Edwards WRG - Fluctuations?

[Dave Xanatos]

Please reference attached scope image...

This is the output from my Edwards WRG (Wide Range Gauge) plotted over a few seconds (500ms/div).

Once my vacuum pump (roughing pump only) was operated down to its lower level - around 6.5V on the gauge, which corresponds to about 7E-3 (.007) Torr if I'm reading their chart correctly, I noticed that the voltage would, every few seconds, jump up about half a volt, then ramp back down. For example, the meter would read 6.49 +/- .05v, then suddenly jump up to 7.0v, then over a second or so, return to the 6.49 +/- volt level.

I checked it on two different meters, and seeing the same behavior, I put it on my scope and observed the attached trace. Note 500mv/div. There appears to be a "not-quite" regular frequency to the jump in voltage.

When I shut off the pump, the vacuum holds very well. It took about 10 minutes for the vacuum to go from .007 T to .010 T, so I don't believe this "pulsing" is due to a leak or outgassing.

Is this pulsing of the output a normal thing to observe?

And is the trace noise (thickness of the trace) and spurious spikes normal for these gauges?

Thanks for any guidance you experienced folks can provide.

Dave

[Nick Peskosky]

Dave,

The same fluctuations occur on my INFICON BPG 400 guage during operation. The voltage spikes are an artifact of the way the gauge head controller works. If you look at the response graphs and datasheets published by the manufacturer for these wide range style gauges, you will notice that the gauge has two separate pressure sensing elements. In the high Px range, the gauge utilizes a Pirani element to measure the presssue but under under a certain maximum operating pressure ceiling, the gauge will in fact switch to an inverted magnetron (your gauge) or Bayard-Alpert type ionization element (my INFICON). This transistion occurs at a certain set-point (likely you are on the cusp of that pressure range at 7-10 Microns). When the controller actually switches to the low pressure sensor element, the analog output voltage will temporarily spike above the equivalent voltage which was displayed with the high pressure element at the same pressure. If you continue to increase the vacuum level present, the gauge will quickly revert to the proper linear analog output (mine does this below 25E-3 Torr).

[Dave Xanatos]

Thanks Nick!

I had wondered about the transition point, but the spec seems to indicate that the switch happens around 1e-4 to 1e-5 (at least they say that if your working vacuum needs to be in that range then you should consider a different model, I'm guessing to avoid the very issue I'm seeing here with the fluctuations).

I have also noted that the frequency of the "pulses" increases as the vacuum goes lower.

Glad to know it's not just a bad gauge... although I'll still want to pick up a few more and different units just to be sure

Dave

[fizz]

Dave

You have not said how old this WRG is, serial # will tell you the first two digits is the year of production, or what sort of usage it has had, it could just be that the Pirani side just can’t reach the Penning changeover pressure of 5-9 E3 although the system could well be below the required pressure, the WRG will not change over until the Pirani tells it is within range.

In this model the Pirani is housed within the body tube assembly, in a tube containing a small tungsten filament it could be filament contamination mostly due to oil back streaming from your vacuum pump.

We at recalvac@aol.com see several of these a year that come to us for cleaning etc.

Robin Whyte

[Chris Bradley]

A Penning trap type (inverted magnetron) gauge works by lighting a plasma and measuring the current across it. Basically, it does what you can do by eye with a fusor - tell the pressure from the way the plasma is behaving!

There could be a variety of reasons for what you are seeing, the majority of which are beyond your control. One that is within your control is to make sure the device is clean. Just like those 'sparkly' events you see off a fusor grid, the same will happen inside a dirty inverted magnetron. You can usually remove the whole vacuum part from the electronics, and then you can give it a clean. Follow the instructions in the guide, if you have one. If it has been used where the vacuum content has had contaminants rather than pure gases, eg in a manufacturing facility, it will probably be coated in some black cr@p which you will have to figure how to get out.

If you are being quite lazy, you could just leave it on for several hours and see if the dirt has burnt itself off. It could even be clean but is picking up on some dust or dirt that got in when fitting it, in which case leaving it powered up for a while would clean this off.