1. Guitar tuners are not set up to tune the temperament of a piano.
Each piano’s temperament is tuned slightly different depending on the piano. Many pianos have different gages and lengths of strings, therefore what sounds in tune on one piano may not sound in tune on another. This is called inharmonicity. Professionals have very sophisticated piano tuning software, as well as aural tests, that calculate how to tune the temperament of each individual piano.
2. Tuning Stability
A piano’s tuning pin and string must be set, it must be secured by the tuner. A tuner will move the pin only as much as they must to get it to pitch, and then they will ensure that the pin stays put. If it is not done right, one fortissmo blow on the key and the note will pop out of tune. This is called tuning stability. It’s the last thing a piano tuner masters and for most tuners takes hundreds of piano tunings to get the hang of.
When I first started tuning pianos it would take me 4 to 5 hours to do one piano, and it wouldn’t even sound that good. Professional tuners tune pianos in 1 – 2 hours time. The longer it takes, the more exhausted your ears get and the more precarious your tuning hammer precision becomes.
4. String Breaking
Piano wire in old pianos can be brittle and become prone to breaking. Should the string be adjusted too far sharp, it may snap. String replacement is another skill that takes practice to develop. A professional tuner will have the appropriate tools and skills to replace broken piano wire should a string break during tuning.
5. Pitch Raise
If you are considering tuning your own piano, it is probable that the piano is nothing close to A=440Hz. Most pianos require yearly tuning, and if it’s been over 3 years, I recommend a pitch raise. A professional tuner will perform a pitch raise in which the piano is tuned very quickly to get it up to A4=440Hz. They will then do a second pass tuning, the fine tuning, which will provide clean unisons and octaves.
If you really are interested in keeping your own piano in tune, buy the necessary tools and do maintenance tuning in between the times your tuner comes. You may hear a unison that has gone out, or a single octave. Simply tuning one or two notes that have gone out, to ensure the tuning lasts longer, can make all the difference to your enjoyment in playing your piano.