[snip]
Sorry should have been more clear. Plus or minus 30 Volts DC is the
range
that I am interested in.
JakeInHartsel
You will definitely need something that qualifies as a standard, although
you may not need laboratory grade instruments. For 0.3% accuracy you need a
standard about two or three times more accurate, or about 0.1%. Most DMMs
are at least that good, but you need to have it certified traceable to NIST
to be sure. You can buy 0.1% resistors, and build a divider from 30 VDC to
a commonly available standard voltage, such as 1.2 V or 2.5 V, but you will
need to make sure it is within the 0.1% accuracy. Then you would need a
sensitive galvanometer to determine that the voltage difference between
your standard voltage and the divider is zero. In this case you would need
about 2 mV resolution on the galvo. This is the old way to do it. A
certified DMM is the easiest, but certification costs about $40 or more.
Paul