[Amps] Arc distance

[Ian White GM3SEK]

Will Matney wrote:
>Ian,
>
>My reference is the Radio Engineers Handbook by Terman. I have several 
>other books here on getters, but I'd have to dig them up to get the 
>names and authors.
>
>Sure, "full of hydrogen gas" wouldn't happen in a split second, nor 
>would it really fill up the tube. Really, I should have said it would 
>have hydrogen gas in place of O2, etc. in some quantity. After the tube 
>was operated over time, more hydrogen would escape. The kicker is, the 
>anode coated with zirconium would actually absorb hydrogen gas while it 
>was heated at 300 deg, but when it started getting over that 
>temperature somewhere, it would start liberating it at operating 
>temperature. It's an oddity in that type of material. 400 deg C would 
>be where the anode would start to show red color in a dark room from 
>heating up. Terman says you need two different pieces of zirconium 
>heated at two different temperatures to work correctly. Accordingly he 
>says, tantalum would be a much better, but more expensive choice. This 
>is in the vacuum tube section of the book under the title "Getters". I 
>have the 1st edition, but it should be in all of them I would think.

Sorry, I don't have that book of Terman's, only the other one.

However, the chapter on Getters in Kohl's 'Handbook of Materials and 
Techniques for Vacuum Devices' confirms the same general picture, and so 
too does a 1950s reference reproduced on the web:
http://www.thevalvepage.com/valvetek/getter/getter.htm

In the section on Zirconium, the second paragraph points out that it 
does sorb hydrogen quite well at temperatures of 300-500C, but releases 
it again at 850-1000C. Kohl points out that properly treated zirconium 
surfaces can also trap hydrogen down to room temperature, though 
obviously less effectively than at the optimum temperature.

Meanwhile, the optimum temperature for most other species of trace gases 
is rather higher than that.

The implication is that in order for high-temperature getter materials 
to handle all the species involved, the tube needs possibly two or three 
different areas of getter, in different locations covering a range of 
temperatures. This is completely consistent with what the Eimac tube 
designer described.

OK, Will, between us we're making sense of how it works.



-- 
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek