[time-nuts] Tek high voltage probes
jimlux at earthlink.net
Thu Sep 22 17:59:13 UTC 2011
On 9/22/11 10:34 AM, NeonJohn wrote:
> On 09/21/2011 04:57 AM, David C. Partridge wrote:
>> Howsabout HFC-236fa - very similar properties to R114 but not banned.
>>> Tektronix used a Freon in their 40 KV High Voltage probes. The Vapor
>>> pressure of some of those compounds is low at 70F, but they do have to
>>> be sealed.
> I use several of the Tek probes in my work (and of course am too cheap
> to buy the newer solid dielectric ones). When my freon ran out, I
> searched around for a replacement fluid and found ordinary butane
> straight from the Ronsonol can to be equal to or maybe even better than
> the original freon.
Yes.. most hydrocarbons make a fine dielectric (viz. transformer oil).
To a certain extent, density is key, which is why the halogenated ones
are nice (F or C are more massive than H). The other thing is that
those halogens are electronegative which tends to suppress breakdown
(why SF6 is great.. not only is it really dense, but it's also non
flammable AND it's got fluorine in it)
The halogenated ones are preferred in some cases because you can get
other vapor pressures and/or they don't burn.
There is, of course, a whole class of nice dielectric fluids based on
halogenating double ring structures: PCBs. They're really inert, great
dielectric strength, immiscible with contaminants (no dissolved water,
which ruins dielectric properties). The problem is that there's an
unavoidable carcinogenic contaminant (dioxin-like PCBs and PCDFs). And,
because they ARE so resistant to breakdown, they persist forever, and,
unfortunately, are fat soluble and taken up by wildlife.
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