/100th of a wavelength can cause considerable leakage.
> Therefore, it is necessary to keep slot lengths as short as
> possible to minimize leakage. Currently, the FCC has
> requirements up to 2 GHz, which, as derived below, correspond
> to a recommended maximum aperture size (vertical or horizontal)
> of about .75 centimeters; for example:" (see PDF for the example
> equations.)
>
> Converting to inches that means, for a nice el-cheapo stamped
> vent, round holes under .295 inches.
Hmmm... you missed section 2.5.7 which says:
"A large number of small holes give better shielding than
a single large hole of the same area. Either large holes
or small holes placed too close together can become
significant slot antennas. Space small holes apart by
a distance equal to the diameter of the hole ( l / 20).
Reduce emissions from large holes by placing screens over
large holes or forming a 'waveguide below cutoff.'"
In other words, a WIRE GRILL over one big hole would
give the same protection against EMI.
> And Intel discussion on EMI
>http://www.intel.com/design/pentiumii/applnots/24333402.PDF
>
> Mostly processor, heatsink, and board layout but the chassis
> is mentioned at the end.
>
> ". Where possible, use round holes instead of slotted holes.
> Round holes provide the greatest airflow volume for the least
> amount of EMI leakage."
>
> Now you know why the holes are typically round.
>
> In the interest of fair time we might as well see what AMD
> has to say about it.
>
>http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/22023.p
df
>
> Same comprehensive discussion centering on the electronics
> but then we also have the chassis section.
>
> Apertures in a Chassis: Apertures (vents, holes, seams, screens)
> in a chassis can cause EMI leakage.
>
> . Apertures radiate at wavelengths equal to or less than their length.
> The length of an aperture is inversely proportional to the leaking
> resonant frequency.
> . Small holes and short seams prevent leakage of low frequencies.
> . Circles have minimal width for a given area, so round vents are
> better than slots (which are typically rectangular).
> . Screens are the best vents because of their small hole size.
> . Apertures should be shorter than one tenth of the wavelength
> to be shielded. For instance:
> . A 300-MHz frequency has a 1-meter wavelength, so boxes
> with harmonics less than 300 MHz can have 10-centimeter
> apertures."
>
> Well, whadda ya know, they say the same thing.
Yup:
"Screens are the best vents because of their small hole size."
It confirms that a simple WIRE GRILL would have the same
effect as a lot of small holes.
> Inexpensive vent hole summary: Stamp enough under .295 inch
> diameter round (so the FCC will stay off your butt) vent holes
> in the metal chassis where you want air to enter. Put fascia on
> front so it looks pretty, making sure there's plenty of room for
> air to get to the vent holes you stamped in the metal chassis.
It says nothing about how air is to get into the case, but only
that it have free flow once inside the case in order to get to
the components to be cooled. That is one of your blind spots -
you confuse free air flow inside the case with freedom to enter
the case.
And besides being pretty, the fascias designed by the big vendors
direct the air NOT smoothly and NOT directly into the holes
with their molded plastic fascias, but at various 90 degree angles
to maximize the turbulence.
And ask yourself why the name brand vendors didn't take
one of those cheap wire grills that they put over air exhaust
fans and put it over one big hole in the front. That one big
hole would allow almost NO AIR RESTRICTION, and the
grill would block the EMI much more effectively. Why did
they do that? Because the entering air would have little
turbulence. Instead, they went with a bunch of little holes
having a much greater edge-to-area ratio - which increases
turbulence.
*TimDaniels*
"David Maynard" switches to MicroATX:
>Timothy Daniels wrote:
>>>>I gave you the NLX and ATX specs, which
>>>>delineate maximum hole size for EMI shielding,
>>>>and an entire article on using waveguide technology,
>>>>for fan vents, to improve EMI shielding.
> [...]
>> C'mon, Davey. Post the link. Let's see the ATX specs
>> which tell about EMI and the maximum size for vent holes.
>> C'mon, c'mon. I wanna learrrrrn. <hee, hee>
>
> Okay, laughing boy, for the sake of variety,
> let's look at the microATX thermal design
> suggestions from formfactor.org
Before we dance off to MicroATX,
we should note that the ATX specs
(http://www.formfactors.org/developer/specs/atx2_2.pdf)
make no mention of size of vent holes - versus EMI
or air flow or anything else.
Here is all the ATX specs say about cooling:
"Adequate venting should be provided in the system
to allow for unimpeded and well-directed airflow to
cool key components such as the processor. One
recommendation that is implicit in the ATX specification
is the placement of the power supply. The power supply
should be placed in close proximity to the processor if
the power supply is expected to cool the processor
properly (but be sure to observe the component height
keepouts over the PC board). Chassis venting should
be placed strategically to allow for proper cooling of
other components such as peripherals and add-in cards.
A system fan should be considered to allow for proper
cooling of all system components."
It says nothing about EMI from vents, nothing about size
of vents, nothing about quality of air flow except that it
should be "unimpeded" and "well-directed" to cool things
like the processor, and that the chassis venting should be
"placed strategically" to cool non-motherboard cards and
hard drives.
Now then, the MicroATX specs:
>http://www.formfactors.org/developer/specs/microatx_thermal_dg01.pdf
>
>"2.8.1 Chassis and Bezel Venting
> Proper venting is a key element in any good thermal design.
> A balanced vent configuration is a critical factor in this design.
> Implementing an insufficient amount does not allow enough
> air into the system for adequate cooling...
>
> Key considerations:...
>
> Front chassis and bezel venting -
> The bezel vent area should be as large as possible because
> it serves as the main air inlet for the system. Ensure the plastic
> bezel vent pattern allows air to enter freely so it does not
> overly restrict airflow into the system."
>
> Note there is no mention to include 'right angle bends'
> in the 'fancy 3-dimensional molded tight tolerance plastic fascia'
> to accommodate 'Tims turbulence theory'. No, it says 'allows
> air to enter FREELY." In fact, you won't find one word about
> 'ensuring turbulence' anywhere, much less 'vent hole turbulence'.
> Everything is eliminate resistance, eliminate resistance, and
> eliminate resistance.
Read it again. It says "allows air to enter freely so it does not
OVERLY RESTRICT airflow into the system". It does not
define "overly", neither does it recommend against turbulence.
That MicroATX says nothing about turbulence of the air
flowing into the case only means that turbulence is hard
to define. How can anyone define how much swirl, what
diameter of vortices, their relative orientation, speed of
rotation, etc.? That is why it takes repeated a cut-'n-try
experimentation by the labs of the name brand vendors to
get it right and why the sellers of empty cases cannot compete.
To expect that such a low level of specification should be
contained in the MicroATX specs is to expect that all PCs
be built exactly the same. Obviously, there is no intention
of doing that. And there is no mention of "eliminate resistance"
of air flow at the entrance to the case. The free air flow
described is that which is WITHIN THE CASE, between
the components to be cooled. The greater efficiency in cooling
afforded by turbulent air is not all addressed by the MicroATX
spec because it is a technique used by the name brand vendors
to give better cooling with less cost and less intake noise.
> For EMI:
> "(NOTE: To eliminate possible electromagnetic compliance issues,
> neither the maximum vertical nor maximum horizontal dimensions
> of ventilation apertures, I/O ports, and open areas along chassis
> seams less than 1/20th of a wavelength of the highest harmonic
> frequency of interest.)"
>
> Well, shazzam, an EMI caution.
Well, shazzam, an EMI caution for MicroATX form factor.
> Well, let's see what the microATX document specific for EMI says.
> http://www.formfactors.org/developer%5Cspecs%5Cmatxemc.pdf
>
> "2.5.2 Apertures
> Keep maximum linear dimensions of ventilation apertures,
> I/O ports, and open areas along chassis seams less than 1/20th
> of a wavelength ( l ) of the highest harmonic frequency ( f ) of
> concern (1/20th rule, see also Figure 2). Absorption and shield
> thickness may contain low frequency magnetic fields, but high
> frequency electric field radiation out of slots becomes the next
> concern. Apertures (or slots) can be viewed as half-wave
> dipole antennas and are thus able to radiate maximum energy
> at dimensions of 1/2 a wavelength. In fact, slots longer than
> 1/100th of a wavelength can cause considerable leakage.
> Therefore, it is necessary to keep slot lengths as short as
> possible to minimize leakage. Currently, the FCC has
> requirements up to 2 GHz, which, as derived below, correspond
> to a recommended maximum aperture size (vertical or horizontal)
> of about .75 centimeters; for example:" (see PDF for the example
> equations.)
>
> Converting to inches that means, for a nice el-cheapo stamped
> vent, round holes under .295 inches.
Hmmm... you missed section 2.5.7 which says:
"A large number of small holes give better shielding than
a single large hole of the same area. Either large holes
or small holes placed too close together can become
significant slot antennas. Space small holes apart by
a distance equal to the diameter of the hole ( l / 20).
Reduce emissions from large holes by placing screens over
large holes or forming a 'waveguide below cutoff.'"
In other words, a WIRE GRILL over one big hole would
give the same protection against EMI.
> And Intel discussion on EMI
>http://www.intel.com/design/pentiumii/applnots/24333402.PDF
>
> Mostly processor, heatsink, and board layout but the chassis
> is mentioned at the end.
>
> ". Where possible, use round holes instead of slotted holes.
> Round holes provide the greatest airflow volume for the least
> amount of EMI leakage."
>
> Now you know why the holes are typically round.
>
> In the interest of fair time we might as well see what AMD
> has to say about it.
>
>http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/22023.p
df
>
> Same comprehensive discussion centering on the electronics
> but then we also have the chassis section.
>
> Apertures in a Chassis: Apertures (vents, holes, seams, screens)
> in a chassis can cause EMI leakage.
>
> . Apertures radiate at wavelengths equal to or less than their length.
> The length of an aperture is inversely proportional to the leaking
> resonant frequency.
> . Small holes and short seams prevent leakage of low frequencies.
> . Circles have minimal width for a given area, so round vents are
> better than slots (which are typically rectangular).
> . Screens are the best vents because of their small hole size.
> . Apertures should be shorter than one tenth of the wavelength
> to be shielded. For instance:
> . A 300-MHz frequency has a 1-meter wavelength, so boxes
> with harmonics less than 300 MHz can have 10-centimeter
> apertures."
>
> Well, whadda ya know, they say the same thing.
Yup:
"Screens are the best vents because of their small hole size."
It confirms that a simple WIRE GRILL would have the same
effect as a lot of small holes.
> Inexpensive vent hole summary: Stamp enough under .295 inch
> diameter round (so the FCC will stay off your butt) vent holes
> in the metal chassis where you want air to enter. Put fascia on
> front so it looks pretty, making sure there's plenty of room for
> air to get to the vent holes you stamped in the metal chassis.
It says nothing about how air is to get into the case, but only
that it have free flow once inside the case in order to get to
the components to be cooled. That is one of your blind spots -
you confuse free air flow inside the case with freedom to enter
the case.
And besides being pretty, the fascias designed by the big vendors
direct the air NOT smoothly and NOT directly into the holes
with their molded plastic fascias, but at various 90 degree angles
to maximize the turbulence.
And ask yourself why the name brand vendors didn't take
one of those cheap wire grills that they put over air exhaust
fans and put it over one big hole in the front. That one big
hole would allow almost NO AIR RESTRICTION, and the
grill would block the EMI much more effectively. Why did
they do that? Because the entering air would have little
turbulence. Instead, they went with a bunch of little holes
having a much greater edge-to-area ratio - which increases
turbulence.
*TimDaniels*
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