Paul, The best probe I have found for the purpose are the old 100X scope probes. They have a very low capacitance on the order of 2.5pF in parallel with 10Meg so circuit loading is minimal. Most have extra voltage capability which is handy for working around transmitters. While designed for AC & DC measurements via scopes, you can use them accurately with DMM's because the 100X divider swamps out the loading effect of the DMM. The numerical accuracy of the DMM compensates for the high voltage division ratio.
Dennis AE6C On 11/17/12, Paul Christensen <w...@arrl.net> wrote: > I finally got around to trying a good 'ole fashion VTVM probe with a Fluke > DMM. Recall that most VTVM probes have a switch that allows for either DC > Volts in one position, then AC/Ohms/mA in the other position. The DC > position typically has a 1-meg isolation resistor, highly useful for VT grid > > measurements. Without value compensation, a modern DMM cannot use such a > probe and is otherwise useless for serious tube receiver work. > > I took a Simpson VTVM probe with a BNC connector and connected it to a > Pomona BNC-to-Banana adapter with standard 3/4" centers. With the adapter, > > the probe easily connects to a DMM. My first measurement was a precise +12V > > source. When using the VTVM probe in the DC position (series 1-meg > resistor), the DMM displays +10.93V. So, +1.07V is being dropped across the > > 1-meg resistor. Assuming the resistor is close to 1-meg in value, the input > > Z of my Fluke 8060A calculates to 10.215 meg. Essentially, a 10:1 voltage > divider is being created between the 1-meg iso-resistor, and the internal Z > > of the DMM. The drop is creating the value discrepancy. VTVMs are > compensated in design and manufacture for this. > > I have several Fluke DDMs, including 8060A (my favorite DMM), and an > advanced model 189. However, peering through the manuals, I see no setup > routine to create a user-defined DC offset. What I want is the ability to > measure a precise DC voltage, then enter a menu that allows me to assign a > new display value to compensate for the voltage drop across the > iso-resistor. I have a lab-grade Keithley bench-type DMM that does allow > for such an offset, but hauling it around is a pain. > > So, does anyone know of a DMM that allows for DC voltage offset? This is > different than the "Relative" button seen on many DMMs. Relative is used to > > "zero the display" for any input value. I want the same thing but instead > of zero, assign a new value of my choice. > > Paul, W9AC > > > _______________________________________________ > Drakelist mailing list > Drakelist@zerobeat.net > http://mailman.zerobeat.net/mailman/listinfo/drakelist > _______________________________________________ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
