On 9/14/2010 9:10 AM, jimlux wrote:
> Roger (K8RI) wrote:
>>
>>
>> On 9/13/2010 8:58 PM, jimlux wrote:
>>> Gary Schafer wrote:
>>>> Simple spark gaps are not so simple. Temperature, humidity,
>>>> contamination by
>>>> dirt etc will all make a difference on what voltage they arc at.
>>>> Gas tubes
>>>> are much simpler and more reliable for repeatability of breakdown
>>>> voltage.
>>>
>>> Indeed, gas tubes are nice for that reason, BUT.. they do cost more
>>> than a couple pieces of wire (not much more), and I'm not sure that
>>> in this application you need that kind of control. After all, the
>>> original intent here was for installation at the tower, *in addition
>>> to* the high quality protection at the shack entrance. All you're
>>> really doing here is protecting the relatively cheap relay, or, more
>>> accurately, saving yourself the hassle of replacing it on a big strike.
>>>
>>> And, the breakdown voltage doesn't change all that much. It goes as
>>> density of the air, so from -20C to +50C, you're talking about 20%
>>> variation or so. Humidity effect is very small.. maybe a couple
>>> percent.
>>>
>> The problem from humidity comes from condensation and that
>> condensation is far from pure, just check the PH. It'll conduct like
>> crazy and with a .020 to .030" gap it's likely to bridge the gap
>> creating a conductive path. OTOH after a slight burp in the
>> impedance at a KW it'll clear...or initiate an arc.
>
> wouldn't that same condensation also bridge the relay contacts?
I don't think it'd be as much of a problem as the relays normally use a
much larger spacing when open.
I think (I'd have to go measure it) that the open frame relays run about
0.050 spacing when open.
73
Roger (K8RI)
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