Guildline 6500 Teraohmmeter Adjustment Problem

[lhiga]

ok folks, this is what I learned.  The solartron multimeter called out in the NASA procedure for the 6500 teraohmmeter has the same input impedance as the Fluke 8508A, Agilent 3458A, Wavetek 1271/1281.  The 100M ohm input impedance was on the NASA procedure equipment list.  I got my information from the Solartron technical manual for the multimeter model called out in the procedure.

[Hawaii596]

I don't debate what you said. But I applied the 100 Megohm load based on that statement in the NASA procedure (which I got from GIDEP). So does that mean that the 100 MegOhm input impedance was not correct in the NASA procedure. Because that is not the input impedance on the 3458A or 8508A. When I have tried running the DCV output adjustment procedure using just the 8508A or 3458A DMM's without the 100 Megohm resistor for a load, I get messed up accuracies on all of the upper output voltages. This being due to the change in input impadances.  The HP 3458A has >10GOhm Zin on 0.1, 1, and 10V, and 10 Megohm on the rest. And on the 8508A it is about the same (ranges a little different).

I also tried using a Ross divider, which does not has the correct input impedances. 

My issues all come back to range to range accuracy. The 6500's accuracy (in part) is based ratiometrically on the verified 100 Megohm reading, and the DC voltage output accuracies (plus the timing in RC time constant of checking the capacitance values).  So the meter works great on the lower DCV output ranges where DMM Zin is >10GigaOhms.  But loses something when going to everything abot 10 or 10 VDC out, due to the Zin mismatch.

Just wondering how others have solved this.

[lhiga]

Well I need rebut your last statement.  I found the 8508A reading high on the 1, 3, & 10 VDC values (as much as 10,000ppm)   and reading low on the voltages from 30 VDC and up.  This is why I will take Guildline up on their offer to purchase a accredited calibration and any repairs will be covered by Guildline. Guildline convinced me maybe something is wrong with the power supply.

[Hawaii596]

It's been a little while since I calibrated ours. A few months, so I may not still have the details memorized. We own two 6500's and a 6520. And believe it or not, the 6520 is the only one not in service (working, but not cal'd).  The 6520 has the BNC and Triax connectors.

I may have gotten some details slightly mixed up, as I am going from memory. It may have been that I strapped a 100 Megohm load (calibrated Guildline 65206) across the DMM inputs on the low ranges (up to <= 20VDC). Then, as I recall, I went over to using the Ross Divider with a 65206 strapped across it's input for the higher voltages. Then after doing the other alignment steps, including doing the the 100 Megohm alignment, I get really good readings on voltage settings up to 20 VDC. As resistance values go higher, though, the meter won't allow the lower voltage settings. 

Since I have Guildline 65206 resistors from 100 Megohms through 1 Teraohm (and I have a Guildline 9337 of 100 Teraohms), I do individual range characterizations, and get really good readings all the way up.  We really don't do too much above 10 or 100 Gigaohms, and rarely do 1 Teraohms. I have yet to have a need for 100 Teraohms.  And pretty much all we use the meter for is to certify our Penn Airborne resistors (and similar), but all in-house items.  And I send my set of 65206 resistors out to a good standards lab.