18 Dec
2008
18 Dec
'08
10:57 a.m.
In this case, I'd have expected a VR tube.
They're far more
rugged. I have several related pieces of equipment from that era
which use VR tubes, though not a DVM per se.
In these older DVMs, the Weston cell is not used as a reference, but
rather as a standard to calibrate against. There is a voltage refernce,
typically a zener diode, which is used as the referece for the readigns.
But it can drift with time/temperature/whatever, so you effecitvely use
the DVM to measure the votlage fo the Weston cell and tweak the
calibration control so that it give the right reading.
-tony