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Re: [APD] Re: cheap pH meters!

I don't know if it got posted yesterday, but with a new chem kit I found my
levels to be 7.0 ph Ammonia 0 nitrites 0 high range PH 7.2.
Looks like I'm in the ballpark so to say. Just added two discus who have
been aclimated to our areas water chemisty for a few months and they are
doing great.
Thanks again for the info, Mark
----- Original Message ----- 
From: "Bill Wichers" <billw at waveform_net>
To: <aquatic-plants at actwin_com>
Sent: Wednesday, March 24, 2004 10:21 AM
Subject: [APD] Re: cheap pH meters!

> >That's pretty inexpensive... however I don't understand why you would
have a
> >resolution of 0.01 when the accuracy is only +-0.2. Seems to me thats
kind of
> >misleading.
> It's not misleading, it is common to specify measurement apparatus that
> Resolution is the same thing as precision in this case, and it means how
> specific the device can reliably (repeatedly) measure a parameter. With a
> resolution/precision of 0.01 the device should be able to make repeatable
> measurements to within +/- 0.01 unit. If you take 3 measurements of the
> same solution, with a pH of 6.0 (6.0 being the average reading across the
> range of measurements you made) for example, the readings should all be in
> the range of 5.99 to 6.01. This parameter is primarily due to limitations
> inherent in the design of the specific device in question.
> Accuracy means how closely the measuring device's result will agree with a
> reference standard. An accuracy of +/-0.2 means that the device's
> measurement should always be accurate to within +/- 0.2 units when
> against a reference standard. If your reference solution was *known* to be
> *exactly* pH 7.0, for example, the unit should read a pH somewhere within
> the range 6.8 to 7.2. This parameter is related to the calibration of the
> device, and also the ability of the device to "stay calibrated" over time.
> Now if you combine the two, it is possible to make *relative* measurements
> to a precision of +/- 0.01 units, which would allow you to make very good
> measurements of the hour-by-hour or day-to-day pH variations in your tank.
> You would be able to state the changes (deltas) from one measurement to
> next knowing that any of the readings were accurate to +/- 0.01 unit
> relative to any of the other measurements. But if you took *all* of the
> measurements and compared any of your individual readings against an
> *absolute* (known) reference standard, THAT difference could be up to +/-
> 0.2 units.
> It can be a tricky concept if you aren't familiar with it :-) I've
> simplified a bit since generally the unit's accuracy will deteriorate over
> time (hence the need for scheduled calibrations for most lab equipment),
> and precision can also have a time component (readings separated by a
> time may agree more closely than those separated by a longer period of
> time) . I hope I've clarified this a bit for everyone.
>          -Bill
> *****************************
> Waveform Technology
> UNIX Systems Administrator
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