Stable Isotope Geochemistry


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John Hill <[log in to unmask]>
Reply To:
Stable Isotope Geochemistry <[log in to unmask]>
Fri, 5 Dec 2008 10:18:30 +0000
text/plain (149 lines)
Hi Alison

Not necessarily, an arc or flash-over will usually occur at a higher
potential. If you have been having to take the source out to change duff 
filaments then it could be that something has been disturbed.
There may also be a defective lead-through on the source base-plate. You 
also need to check there are no metallic deposits on the ceramic 
insulators of the lead-throughs. This could come from misaligned 
filaments which glow like a light bulb when the trap and emission are 
zero and you can get a sputtering effect.

Sorry, I thought the source of 252 was similar to the 253 which have the 
light bulb source heaters at ground potential.


John Hill
Mass Spectrometry Facility
University College London

Alison Pye wrote:
> John:
> If I were getting discharge inside the source, wouldn't it trip at other
> voltage settings as well? It seems to have long term stability at 8KV. I
> don't think there is a crack in the ceramic tube. If there was, I don't
> think I'd be able to get the gas sensitivity that I have now (when it
> works). And the source is not heated.
> If it continues to trip, I will haul it out again and have another look.
> --Alison
> ________________________________
> Alison Pye  ē  [log in to unmask] 
> Stable Isotope Lab Coordinator
> CREAIT ē TERRA Facility ē Stable Isotope Lab
> Alexander Murray Building
> Memorial University of Newfoundland
> St. John's, NL ē Canada ē A1B 3X5
> ph: (709) 737-3217 ē fax: (709) 737-2589
> -----Original Message-----
> From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf
> Of John Hill
> Sent: December 4, 2008 11:21 AM
> To: Alison Pye
> Subject: Re: help for tripping source and pumps - MAT252
> Hi Alison
> Sounds to me that you could be getting a discharge from the HT inside 
> the ion source which is causing the vacuum to trip. This is due to a 
> transient spike going round the earth line. The discharge could be 
> caused by the connecting wires inside the source perhaps very close to 
> the grounded heater wires or possibly a crack in the ceramic tube that 
> connects the inlet line to the ion source itself. You will need to take 
> the source out and thoroughly check all the connections.
> Regards
> John Hill
> Mass Spectrometry Facility
> Chemistry Dept.
> University College London
> Alison Pye wrote:
>> Iím hoping someone can help me with a problem that has developed on my 
>> MAT252. I recently went through a flurry of bad refurbished filaments 
>> (no trap emission) and now that I have a nice new filament in there, I 
>> canít get the source, or pumps, to stay on more than a couple minutes 
>> after turning the voltage up to the full 10 KV. I havenít yet openedthe 
>> ConFlo, so it is in dual inlet mode. I left it overnight with the HV at 
>> 8 KV, and the pumps were still going this morning, but the HV was off 
>> (source control still on). HV reset OK. Vacuum was at 3.6 E-9. Within 2 
>> minutes of turning on the HV to 10 KV, the pumps tripped again. Iím 
>> lucky now in that after half a dozen trips over the last three days, 
>> Iíve at least figured out how to reset the pumps before they vent so 
>> that I donít lose vacuum. As long as I am in the room.
>> Iíve had a similar problem to this in the past, and the problem was with 
>> a couple capacitors the vacuum gauge monitor, which was giving faulty 
>> signals causing the trips. At that time, the needle valve on the 
>> analogue vacuum display had been ďjitteryĒ and then outright failed. And 
>> the inlet forevacuum display in Isodat had been jumping around. But I 
>> havenít seen the same symptoms this time, except perhaps that the vacuum 
>> during CF mode seemed slightly lower than normal (2.6 E-6 as opposed to 
>> 4 E-6). I note that the vacuum gauge display is not responding at all 
>> when the pumps trip, until I can reset the MS, restart Isodat system, 
>> and toggle the vacuum display buttons. Toggling the buttons is a trick 
>> that has always been necessary from time to time to get the analogue 
>> display to respond. Until the analogue vacuum gauge display returns to 
>> an active reading mode, the pumps will not restart. As with my earlier 
>> problems, this suggests a possible problem with the vacuum gauge monitor 
>> again. However, I donít recall there being a link before to having this 
>> happen only with HV at the full 10KV.
>> And here is another piece to the puzzleÖ I just decided to keep an eye 
>> on the source forevacuum (through the analogue display since I canít 
>> figure out how to get it to display through Isodat) to see if it was 
>> displaying any odd behaviour. Since I have switched it to this setting, 
>> still at 10 KV, it hasnít tripped yet. As I write this itís only been 1 
>> Ĺ hours, but it hasnít tripped yet. I couldnít get more than 2 minutes 
>> of run time this morning and yesterday when set at 10KV while monitoring 
>> source high vacuum.
>> Does anyone out there recognize these symptoms? At this point Iím still 
>> thinking it is the vacuum gauge monitor. I should know by the end of the 
>> day whether this is directly related to the analogue display being set 
>> to the source high vacuum at 10 KV only, or whether it is a fluke that 
>> it hasnít tripped again.
>> Thanks for any insight.
>> --Alison
>> ------------------------------------------------------------------------
>> Alison Pye  ē  [log in to unmask] <mailto:[log in to unmask]>
>> Stable Isotope Lab Coordinator
>> CREAIT ē TERRA Facility ē Stable Isotope Lab 
>> <>
>> Alexander Murray Building
>> Memorial University of Newfoundland
>> St. John's, NL ē Canada ē A1B 3X5
>> ph: (709) 737-3217 ē fax: (709) 737-2589