Helium infiltration, NOS Valves

[Tyso_Bl]

Has anyone had any experience of Helium infiltration into NOS valves as opposed to CRTs?

T

[G8HQP Dave]

Do you mean helium getting in, or alpha particles failing to get out?

[Herald1360]

I wonder if it might affect the odd valve in my lab drawer at work? I suspect the atmospheric helium concentration here might be way above average- we use 1000's of litres of the liquid variety in our magnets and a significant amount ends up as boil off during some processes.


Helium gas is excellent at finding its way through the smallest of holes- it's used as a very sensitive leak detecting agent whilst checking vacuum vessel welds.

Is helium collection rather than infiltration a problem with thoriated cathode/filament type valves in particular?

[ukcol]

Originally Posted by Tyso_Bl

Has anyone had any experience of Helium infiltration into NOS valves as opposed to CRTs?

I didn't know about Helium infiltration in CRTs let alone valves. What is the mechanism? Are the symptoms the same as for any other gas, ie., softening of the vacuum.

[Mr Moose]

Hello,
This document (http://www.et-enterprises.com/files/...20lifetime.pdf) although referring to photomultiplier tubes says that the rate of helium infiltration varies according to the type of glass used. The time for helium infiltration to cause problems varies from 8 years for pure silica, 30 years for Pyrex, 230 years for borosilicate glass and 144,000 years! for ordinary lime soda glass.
So with ordinary glass valves you might want to put a note in your diary to check them in 146,017.( So not really likely to be a problem.)
I think I have a couple of NOS Mullard silica transmitting valves made for the Royal Navy I think for WW2 so they might have a problem.
Yours Richard

[turretslug]

Intriguing thread.... So pre-war Pyrex CRTs could be prone to acquisition- hence the post#1 qualification? Radon decays to polonium with alpha release- would this phenomenon thus be any more significant in Grampian or Cornwall environs?

[Tyso_Bl]

Thank you Mr Moose for the link, its been very helpful,

T

[ms660]

Originally Posted by turretslug

Intriguing thread.... So pre-war Pyrex CRTs could be prone to acquisition- hence the post#1 qualification? Radon decays to polonium with alpha release- would this phenomenon thus be any more significant in Grampian or Cornwall environs?

Sure would, granite is a source of a lot of radon, I was present when there was a large release of radon in Cornwall, long story short, back in the early 1970's I was working on the 310 fathom level (below adit) at South Crofty tin mine down here in Cornwall, we were heading a tunnel to break through into the old East Pool mine workings that had been drained of water down to the 310 fathom level, when the tunnel was finally blasted through the water level in the old workings was about 1" higher than the floor of the breakthrough tunnel (hats off to the surveyors) lot's of radon was released, a few weeks later my work direction changed as I was to go gold mining abroad, a few years later upon my return I tried to look up the blokes I worked with who continued to work in that section, most of them had gone to an early death.

Nasty stuff is radon...avoid.

Lawrence.

[dave cox]

Even a piece of tissue paper will stop most alpha radiation so no chance of it going through glass (helium permeability is different mechanism).

dc

[turretslug]

An alpha particle is a helium nucleus and radio-active decay is the source of most terrestrial helium, thus the supposition that areas of known radon presence might also exhibit higher-than-usual helium capture in evacuated thin glass vessels.

[ms660]

Here's a Radon map showing the hot spots, as said, lots of Radon down here, the only cure for in building Radon is either ventilation or sub floor radon trap or both, done a couple of them back in the day, basically four blocks with a paving slab on top, underneath of slab vented to outside via 110mm pipe, DPM on top of the slab then insulation/oversite concrete, back then standard DPM was used, these days it's branded as Radon membrane and attracts a premium as it's a bit more duty heavy and apparently engineered to be gas proof.

A few years ago one house down here had a Radon reading of crazy o'clock off the scale, a bit like South Terras :

http://www.ukradon.org/information/ukmaps

Lawrence.

[Philips210]

Hi

Could the weak link be in the glass to metal seal in a valve or CRT? If this is the case, then could the helium infiltration be through this route ie between the pins and surrounding glass?

Regards
Symon

[ms660]

Uranium glass was used to seal some valves.

Lawrence.

[Philips210]

Hi

Regarding the Radon problem, and sorry mods for briefly going OT again, but a good project, Radon Monitor can be built from here http://www.americanradiohistory.com/...ics%20_Now.htm in the Jan 1994 issue . I built this a few years ago and it is very useful.

Regards
Symon.

[David Simpson]

Thanks Lawrence for the mapping link. I shan't sleep tonight as it looks like my whole valve testing and collecting endeavours are in serious jeopardy - my OS map reference is covered by Dark Brown !

Regards, David

[Argus25]

Originally Posted by turretslug

Intriguing thread.... So pre-war Pyrex CRTs could be prone to acquisition- hence the post#1 qualification? Radon decays to polonium with alpha release- would this phenomenon thus be any more significant in Grampian or Cornwall environs?

I have a very disturbing experience to report with pre-war and wartime pyrex CRT's.

I acquired a large number of 5BP4 tubes for test, many exhibited exactly the same problem. These have a much thinner pyrex wall than the thicker one seen on VCR97's (if they are indeed pyrex).

Initially on power up they produced reasonable emission and screen light output and focus. But after a few minutes it all changed with the light output dropped and the focus deteriorated too and there is a violet glow noted around the gun near the control grid to varying extents. It appears as though they had absorbed gas and this had poisoned the cathode emission surface.

I tested at least 15 tubes, more than 10 were like this, the worst being the wartime VT equivalent ones. In the end the only 5BP4 that I could get to work perfectly for a TV image was a late 1950's/early 60's manufacture one, probably the last production run ever made by RCA. So 4 or 5 good ones out of 15 or so is a pretty poor result.

Many of these affected tubes would still pass for a single line radar display, but not a television raster.

I had more luck with 5AP4's which are a shorter version of a 5BP4 made by National union for the Andrea pre war KTE-5 set.

[Philips210]

Hi

It's fascinating the property of helium being able to permeate through an apparent impenetrable material. Going slightly OT again, sorry, I've noticed that some balloons are not suitable for helium fill so wondered if the helium actually passes through the latex or does that only occur around the neck seal?

Regards
Symon

[kalee20]

Definitely does. Even air does slowly permeate through the thin rubber.

I'd expect hydrogen to go through even more, but that's based on simple kinetic theory of A level standard only.

Regarding Argos's experiences, they're puzzling! If they are similar tubes, stored in the same place, I'd expect any inward gas leakage to be the same (unless most had an actual defect such as poor glass-metal seal).

Helium can't poison the cathode as it is an inert gas - though feasibly it might spoil the surface by ion bombardment. I don't believe it's absorbed readily by metals etc, again because it is inert. And a small amount of gas gas even been used to improve focussing, by neutralising the natural tendency of an electron beam to diverge due to mutual repulsion. So, maybe it was some other effect that caused Argos's problem? Latent air absorption not properly baked out prior to sealing?

[Argus25]

Originally Posted by kalee20

Regarding Argos's experiences, they're puzzling! If they are similar tubes, stored in the same place, I'd expect any inward gas leakage to be the same (unless most had an actual defect such as poor glass-metal seal).

The tubes were from multiple sources/locations.I also do not know what the gas is that glows blue, it looks similar colour to that seen in power pentodes.

[dave cox]

I would be more worried about exposure to natural gas than radon, as this can contain several percent helium. Given radon's short half-life, a precursor (U238) with a very long half-life in addition to the chance of infiltration of the created helium I would be surprised if the effect was as big as negligible!
It’s a very different matter breathing in radon, that puts a very potent alpha source (half-life in days) right where you don’t want it. Even 1 damaged cell can kill you …

dc