They are good for what they're good for. [quantifying repeatablity]
They're just not good to fuel an arguement. :-)
But they could be cheaper.
Ken
At 05:36 PM 10/26/02 -0700, you wrote:
>>>>
Sounds like I just wasted $50 on a PWT meter.<<<<
Ode Coyote <[email protected]> wrote:
Both TDS meters and the PWT do the same thing the same way. Unless
colloidal particles are actually touching, or perhaps very very close
together, they won't register at all with either meter.
The best you can do is guesstimate the particulate content with a laser
pointer and add a fudge factor to the reading.
Further, the Hanna people have seen fit to mix metaphors with their specs
for the PWT.
It says "Range is .01 to 99.9 uS" and "Accuracy is +/- 2% Full Scale"
[but don't mention what full scale is]
Now, you'd think that range and full scale would be the same thing, BUT,
stick the PWT in water and add salt to see where the scale ACTUALLY pegs
out. That would be at 999.9 NOT 99.9.
If the meter were accurate to +/- 2% in its "range" they would have said
it that way.
That said, at least the meters are repeatable within themselves, but don'texpect any two to read the same thing any more than two different
laboratories have a tendency to tell you the same thing within +/- 10 PPM.
If any two PWTs get within 10 microsiemens of each other, that would be
very very good and at least somewhat accidental.
I guess that "state of the art" means just that....Art "Looks like" an
object.
I asked my researcher friend about the discrepencies [He has a huge muliti
million megabuck lab at his disposal including mass spectrometers, TEMs and
such]. Basically, he says that to get any sort of 'accurate' measurement
in the lower parts per million ranges, it takes huge samples and the
averaging of several tests.
..and the term 'accurate' is STILL subject to definition.
It's best not to use any those 'standards' to flesh out an arguement.
[everyone will be wrong or if someone is right, no one will know who that is.]
In our case, where there are no dosing standards at all and
el! imination/adsorption rates vary minute to minute, +/- 2% of 999.9 is OK.
Your taste buds, a laser pointer and a 'strong...not so strong' resolution
might do just as well.
It's Not very objectively informative but well within the 'one glug or
three' range of practical application.
No matter how tall a mountain actually is, no matter if it's measured from
sea level or it's lowest accompanying valley in millimeters or feet.....
the top is still the top and it still looks like a mountain.
Ken
Why is it that professionals get to practice but everyone else has to get
it right or get fired?
Could it be that a professional needs only to 'profess' that he knows
what's happening?
What about the 'professor' who teaches and certifies the professionals,
anyhow?
Right or wrong....I profess to be certain? [I'm licensed to do that
....I've been 'certified']
Well, I know what a rubber room looks like...maybe not what it IS,! but I do
know what it looks like. [ A good dose of Tyrosine makes the vision clearly
fuzzy]
At 03:04 PM 10/26/02 +1300, you wrote:
>Paul has led you astray a bit in his answer :-)
>
>Both the TDS (total dissolved solids) meter and the PWT (pure water
>tester) are conductivity meters. The difference is in the scale and
>calibration. A TDS meter is calibrated to convert the conductivity
>reading (micro-Siemens uS) to reflect the amount of dissolved calcium
>carbonate (in most cases) that would give rise to the conductivity
>measured. This calibration is about 1ppm TDS to 2uS for CaCO3. That
>would be fine if one could just multiply the ppm TDS x 2 to get the
>equivalent conductivity reading in uS, however, most TDS meters have a
>scale 000 - 999 ppm TDS, and generally have a error of 2% of the full
>scale reading. The error is therefore +/- 20ppm TDS or a range of
>80uS, obviously too! high if one is measuring a 10ppm CS solution which
>will have a conductivity reading of about 8uS.
>
>The PWT scale is much more suitable, being 00.0 - 99.0 and the readout
>is in uS. The error is +/- 2uS or a range of 4uS.
>
>Regards
>Ivan.
>
>-----Original Message-----
>From: [email protected] [mailto:[email protected]]
>Sent: Saturday, 26 October 2002 1:52 p.m.
>To: [email protected]
>Subject: CS>TDS/PWT meters
>
>
>please explain to me as simply as posible the difference between the
>tds meter and the pwt meter when testing CS... we have two tds meters,
>and they both give totally different readings... if i understand
>correctly, i need to invest in a pwt meter but i'm not totally
>understanding why it is better... thanks!
>
>
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