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RGA Bottoming out?
Amanda2482
Member
01-29-2013 01:48 PM
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I am using LabVIEW to read certain molecules' partial pressures continuously. I have utilized the MR VI and some of the VIs in the SRS RGA Development Kit and the pressures for the three molecules I am interested in are read fine. When the system is vacuumed and the pressures reach about E-10 Torr the value then alternates from 0.00 to the previous value; as if it was bottoming out. Could anyone please help me get accurate readings (about E-11 or E-12)?
Eric-J
NI Employee (retired)
01-30-2013 04:32 PM
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Hi Amanda,
It appears that you are using a driver developed by Stanford Research Systems. Have you tried contacting them about the issue?
Also, what hardware are you using?
Since the value alternates between E^-10 and 0 at very low pressures, it seems that precision may to be the issue. This can be caused by the data type you are using (single vs. double). Also, it could be that you are having issues with the precision of your hardware. Do you know the precision of the instrument you are using?
Applications Engineer
National Instruments
National Instruments
01-31-2013 02:41 PM
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Yes, I have contacted SRS about this issue and they believe it is a precision problem also. They suggest it is because the age/condition of the RGA and would like a sample of the program running. Last time I ran the RGA200 on the Windows Software, it ran fine and could read down to E-11. I will send you and SRS the sample run as soon as I can.
Regarding your suggestions, the VI I am using returns the number with data type double. I have tested that the type can return under to E-100 so I assumed that was not the problem. I have attached the VI Library that I have downloaded from the SRS website. I am mainly using the SRSRGAc MR Single Mass Command.vi to read Ion current of a certain molecule, the SRSRGAo GET Calibration.vi to get the parameters, and the SRSRGA Convert String to Units.vi to convert the Ion Current into pressure in Torr. I am also using some other VIs to open the session, turn on the filament and CEM (I will send you the actual VI I created as soon as possible, my access is limited).
I believe it could also be a hardware issue with the communication. To communicate we are using an NI card PXI-8430/8 with the breakout cable (SCSI-68 to eight DB9 male) it came with. I assume RS232 is suitable for the precision we need, right?
Thanks for helping me!
01-31-2013 05:35 PM
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Here are screen captures of the VIs. Sorry I could not upload the actual VIs.
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chaka1234
NI Employee (retired)
02-01-2013 10:19 AM
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Hi Amanda,
I believe that the problem might be in the RGA since the PXI card is just reading the transmission bits that this device is sending to the card.
Also, I found on this link that there is an example VI to Measure Partial Pressures, you can take a look of it and check if this works for you too.
Best,
Carmen C.
