On 03/07/2017 08:50 PM, John Larkin wrote:
> On Tue, 7 Mar 2017 12:17:40 -0500, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>
>> On 03/07/2017 11:49 AM, John Larkin wrote:
>>> On Tue, 7 Mar 2017 06:51:48 -0800 (PST), George Herold
>>> <gherold@teachspin.com> wrote:
>>>
>>>> On Tuesday, March 7, 2017 at 9:33:38 AM UTC-5, Phil Hobbs wrote:
>>>>> On 03/06/2017 05:17 PM, John Larkin wrote:
>>>>>> On Mon, 6 Mar 2017 09:42:27 -0500, Phil Hobbs
>>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>>>>
>>>>>>> On 03/04/2017 07:22 PM, John Larkin wrote:
>>>>>>>> On Sat, 4 Mar 2017 11:45:26 -0800 (PST), George Herold
>>>>>>>> <gherold@teachspin.com> wrote:
>>>>>>>>
>>>>>>>>> On Friday, March 3, 2017 at 6:19:23 PM UTC-5, John Larkin wrote:
>>>>>>>>>> On Fri, 3 Mar 2017 20:20:11 -0000 (UTC), Chris <cbx@noreply.com>
>>>>>>>>>> wrote:
>>>>>>>>>>
>>>>>>>>>>> This may be a stupid question, but here goes.
>>>>>>>>>>> We all know that cable is graded for its current carrying capabilities
>>>>>>>>>>> according to its cross-sectional area. BUT, could one conceivably pass
>>>>>>>>>>> excessive amounts of current through a cable not rated to carry it by
>>>>>>>>>>> pulsing the current in short bursts at a very low duty cycle?
>>>>>>>>>>
>>>>>>>>>> Sure. The cable heats up from the current (current squared,
>>>>>>>>>> approximately) and has some heat storage capacity. So you can really
>>>>>>>>>> whack it for a short time, milliseconds to tens of seconds maybe,
>>>>>>>>>> before the copper gets too hot.
>>>>>>>>>>
>>>>>>>>>> Wire can handle a lot of current if you cool it, too. Most power
>>>>>>>>>> wiring stuff assumes that wires are inside jackets, inside walls
>>>>>>>>>> maybe, where there's not much cooling. So power wire is conservatively
>>>>>>>>>> rated for current.
>>>>>>>>>>
>>>>>>>>>> Pulse bursts don't increase the long-term RMS current capacity of a
>>>>>>>>>> wire. They actully reduce it.
>>>>>>>>>>
>>>>>>>>>> (Which could restart the argument about "average RMS current.")
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> --
>>>>>>>>>>
>>>>>>>>>> John Larkin Highland Technology, Inc
>>>>>>>>>> picosecond timing precision measurement
>>>>>>>>>>
>>>>>>>>>> jlarkin att highlandtechnology dott com
>>>>>>>>>> http://www.highlandtechnology.com
>>>>>>>>>
>>>>>>>>> For thing like magnet coils (we do mostly air coils)
>>>>>>>>> you can totally run 'em high, we have one instrument,
>>>>>>>>> that limits the duty cycle... up to a 20 second period.
>>>>>>>>
>>>>>>>> Big (non-superconducting) electromagnets are usually water cooled.
>>>>>>>> They have a lot of copper volume to surface area ratio, so get hot.
>>>>>>>>
>>>>>>>>>
>>>>>>>>> For a hunk of copper, there should be some current, that
>>>>>>>>> raises the piece 1 deg K/ sec. (Well at least for small changes in T)
>>>>>>>>>
>>>>>>>>> A related question, (and currently of more interest to me. NPI)
>>>>>>>>> is how much current can a wire carry in vacuum.
>>>>>>>>> I've got some graphs on my computer at work, but I'm not sure I believe
>>>>>>>>> them.... The wire is phosphor-bronze,
>>>>>>>>> this looked good, but they didn't model radiation..?
>>>>>>>>> https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20090032058.pdf
>>>>>>>>>
>>>>>>>>> I've never heard of this, but do people paint their
>>>>>>>>> wires black? Better radiators.
>>>>>>>>
>>>>>>>> Most shiny metals have low emissivities at thermal wavelengths. Copper
>>>>>>>> is an almost perfect mirror at thermal IR. So in a hard vacuum,
>>>>>>>> practically the only cooling will be conduction to the end
>>>>>>>> terminations.
>>>>>>>>
>>>>>>>> Making the wire black (at thermal wavelengths!) would really help.
>>>>>>>> Smashing it into a ribbon would increase the surface area, too.
>>>>>>>>
>>>>>>>> Painted or insulated wire is better than bare metal, unless you can
>>>>>>>> run literally red hot. Most organics have high emissivity.
>>>>>>>>
>>>>>>>> Interesting experiment: try bare copper wire vs magnet wire, same
>>>>>>>> sizes, same current, in vacuum. Inferr the temperature from the
>>>>>>>> resistance.
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>> Thick black copper oxide has an emissivity of about 0.78 in the thermal
>>>>>>> IR, according to
>>>>>>> <http://www-eng.lbl.gov/~dw/projects/DW4229_LHC_detector_analysis/calculations/emissivity2.pdf>
>>>>>>>
>>>>>>> Cheers
>>>>>>>
>>>>>>> Phil Hobbs
>>>>>>
>>>>>> I guess that copper will get hot and tarnish some.
>>>>>>
>>>>>> Not to change the subject, but regular office white-out has a very
>>>>>> high emissivity. So you can dab it on shiny things, like the metal
>>>>>> tops of some FPGAs and such, to read the temps better.
>>>>>>
>>>>>> Kapton tape is pretty good.
>>>>>>
>>>>>> https://dl.dropboxusercontent.com/u/53724080/Thermal/Cool1.JPG
>>>>>>
>>>>>> https://dl.dropboxusercontent.com/u/53724080/Thermal/Cool2.jpg
>>>>>>
>>>>>>
>>>>>>
>>>>> Just about any dielectric at least a few mils thick has a thermal
>>>>> emissivity of about 0.95. The rest is Fresnel reflection at the surface.
>>>>
>>>> Fresnel reflection... I had to look it up. (dielectric mismatch)
>>>> Is this for IR wavelengths? Certainly off white paint is higher than
>>>> that in the visible.
>>>> Maybe I can get the wire with a triple or quadruple build
>>>> of insulation. That adds... (checks MWS catalog) ~30 mil to the diameter.
>>>>
>>>> George H.
>>>
>>> The more insulation, the cooler the wire! A universe full of plastic
>>> conducts heat better than a universe full of vacuum.
>>>
>>>
>>
>> T'other way round. Near room temperature, any thickness of vacuum looks
>> like about 5 mm of air, iirc, which is 0.5 mm of plastic.
>>
>
> I don't get that. Vacuum conducts heat better than air? 0.5 mm of
> plastic conducts heat as well as 5 mm of air?
>
> But it is complex. More insulation increases the radiation surface,
> which works against the T^4 radiation curve. And more insulation
> conducts heat out to the surface of a given radius, better than a
> vacuum gap would... depending on the thermal conductivity of the
> insulation. All that math is way past my pay grade.
>
>
Yup. In vacuo, the heat just radiates away into space, so any thickness
of vacuum has the same thermal resistance, namely the derivative of the
Stefan-Boltzmann law, i.e.
L = epsilon sigma T**4,
where epsilon is the thermal emissivity. Two parallel surfaces with
different temperatures will exhibit a power transfer per unit area of
Delta L = epsilon_1 epsilon_2 sigma (T_1**4 - T_2**4),
which for small delta-T is
alpha = dL/dT = 4 epsilon_1 epsilon_2 sigma T**3.
It's modified some by the thermal emissivity of the emitter and
surroundings.
That's how superinsulation works--you have many layers of metallized
Kapton, spaced out so that they don't touch. Works great, but it's an
absolute bear to bake out--all that surface area, the constricted space
for gas to diffuse out, and the superior insulation making it hard to
get it all hot.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net