Sam,
It was in Tubes 201. I misquoted it, forgetting the negative value. It was 10 ^
-10 atmospheres or 10 ^ - 7 mm mercury. That would be about 0.000000002 PSI if
my conversion calc is correct. Sorry about the mis-quote, I read over the text
once and could have swore that's what it said. Below is a quote;
"Vacuum (lack of it) and its effect"
"In theory, the inside of a tube?s envelope is a vacuum. In practice, a perfect
vacuum is unachievable, and a certain level of residual gas has to be accepted.
The pumps that are used to evacuate tubes can typically get down to about 10 ^
-7 mm of mercury, or about 10 ^ -10 of atmospheric pressure. However once the
tube is sealed, gas can still get into the space inside it. First, the
glass-to-metal seals around the pins or lead-out wires are not perfect, and
allow small amounts of air to pass. Secondly, a certain amount of gas is
adsorbed at the surface of the metals, mica and glass inside the tube. Under
the high vacuum, this gas is slowly released, particularly if the surface is
raised to a high temperature. This is the main reason why allowing the plate of
a tube to overheat is bad for it ? it results in a sudden release of gas which
interferes with the tube?s operation and can rapidly result in destruction of
the cathode".
Best,
Will
*********** REPLY SEPARATOR ***********
On 7/28/06 at 10:46 PM S. J. Blackwell wrote:
>Vacuum is usually measured in absolute pressure, ie a perfect vacuum
>would
>be 0 atm and exists no where in the universe. A really good vacuum in
>scientific equipment would be in the range
>of 1x10^-12 mmHg. A vacuum of 10 atm is a non-sequitur and would cast in
>doubt any thing else the text may have in it..
>Best
>Sam
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