Fusor Forums

Hooked up my Edwards RV3 to the chamber last night, along with Pirani sensor and display to see how it performed.

Quickly ran into trouble with my setup though, as the display didn't show any change in pressure, it just sat around its normal atmosphere point. If I gave the display box a firm (light) whack, the needle would swing down from atmosphere to somewhere in the 100 micron range, but would return straight to atm. If I whack the display box with the sensor at atmosphere, there is no swinging of the needle. If I blow into the sensor in open air, the display registers some increase in pressure, swinging beyond the atm mark. The orange light on the display (see picture) flickers a little bit when the sensor is under vacuum, no flickering when the sensor is at atmosphere - I don't know if that 's relevant.

I know there's some level of vacuum being produced by the pump - When I open the KF flanges, there's quite a bit of sucking and whooshing as the chamber comes back up to atmospheric pressure - the problem isn't the pump.

So, the problem seems to be that the readout isn't properly showing what the sensor measures. The reason I don't suspect the sensor as much is that there is some response on the readout to changes in pressure (ie swinging from atm to 10 - 100 micron and back with a tap), so the sensor seems like it's sending back a signal, just the display isn't showing it properly.

I guess my next step would be to confirm the sensor is working. From reading old posts, Chris Bradley described these as 'glorified voltmeters' and advised a voltage across the two appropriate pins on the lead and read the voltage signal output, seeing how it changes with pressure and checking against the calibration curve. http://fusor.net/board/viewtopic.php?f= ... ani#p26491 The connector on the sensor is a weird one - have attached a picture, anybody able to identify it?

Is this the right way to proceed? Would anybody have any goto suggestions for what could be going wrong and how to fix it?

Thanks,
Tom

It's a cinch jones connector. Still available but not used much anymore. I would be pretty surprised if it is not a bad pirani tube, they are the most delicate part of the unit.

Maybe a bad solder joint in the box electronics or internal plug to circuit board. Just a thought since whacking the box made a difference.

Richard Hull

Hi Tom

Try putting the Head directly on the pump inlet then watch the needle go across the meter, so long as you pump is ok !

Robin

Thanks Jerry, you might be right. I'm unsure though, will see how things go with help here!

I thought the same Richard, have taken a quick look at the innards, doesn't seem like any glaring issues at present (although I'm no expert).

Robin - I was about to email you before I saw your post, speak of the Devil!

I deadheaded the gauge and the needle did go across meter, but went straight from atm to 10^-3 torr and even further past that, the needle did slow down as it approached the end of the meter. When the pump is switched off, the needle immediately heads back towards atmosphere. When the gauge is returned to atmosphere pressure, the needle does a little jump back to about 6 torr before returning to sit at atm. Does that point to anything glaring?

The pump is an Edwards RV3, rated for 1.5 microns, so going past 1 micron seems unlikely.

Tom

There is a slight possibility it just needs to be calibrated. But to do this you need to have it attached to a system that will go down to at least the -4 range.

Thanks for thinking of me Tom !

What you have described is normal except for the head reading off scale @ 10-3, your pump is rated @ 3.7m3h-1 so its pumping a tiny
Pirani tube with a volume of 8 cm3 its just like one big gulp of gas to get to 10-2 then quickly gets to the pumps ultimate of around 5x 10-3 mbar, if you are useing Ultragrade 19 Oil, I would reset the Pirani heads Vac pot to read just short of the 10-3 reading.
I hope that helps you.

Robin

No problem - Thanks for the help!

Is it normal that the needle heads back to atm when the pump stops running? There's still vacuum in the deadheaded volume when the pump is turned off, so that seems strange - Unless I have an abominable leak, but this seems unlikely as if I leave the pump off for a while and then open up the KF flange, there's a noticeable resistance to opening up the flange, and I can hear the air whooshing in.

So to reset - Just turn the potentiometer on the head when pump's running until it reads at about 10-3? Found a document here for Pr-10 gauges: http://mfc.engr.arizona.edu/documents/E ... 157880.pdf

section 4.1.1 outlines the reset procedure. As Jerry mentioned, if I can get someone who has a confirmed sub 10-4 system, they could reset it accurately for me. I'm not sure my pump is actually doing the 1.5x10^-3 torr it's rated for, so I'm not sure how accurate resetting it myself would be.

I am using Ultragrade 19.

Jerry, thanks too for your help.

Tom

No, most pumps should hold a significant portion of their ultimate vacuum when shut off. There is a anti-suckback valve that closes when the pump stops. If it did not close you would have a sensor full of oil.

Hi Tom

Have you used the Gas ballast on the RV3 ? If not and the oil is still a clear through the sight glass open the GB switch on ll (full ) for a couple of hours , just in case there are contaminants there ,do this with the pump inlet sealed off, once the pump has done its time close the GB switch and allow to cool half hour should be OK.

You have never said that the pump has been tested for its ultimate pressure anywhere I can see,
but the Pirani head showed <10-3, if you adjusted the Vac pot like I said to just above the 10-3 line.

Once the pump has cleaned itself and cooled, put the head back on and see what you can obtain again, it should be the same reading or may be a little better, if it again shows 10-3 adjust the pot to appx 2 x 10-3 this lies at half a millivolt on the bottom scale on the meter, with that done you should be very close to true, always remembering that Edwards quote an accuracy of +- 30% of reading when new! Anyway why do you need to know to the nearest mbar as these instruments are normally just for indication between 1 mbar & 10-2 to run a diffusion pump.

The RV3,5,8 pumps all have a quoted total pressure measurement of 2 x 10-3 Mbar when new.

Regarding the pressure rise it could be a leak but I doubt it, but just vapour, I always advise people to have there gauge heads adapted to an isolation valve like an Edwards SV10 so that you can protect the filament from an inrush when dismounting the head.

Robin