Topband: Verifying integrity of 75 ohm coax.

[Tom W8JI]

> Thanks for your thoughts on this. But all this tells me is that some
> silicone dielectric greases may not be abrasive. I am certain that some
> are, because I am not the only one who has experienced this.
>

I have no doubt you experienced contact wear, which is not unusual for 
contacts that arc when switched when any petroleum based grease is used. 
That is a well-documented failure, but it is not caused by silicone grease 
being abrasive. It is caused by the arc carburizing the grease into a 
silica-carbide, which is a well-known abrasive.

> The stuff I used was either from Radio Shack or an automotive parts store.
> It was in a small gray squeeze tube. IIRC, the brand was Permatex, but I 
> am
> not 100% certain. (In my last post, I stated it was labeled "silicone
> dielectric grease", but that may be wrong; it may have had "tune-up 
> grease"
> on the tube.
>
> Clear liquid silicone oil is not the same as the translucent grease. 
> People
> have successfully used silicone oil as a lubricant for many decades.
> And what kind of silver-plated rotary meter switches needed silicone 
> grease
> in order to make them last? That's puzzling. Was it grease or oil?

It is grease, and it is one of the more commonly used greases in many wiping 
switch contacts. It cannot be used in higher voltage switches, or with 
contacts that arc. Actually, very few greases are acceptable in hot switched 
contacts.

Almost everyone with low-current low-voltage switch or potentiometer 
applications knows they need a lube that also blocks air. While silver oxide 
is somewhat of a conductor, and while gold resists any tarnish, the platings 
are normally in ten thousandths of an inch thick. Without lube they will 
quickly wear away.  Silver in contact with air in typical environments forms 
silver-sulfide, and silver sulfide and other oxides of silver are not good 
conductors. They are, at best, semi-conductors.

That is why they are normally lubed. The exception is generally when 
contacts are in applications that have arcing.

We should be careful attributing wear to an abrasive grease, when almost 
always wear in lubed contacts comes from an arc carburizing the grease. 
Here's a quote:

"Silicone grease should not be applied to (or next to) any switch contact 
that might experience arcing, as silicone can convert to silicon-carbide 
under arcing conditions, and accumulation of the silicon-carbide can cause 
the contacts to prematurely fail."

73 Tom