Gas Discharge Tubes GDT

[Wallace Gasiewicz]

Does a GDT degrade over time or is degradation only related to number or magnitude of over voltage events?

[Ian.M]

It depends - some older GDTs used a radioactive doped gas fill (or radium doped electrodes which emit traces of radon gas) to minimise dark effect.  If the radioactive isotope used had a half-life of a few years, the breakover voltage of an antique N.O.S  GDT may have increased significantly with age.  However IIRC radioactive GDTs have been banned in most jurisdictions and most datasheets for recent and current parts will state the tube is made with non-radioactive materials.

[T3sl4co1l]

I don't know if GDTs are manufactured to avoid it, but neon tubes notoriously degrade over time and use, due to oxidation and other contamination of the electrodes, or dilution of the gas (which, usually the bulb is sealed fine, but if helium is used, it diffuses very slowly even through solid material).  It's possible the same effect applies, but it can be alleviated (giving longer, if not unlimited, life) with proper manufacture.

Also, needless to say, GDTs don't discharge very often, whereas neons can continuously.  So it might be effectively irrelevant in practice -- if it lasts say millions of surges (or whatever the datasheet says), you're already well outlasting a MOV.

Which is kind of a good reason to avoid a radioactive type anyway, if you need super long life equipment (20yr+).  Just as well, eh?

Tim

[Terry Bites]

Read this:

[TimFox]

I don't know if GDTs are manufactured to avoid it, but neon tubes notoriously degrade over time and use, due to oxidation and other contamination of the electrodes, or dilution of the gas (which, usually the bulb is sealed fine, but if helium is used, it diffuses very slowly even through solid material).  It's possible the same effect applies, but it can be alleviated (giving longer, if not unlimited, life) with proper manufacture.

Also, needless to say, GDTs don't discharge very often, whereas neons can continuously.  So it might be effectively irrelevant in practice -- if it lasts say millions of surges (or whatever the datasheet says), you're already well outlasting a MOV.

Which is kind of a good reason to avoid a radioactive type anyway, if you need super long life equipment (20yr+).  Just as well, eh?

Tim

With conventional neon indicator bulbs (NE-2, NE-51), my experience with degradation in continuous use was that metal from the electrodes sputtered to the interior surface of the glass, darkening the bulb.
A bit of radioactive material was also used in some gas-discharge voltage-regulator tubes to ensure starting the discharge.
An interesting list:  https://www.royalsignals.org.uk/articles/radioactive-valves.pdf

[Wallace Gasiewicz]

Thank you all for your comments. I was just thinking about a 145 Volt GDT as a protective device on a input circuit on a HP 3456. I suppose that with the essentially non triggering  of the tube, it is not necessary to replace it. I also bought some NOS 145 Volt GDTs that are probably 20 yrs old. I suppose these are still good and will outlast me.
I believe these are more modern tubes without the radioactive gas and that for this particular purpose, unless there is a leak the GDT is good.