[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: pH measurement (was APD V3 #986
Hello James,
What you should be after is greater *accuracy* in pH measurements. The
precision will follow. Using the common definition,
Accuracy = how close is your reading to the "true" value,
Precision = how close to each other are the readings,
it follows that if you have good accuracy, i.e. your reading is very
close to the true value, then, of necessity, the individual readings can
not be too far apart. The most misleading instrument is one that is
precise, but NOT accurate!
Translating that to pH measurement, you want an instrument that is
accurate, precise, has good resolution and is stable = does not drift
neither from electronic noise, capacitance changes, nor from streaming
currents.
A resolution of +/- 0.01 is *always better* than +/- 0.1. That is
because the best reading, the most accurate reading that one can
theoretically make (excluding averaging, etc.) is +/- 1(one) smallest
division (+/- 0.1 vs. 0.01). In practice, one has to assume that +/- 2
divisions will be closer to the truth. In a "0.1 instrument" that
translates to +/- 0.2 = 0.4 pH units difference!
I am NOT familiar with the Piccolo Plus instrument.
In ANY pH meter the "weak" point is the pH electrode. There must be a
dozen reasons why an electrode "goes bad". In practice it most often
happens from neglegt, followed by "poisoning" (although few
manufacturers dwell on *that*!)
The manufacturer's electrode maintenance directions -- they sometimes
vary -- MUST be followed exactly. If there are no electrode maintenance
directions included, don't buy the instrument -- the manufacturer does
not expect it to last very long! :-))
An interchangeable electrode is a good feature, it will significantly
prolong the useful life of the instrument. A pH meter is, in principle,
just a high impedance volt meter. Only the pH electrode "goes bad" in
use.
Best,
George
> Here's one for the scientifically inclined...
>
> Currently, I use a LaMotte pH test kit which is calibrated in divisions of
> 0.2 pH units. By extrapolation, it isn't too difficult to read the
> comparator to 0.1 pH.
>
> I've been considering buying a Hanna Instruments Piccolo Plus pH tester,
> which has a claimed precision of 0.01 pH and an accuracy of ±0.01 pH. It has
> manual 2 point calibration and will automatically compensate for temperature
> variations.
>
> Years ago, when I was still doing salt water tanks, I picked up an el-cheapo
> "pH pen" at a club auction and promptly threw it away due to it's lack of
> accuracy (or was it just because I didn't realize that the thing had to be
> re-calibrated constantly and I was just too much of an idiot to know any
> better at the time?). So I'm rather leery of electronic meters.
>
> Anyway, the Piccolo Plus looks to be a quality piece of equipment and I'm
> wondering if anyone with more experience dealing with electronic pH
> measuring devices can give me an opinion. I'm favouring it over cheaper
> devices due to it's higher precision (0.01 pH for the Piccolo Plus vs. 0.1
> pH for most other units). Is the extra precision worth it?
>
> The pH electrode on this unit is replaceable - how long a service life
> (given proper storage and maintenance of course) can I expect of the
> electrode.
>
> Thanks,
>
> James Purchase
> Toronto