HP34401 needs to msr dc volts with input impedence >10 gigaohm

[rdj]

I want to measure dc volts at >10 gigaohm input impedence with an HP34401A dmm, using LabVIEW 8. HP manual says: "With AUTO ON, the input resistance is set to > 10 Gohm for the three lowest dc voltage ranges [100 mVdc, 1 Vdc, and 10 Vdc]. The CONFigure and MEASure? commands automatically turn AUTO OFF."

In the block diagram, I put "Configure DC Input Resistance.vi" in between "Configure Measurement.vi" and "Read (Single Point).vi". When I run this, it resets AUTO to OFF resulting in a garbage number from our pH electrode, from which we have measured 40 mV at pH=7 and 200 mV at pH=4 with a pH meter (>1 gigaohm input impedence, but no comm capability).

[Steven_T]

Hello RDJ,

The VI sequence you are using should reset the input resistance to auto.  Would you be able to make a small example where you just initialize, configure measurement, configure dc input resistance, read, and finally close?  You can then make an NI-Spy capture of this VI executing.  Maybe you could post the spy capture and vi so we can see where the problems are specifically taking place.  If I understand you correctly, the input resistance auto setting is set to off during the configure measurement and read vi, and it is being set to on by the configure dc input resistance vi. 

This link will show you how to get a good spy capture  for debugging this problem. http://digital.ni.com/public.nsf/websearch/8D890EC09B15C05A86256E6F007E3E86?OpenDocument

Steven

[rdj]

Attached is a zipped Capture.spy written by NI-Spy during one run of "Agilent 34401 Read Single Measurement.vi" with the sole change of setting AUTO ON by inserting "Configure DC Input Resistance.vi" between "Configure Measurement.vi" and "Read (Single Point).vi". Before Run, I manually set AUTO ON (10 Gohm input impedence) and read 45 mV from the pH electrode in pH=7 buffer solution, the expected result. During Run, the vi changed setting to AUTO OFF resulting in a 3 mV reading. After Run, the 34401 was left in AUTO OFF and the readings were 1 mV to 5 mv, dancing all over.

Message Edited by rdj on 05-11-2006 01:19 PM

[Steven_T]

Hello RDJ,

After looking at the Spy Capture, I noticed that the command to set Input Impedance Auto On only occured one time and no other commands turned it off.  You may be modifying other parameters that require this setting to be off.  We tried this VI on one of our instruments and noticed that Input Impedance Auto was on the whole time.  Please try the attached VI and modify yours if necessary.

Steven T.

[rdj]

Just ran your Input Impedence Tester vi and got a timeout error.
Error -1073807339 occurred at VISA Write in Agilent 34401.lvlib:Fetch (Internal Buffer).vi->Agilent 34401.lvlib:Read (Single Point).vi->Agilent 34401 Input Impedance Tester.vi
Do you want Spy output?

[rdj]

Here is the NI-Spy file created from running Impedence Tester.vi
Altho it timed out and did not report a msmt, the dmm remained at 10 Gohm during and after the run.

[rdj]

Woops.  Here's the attachment.

[rdj]

Steven,
Sorry, we messed up.  Your vi works fine, but  when we first ran it, it was calling our sub-vi's, some of which were corrupted.  Everything is ok.  Thank you very much.  -Rich and Bernie