What does tuner measure?

Or more specifically, what are the units on the display? Most I've seen including mine (Kala) have a scale of -50....+50 and I'm just wonder what the units are, i.e., 50 what? Is that percentage of a whole tone or Hertz or something else? I read of folks saying a certain string was "20 (some units) off". Is that 20 on this scale of 50?

Thanks in advance
Jim B

Hi!

Hertz (Hz). This unit mesures the frequency of a wave.

For example, the A string in a guitar is 440Hz (that's why it's known as A440).

If it's reading +/- 50 then the units are "cents" or 1/100 of the distance to the next note.

But notes are not linearly spaced in frequency, each note is approximately 1.05646 times the frequency of the preceding note.
For a middle C that's 1/100 * 261.625565 Hz * 1.05646 = 2.76 Hz
But an octave up a cent is 1/100 * 523.25113 * 1.05646 = 5.53Hz

It's a little more subtle than that if you want to get into the deep science and math. Here's a wikipedia page:
http://en.wikipedia.org/wiki/Cent_(music)

And to make matters even worse, most inexpensive tuners are not very accurate.

Most tuners are only accurate to within +/- 3 cents

So if one string is flat by 3 cents and another is sharp by 3 cents the tuner show them both in tune but they are actually 6 cents apart from one another

and being out of tune 6 cents is easy to hear.

Strobe tuner is the way to go.