On Tuesday, March 7, 2017 at 10:07:38 AM UTC-5, Phil Hobbs wrote:> On 03/07/2017 09:51 AM, George Herold 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. > > Emissivity, not reflectance. Blackness is what you want for thermal > measurements.Sorry I spoke backwards.. which happens a lot. George h.> > 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
Cables and Currents
Started by ●March 3, 2017
Reply by ●March 7, 20172017-03-07
Reply by ●March 7, 20172017-03-07
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. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●March 8, 20172017-03-08
On Tuesday, March 7, 2017 at 8:50:58 PM UTC-5, 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?I think Phil is calculating the thermal conductivity of air, then equating that to some thermal loss from radiation in vac. But he has to pick some temperature rise... 1 deg K or something. And the air number has to be for low temperature differences.. or convection kicks in...> > 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.Grin... right do the measurement and use that to check/adjust your math. :^) George H.> > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
Reply by ●March 8, 20172017-03-08
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
