Although I have observed some old timers (which I am quickly becoming)
use VTVM for digital meters, I do not. There are several reasons for
this.
First, not all DVM are equal. A reasonably high quality DVM will have
an input resistance (for DC voltage ranges) of 10 megohms. But less
expensive meters only have an input resistance of 1 megohm. It turns
out that Triplett Corporation is located right here in Bluffton, and I
know several members of the Triplett family. I asked one of them about
this, and he confirmed that an accurate 10 megohm voltage divider is
one of the more expensive parts of a meter. A 1 megohm divider is less
expensive and might be used in cheaper meters.
We found this out the hard way by not paying attention when we bought a
couple dozen meters at a very attractive price. One megohm meters are
fine for some things, but in our physics labs for science majors we
prefer the 10 megohm meters.
But even a 10 megohm meter is not high enough resistance to compete
with a VTVM, nor is it high enough to measure some high-impedance
sources such as a pH electrode. Solid-state meters that are more
equivalent to VTVM would be FET-input or MOSFET-input meters.
For high input impedance meters using active input components,
regardless of whether they are vacuum or solid state. I am inclined to
use the word... electrometer.
Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817