# [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 +/- 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 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.

-Bill

```*****************************
Waveform Technology
```_______________________________________________