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[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
It's not misleading, it is common to specify measurement apparatus that way.
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 compared
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 the
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 +/-
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 short
time may agree more closely than those separated by a longer period of
time) . I hope I've clarified this a bit for everyone.
UNIX Systems Administrator
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