Re: [APD] cheap pH meters!

["S. Hieber" <shieber at yahoo_com>]

It's not really quite that bad. E.g., the pH meter might
tend to read about 0.2 units too high, but each indicated
change of 0.01 units is very close to the correct change.
It's not as if for a reading of, say, 4.5 the correct value
is, say, 4.3 AND when the reading is 4.51 the value is,
anywhere between 4,31 and 4.71. The degree of inaccuracy
tends to be linear.

So, if you are interested in small changes, the higher
resolution might be helpful to you despite the degree of
inaccuracy. YOu'll find this same situation with, for
further example, aquarium thermometers.  Find a handle on
the shelf and several will differe by several degrees. But
raise the temp 3 degrees and each goes up 3 degrees.

Making things worse (depending on how the degree of
accuracy is assessed) on any digital display you are never
closer than the amount of the last/least digit -- at
0.x99999999999 it still reads 0.x but the real value is
virtually 0.x + 0.1. So a device that only reads to 0.x
units could be off very nearly 0.1 units due just to the
display being digital, nevermind the accuracy of the
sensor. A digital watch shows the correct time manys times
a day but most of the day, it's off by a digit  ;-)

Scott H.
--- revance at indiana_edu wrote:
> Quoting "S. Hieber" <shieber at yahoo_com>:
> 
> > Resolution and accuracy are two very different things. 
> > Many things have a higher resolution than accuracy.
> > 
> > Never mistake the number of digits on the screen for
> the
> > degree of accuracy. ;-)
> > 
> > Scott H.
> 
> 
> My point is that there is no reason to have a higher
> resolution than accuracy. 
> If the accuracy is +-0.2 and the resolution is 0.01, the
> hundredth place means 
> nothing. This is well understood in scientific research
> practices. . . .

=====
S. Hieber
-  -   -   -   -   -   -   -
Aquascaping by Takashi Amano and more at
the AGA 2004 Annual Convention
Nov 12, 13 & 14; Marriott Crystal Gateway, Arlington, Virginia, USA
Aquatic Gardeners Association www.aquatic-gardeners.org

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