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Can anyone help with info on this great find ??

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Patrick

Mon Jul 12 2010, 08:20PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Proud Mary wrote ...

The Jennings instrument is an antique, and dates to a period of minimalist radiological protection standards.

thats a good point Proud mary, back in the 50's/60's radiation was the new "it" thing. radioactive food and "energy drinks" ( which caused one mans lower jaw to fall off ), and nuclear weapons used for civil construction pojects like roads and dams. the relative safety was just presumed to be acceptable, but i think now that has been disproven with the nevada/utah downwinders in america and the Semipalatinsk Polygon downwinders of the soviet era.

Adam Munich

Mon Jul 12 2010, 08:31PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

How about making cardboard tubes that fit around the caps, then wrap a layer of lead sheet around the tubes. If you ever need to measure anything 15kv+, then just slide on the cap tubes.

IntraWinding

Tue Jul 13 2010, 10:09AM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

I think the way to go is to get/make an X-Ray measuring device so you can see if there is a hazard when you use it.
You'll then also be able to see if you've solved it!

Patrick

Tue Jul 13 2010, 09:56PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

a scintillator would do nicely, to prevent harmful/fatal exposure.

Adam Munich

Tue Jul 13 2010, 10:19PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Doubt he wants to spend any money on a detector though.

plazmatron

Tue Jul 13 2010, 11:35PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

It is unlikely that in normal operation, that the vacuum capacitors on the meter should produce any detectable x-rays.

Purposely designed cold cathode x-ray tubes are finicky at best, and will only operate in a certain pressure range (yes they contain gas). If the pressure drops too far, x-ray emission will cease.

A vacuum capacitor should have a hard vacuum, and whilst the vacuum can break down, and conduct, it should only be transient. Vacuum breakdown is poorly understood, however it is sometimes deliberately induced during processing of high voltage tubes, in order to smooth off any microscopic projections off of otherwise smooth electrodes (a process known as "spot knocking").

If the vacuum were to go "soft", a detectable amount of x-rays may be produced, but then, the meter wouldn't work either.

If you are really concerned, you could use thick chlorinated perspex sleeves over those caps. But personally I wouldn't be sitting for any length of time with my face up to a meter that has 100kV exposed terminals on top!

Les

teravolt

Wed Jul 14 2010, 02:54AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

I believe that is a capacitive voltage divider with a diferensioal input for measuring peak HV up to 50kv. The unit will not emit x-rays. The nice thing about it is that it has a scope port and if you have a DRSSTC around and it has the frequency response then you can see the tank voltage across L or C or use the meter to tune it by looking for the largest VCpeak or VLpeak

Bored Chemist

Wed Jul 14 2010, 06:39PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Surely a capacitor that leaks (significant numbers of) electrons isn't a capacitor?
If it leaked enough to produce any decent current it wouldn't work. If it doesn't draw a current then it can't generate Xrays.

radiotech

Wed Jul 14 2010, 09:28PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The meter measures the charging current flowing from the high side of one vacuum capacitor though an impedance to the low side of the other vacuum capacitor. The voltage across the impedance is scaled for the meter range selected.

Not wishing to start a debate, x-rays require kinetic energy as Bored Chemist stated. If the input vacuum capacitors had any power factor, some of that energy loss might produce x-rays .

Proud Mary

Wed Jul 14 2010, 10:46PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Bored Chemist wrote ...

Surely a capacitor that leaks (significant numbers of) electrons isn't a capacitor?
If it leaked enough to produce any decent current it wouldn't work. If it doesn't draw a current then it can't generate Xrays.

I'm sure you must be having a joke with us here?

Looking at Comet AG's
SERVICE BULLETIN SB-52
Technical Recommendations and General
Instructions for Vacuum Capacitors

we read:

Combined DC plus RF, or DC blocking applications, present a particular problem for
vacuum capacitors because of the field emission current.
COMET however has pioneered manufacturing processes that reduce field emission,
as well as total leakage current of conventional capacitors for DC applications to less
than 0.1 Î¼A, a level which is significantly lower than what is generally accepted for
regular RF applications. The standard leakage current is as follows:
Capacitor type DC leakage current
at peak working voltage (Upw)
Fixed vacuum capacitors < 1 Î¼A
Variable vacuum capacitors Â£ 15 kV < 10 Î¼A
Variable vacuum capacitors > 15 kV < 1 Î¼A
In praxis DC leakage current as low as 0.1 Î¼A is frequently achieved.
To achieve excellent operating performance COMET recommends that the combined
DC plus RF peak working voltage should not exceed the rated peak working voltage.
As a good engineering practice the DC voltage should not exceed 25% or better yet
20% of peak test voltage.

But this is from a top of the range, brand new Swiss vacuum capacitor specially designed for low field emission @ just 15kV in the year 2010.

If we are extremely conservative, and assume that the elderly vacuum capacitors in
this Jennings meter draw as little as 10uA field emission current at 100kV, then we would have about
2.5 Gy/hr at a distance of 10cm from the tube, a non trivial quantity.

But you guys know what beliefs work best for you, so I'll leave you to it!

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