Good afternoon,
One thing springs to mind: not withstanding a problem with the UPS it would appear that the mass spec and penning were working prior to cleaning so to get back to square one I would look at the penning first. Stating the obvious without the gauge working you will require the mass spec to solve the problem!
With regard to the original problem: when the system is up and running again and if the unstable vacuum reading is still apparent I would look for a leak. 

Bob Cokayne
IRMS engineer (retired)

-----Original Message-----
From: Stable Isotope Geochemistry <[log in to unmask]> On Behalf Of Geldern, Robert van
Sent: Friday, September 28, 2018 8:26 AM
To: [log in to unmask]
Subject: [ISOGEOCHEM] Unstable vacuum readings and signal on a Delta V

Dear friends,

after a week of struggling it is now time to ask four the wisdom of the community.

I am in South East Asia trying to get a Delta V Advantage running. The main issue was originally a regularly oscillating (2 sec interval) high vacuum reading (fast changes between 5.8 and 7.9 x 10^-8) together with fluctuations of the box/trap ratio and signal intensity. All four oscillated together, visible as an up and down pattern in a time scan. For the signal intensity it seems that the amplitude of the intensity changes depended somehow on the m/z measured and were smaller for m18 (50mV)  but extreme für e.g. m28 (50V).

With ion source off we did no see any noise anymore in the detector and/or the filament but the vacuum reading was still flickering.

What we did so far:
- replaced the fore vacuum rotary pump
- cleaned all tubings from the fore pump to the turbo and removed all oil contamination
- changed the oil wick of the large turbo (small is not installed)
- checked ion source connections and changed filament, just in case
- removed the Penning gauge and cleaned the tube mechanically

After re-assembling everything the instrument started smoothly within a couple of minutes. The turbo and fore vacuum sound is OK.

However, vacuum readings are now far off and still flickering. Reading on Isodat is now 6 x 10^-10 and 2 x 10^-9. This is totally unrealistic and we do not get a green light on the vacuum LED on the front panel. Obviously Isodat also thinks that is out of range? I did not find a documentation about a lower end trip point, only about the upper set point (meaning when your vacuum is not good enough).

We measured the Penning voltage and the trip point on the access points MP11/12/13 on the power distribution board of the Delta V. Voltage is between 2.1 and 2.2 Volt, which Isodat interprets as an extreme goor (or unrealistic?) vacuum. We are not able, of course, to switch on the ion source to see if we still have fluctuations on the signal. Wa also exchanged the cable between the Penning’s magnet housing and the board.

From my current interpretation we might have 3 options:
- a broken Penning gauge
- a broken/not correctly working magnet housing
- a problem with the power supply or reading of the voltage on the Delta V board.

What I would appreciate are thoughts about further tracing down the problem. Unfortunately we do not have a new Penning gauge or Magnet housing at hand. Any options to cheat on Isodat or how to check if board supplies the correct power to the Penning or to somehow calibrate the Penning?

I also fully open to new suggestions regarding the original problem of fluctuations in vacuum-signal-filament readings.


PD Dr. Robert van Geldern
Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) | GeoZentrum Nordbayern Schlossgarten 5 | 91054 Erlangen | Germany [log in to unmask] |

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