23 Mar
2006
23 Mar
'06
4:41 p.m.
On 3/24/2006 at 12:05 AM ard at p850ug1.demon.co.uk wrote:
IIRC the Weston standard cell is a mercury/cadmium
couple.... Although to
be fair a good bandgap voltage reference is accurate enough for anything
I am likely to need, and a lot less hassle.
The Weston cell also has some saturated solutions of mercury and cadmium
salts in addition to the metallic form. Sort of a salt bridge setup. Can
source very little current, though, what, maybe 0.1 ma for the typical 4"
high H-cell? So it's always used as a reference to adjust a primary
source. Of course, if the primary source went kaput, there was nothing to
adjust. In the mills, we'd go around periodically replacing the primary
cells in the chart recorders with those big rectangular black National
Union cells.
My last year on the job was when Honeywell came out with solid-state
voltage references that could actually source some current for their
Electronik series of recorders and controllers. You got rid of the
calibration helipot, primary cells and Weston cells and a bunch of other
miscellaneous parts (recorders would automatically go through a calibration
cycle every few hours). L&N followed somewhat later with their own
version.
AFAIK, the used H-cells just went into the main trash stream.
Cheers,
Chuck