Best tube material for VDG?
Legrand, Mon Feb 12 2007, 04:17PMCan anyone tell me what might be the best type of plastic to use for a Van de Graaff generator tube?
I'd like the plastic to be clear, however I'm not sure which type of plastic tube material will best keep the static charge from traveling down it. I'm currently looking at clear ABS, clear PVC, acrylic, butyrate and polycarbonate. It's difficult to find specs on these plastics, and even when I do, I see volume resistivity, surface resistivity, dielectric constant, etc. Which would be the proper specification to optimize?
Thanks!
Re: Best tube material for VDG?
Dr. Slack, Tue Feb 13 2007, 12:47PM
Compared to voume or surface resistivity specs intrinsic to any of those plastics, the best two parameters to get right are the surface cleanliness (no finger prints etc), and the humidity (dry as a bone, static weather). (My son's school had an open evening a while ago and he went in with instructions to blow-dry the VDG, whatever the teachers said. For the first time in years they had a decent demo.)
The next parameter which does have something to do with the plastic is the surface finish. Acrylic can acquire fine surface cracks which attract moisture, so it's generally not the first choice for HV use, but a new clean specimen can be OK.
Dr. Slack, Tue Feb 13 2007, 12:47PM
Compared to voume or surface resistivity specs intrinsic to any of those plastics, the best two parameters to get right are the surface cleanliness (no finger prints etc), and the humidity (dry as a bone, static weather). (My son's school had an open evening a while ago and he went in with instructions to blow-dry the VDG, whatever the teachers said. For the first time in years they had a decent demo.)
The next parameter which does have something to do with the plastic is the surface finish. Acrylic can acquire fine surface cracks which attract moisture, so it's generally not the first choice for HV use, but a new clean specimen can be OK.
Re: Best tube material for VDG?
cbfull, Wed Feb 14 2007, 06:17PM
The VDG builders that I know generally use whatever they can afford. Clear acrylic is generally sought after, but it can crack if not handled properly.
The key is (as NeilThomas stated) making sure the column is very clean and dry. If cost were not such a factor, I would go for polycarbonate since it is crack resistant and lightweight. However, odds are it will not make a measurable difference in your machine's performance.
cbfull, Wed Feb 14 2007, 06:17PM
The VDG builders that I know generally use whatever they can afford. Clear acrylic is generally sought after, but it can crack if not handled properly.
The key is (as NeilThomas stated) making sure the column is very clean and dry. If cost were not such a factor, I would go for polycarbonate since it is crack resistant and lightweight. However, odds are it will not make a measurable difference in your machine's performance.
Re: Best tube material for VDG?
Tesladownunder, Thu Feb 15 2007, 01:31AM
My VDG only did 2 inch sparks, so I am not the person to comment really but...
You need to realise that surface tracking over any nonconductive surface can be up to 3 times the spark length in air. This is one reason why insulators have a corrugated surface. it is hard to make a cheap light and structurally strong corrugated VDG support however.
Don't expect to get a spark 3 times the secondary length like with a TC in which spark growth is way above what is expected from peak voltage, more like the other way around ie 3ft high for a 1 foot spark.
Here is an ignition coil giving 3 inch sparks but the surface tracking is 9 inches on an alcohol cleaned surface.
TDU
Tesladownunder, Thu Feb 15 2007, 01:31AM
My VDG only did 2 inch sparks, so I am not the person to comment really but...
You need to realise that surface tracking over any nonconductive surface can be up to 3 times the spark length in air. This is one reason why insulators have a corrugated surface. it is hard to make a cheap light and structurally strong corrugated VDG support however.
Don't expect to get a spark 3 times the secondary length like with a TC in which spark growth is way above what is expected from peak voltage, more like the other way around ie 3ft high for a 1 foot spark.
Here is an ignition coil giving 3 inch sparks but the surface tracking is 9 inches on an alcohol cleaned surface.
TDU
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