Sound pitch is measured in ‘Hertz’ (Hz) and is the number of vibrations per second.
In the accordion world it is usual to measure in ‘cents’. A cent is a 1/100th part of a semi-tone. At 440Hz a cent is about 0.25Hz.
1)Accordions are normally tuned to the ‘equally tempered scale’ (ETS) and as such the pitch doubles at each octave. That is, the increase is not linear but exponential. (More on this at the ’tuning’ page.)
2)Because of the need to create beats to achieve tremolo or musette in accordion tuning some of the reeds need to be set ‘out of tune’.
Using cents to measure pitch rather than Hertz make both these conditions easier to deal with.
Conversion charts are available on-line (to convert from cents to Hertz) and vice-versa. The Peterson Tuner website used to provide one.
The measurement of pitch is usually done with an audio tuner.
Tuners come in various forms. They are available as:
1) hand-held, battery powered units,
2) mains powered bench units,
3) computer based software programmes,
4) mobile phone based applications (apps).
Whichever one you choose it should be sensitive, stable and accurate. It is useful if it can read to 1 cent accuracy (+/- 1 cent on the higher notes of an accordion significantly affects the tuning.)
The stroboscopic tuners made by Peterson remain popular and are the standard tools of many professional tuners.
There are ample PC/computer based software programmes (many free to use) which are an economic alternative and perfectly capable of getting a reed into the area where it you want it to be.
A mobile phone will run many tuner ‘apps’ and one called ‘DaTuner Lite’ seems to perform well. It is accurate against all the pitch references I have at my disposal.