Re: Media Testing for maclife.de
Re: Media Testing for maclife.de
- Subject: Re: Media Testing for maclife.de
- From: David Scharf <email@hidden>
- Date: Thu, 18 Sep 2008 20:20:31 -0700
Chris Cox wrote:
David, you're just confusing things further.
Well someone is.
Apparently you're missing the concept (I think you are just mixing the
measurement with the interpretation) , and always trust calibration to
always be perfect.
Ah well, all these wasted years doing test and measurement just to miss
the whole concept. But who ever said that I always trust calibration to
be perfect? As I said, you can take the position that most all
measurements are incorrect (wrong, imperfect), its just a matter of degree.
And your insults are completely misdirected.
Chris, I don't even know you. I have no intention of insulting you.
However you post you ideas and opinions here on a public forum for all
to see. And I do disagree with some of your ideas (obviously).
Yes, things are much easier if you assume perfect calibration -- then
your measurement and interpretation should always match. And if
you're doing clinical work (repeating a procedure, not trying to
define new experiments) it might be ok. But in the real world, and
certainly in experimental sciences -- you don't always have perfect
calibration. The closer you get to the cutting edge of research, the
worse it gets. Scientists have been doing post-measurement
calibration for as long as science has existed.
I don't know which R&D labs you have worked at (have you?), but I have
been working in both experimental (R&D) research as well as engineering
for several months now (actually a lot of months), and I can tell you
that when you are doing R&D work, the cornerstone of understanding and
advancement of knowledge is having instruments that you can rely on to
give the proper readings under the circumstances that they were designed
for (duh)--and a properly designed experiment. And we don't "assume"
perfect calibration, we calibrate, using (NIST) traceable standards.
Again: a measurement itself cannot be wrong.
Again: that is wrong.
The interpretation of the measurement can disagree with a standard or
another measurement. And that just means that you either didn't
understand the experiment or the instruments correctly.
This can be true.
I already gave you examples of perfectly valid measurements that did
not appear to agree at first. The temperature example could have been
3 readings of boiling water on different scales, or could just as
easily have been 3 readings from instruments with the same calibration
inside a broken kiln. The measurements are just numbers, they only
appear to disagree if you assign the same meaning to all the numbers.
But the meaning depends on the instruments, and the experimental
setup. This is a very, very fundamental concept in experimental sciences.
So lets just see if we can end this with a point of agreement. It is
true, as I've stated previously, that measurement itself can affect the
value of something being measured. Take a very high impedance DC
circuit. We measure the voltage drop across a 1000M resistor. The
reading is 2.000V. However the impedance of the voltmeter is also 1000M,
so we have actually seriously affected the reading by loading down the
circuit. If this is in a voltage divider circuit, we can now calculate
the voltage that is across the resistor when we remove the voltmeter.
But we're back to relying on the assumption that we really know what the
input resistance of the voltmeter is. So we come back to having to rely
on some standard to establish this with accuracy. You still NEED to know
the actual value of something, otherwise you just go around in circles.
And THIS is a very, very basic concept!
Regards,
David
*DAVID SCHARF PHOTOGRAPHY*
Scanning Electron Microscopy
Phone 323-666-8657
Los Angeles, CA 90039
http://www.electronmicro.com
_______________________________________________
Do not post admin requests to the list. They will be ignored.
Colorsync-users mailing list (email@hidden)
Help/Unsubscribe/Update your Subscription:
This email sent to email@hidden