Talk:Hot-filament ionization gauge

apllication of hot filament ionization guage precaution and sources of error

Bayard-Alpert

Electrons hitting the grid produce X-rays that produce photoelectric noise in the ion collector. This limits the range of older hot-cathode gauges to 10-8 Torr and the Bayard-Alpert to about 10-10 Torr. Additional wires at cathode potential in the line of sight between the ion collector and the grid prevent this effect.

This text needs improvement.

• If the effect can be prevented by additional wires, then guages with those extra wires should be effective to an even lower pressure (say, 10^-11 Torr) and their limit should be caused by something other than photoelectric noise. Is this true? (And if it were, what would the similar gauge with just those extra wires be named, what would its limit be, and what would be the cause of its limit?)
• It doesn't say how the BA is different from older gauges. If both are limited by photoelectric noise, why is the limit different? Does BA have the aforementioned extra wires? If BA is specifically designed to avoid photoelectric noise, why is photoelectric noise still the limiting factor of BA?
• "Photoelectric noise" is such as opaque term, suggesting some arcane incomprehensible effect. When really, it's such a simple matter: electrons smacking onto electrodes produce x-rays, which strike and eject new electrons from other electrodes, these photoelectrons form a current which is mistaken for the positive ion current (to which the photoelectrons are of course drawn in the opposite direction by the applied voltage). And naturally this unwanted signal-component might be avoided simply with x-ray shielding to ensure the surfaces that collect the thermal electrons are obscured from the electrode which collects the ions.. although whether that approach is taken by BA isn't explained yet in any detail..

This article could do with a much better exposition of the Bayard-Alpert detector. Supposedly it was critical in the history of vacuum, which (in the 20th C.) had been stalled for three decades purely due to the vacuum experts being ignorant regarding the photoelectric effect (and erroneously presuming the failure to be in their pumps rather than their meter).[1] Cesiumfrog (talk) 03:24, 10 July 2012 (UTC)

I correct a mistake: the relevant ionization takes place inside the helical grid. (Positive) ions formed outside are retarded by the positive grid and cannot reach the central cathode.Petersteier (talk) 09:31, 5 February 2013 (UTC)