17 Apr
2011
17 Apr
'11
11:31 a.m.
On 4/15/11 9:18 PM, Brent Hilpert wrote:
In genral to measure a quanity you can eitehr compare it to a standard of
the same time (e.g. using a bridge circuit (in many cases), a ruller, a
2-pan balance...) or you can convert it to some other quantity and
compare that against a stnadard (e.g, a spring balanace which converts a
weight (not a mass!) to a change in length which you then compare against
a 'standard' length scale).
Most of the books I have read on measurements say that unless there are
otehr problems, you get the most accurate reuslts if yourt stnadard is
the same type as the quantiity being measured (if only becasue of
possible errors and non-linearities in the conversion).
Obviously there are many times when you can't do this, or don't want to
do this. And there are times when you really do want to check against the
fundamental SI stnadards. Hower, to claim, as said physics teacher
claimed, that a comparison against a standard of the same type is not a
measuremnt is plain wrong.
We all agree there are always comparisons to some
reference or other,
but the point I was attempting to make of the distinction between
approaches remains. The calculable capacitor situation relies on another
definition of C to calculably take the measurement back to that of the
more fundamental measurement of distance, as opposed to simply
calibrating it as another C (more turtles).
Ahh ok, I understand what you mean now.
I once had an argument with a fellow who didn't believe
potentiometric voltage measurement systems ever had any value, because
there have been "multimeters", VOMs and VTVMs for many decades. "Why do
ARGH!
I used a potentiomenter (in the sense of a voltage measuring device) not
that long ago. I wanted to measure the programming voltage in a
programmer for ffusible-link PROMs. The problem was that if I tried to
program a PROM I could proably measure the voltae applied, but if iy was
way off, it'd progbably wreck the PRON. Anyway, Ididnt; want to waste one
or more PROMs testing the programmer.
If i asked it to program a chip and there was nothign in the socket, it
would attempt to program the first location and then fail the
verification and bomb out. Aha!. It was at least going to apply the
programming voltage pulse of that first location.
I didn't have a sotrage 'scope, so I couldn;'t sample that pulse and
measure the amplitiude.
What I did was connect a 'Helipot' multi-turn potentiometer accross a
stable DC power supply, and then montiored the votlage on the slider of
that pot with an DVM.
I then connected that to one ipnput of a 339 comparator IC, and the other
input to a pin on the progmrammer's socket that would gt the progrmaming
pulse. The output of the 339 went to a logic probe so I could see if it
was changing state.
By reppeatedly 'programming' a non-existant chip and adjusting the pot, I
could find a settign where the comparator _just_ didn't toggle. That man
the outptu from the helipot was just higher than the progammign pulse
voltage. And I could measure that constant voltage with the DVM.
-tony