TFA Thermo-Hygrometer WH Set of 4

The humble hygrometer, where to start?

I believe the hygrometer is an essential tool for maintaining your guitars, and I bought these ones with the intention of keeping them in my acoustic guitar cases.

I'll start with the good things first.

Positives
They are small, and will fit in any guitar case without any trouble.

Negatives
I wasn't expecting all four hygrometers to have exactly the same reading, but sadly the disparity within the 4 themselves is far too great, sometimes with it being 5% with each other, and a few degrees too.
In comparison to the other hygrometer I own, all these TFA ones are between 5 to 10% greater in humidity reading.

Also, there's a trick to 'calibrate' your hygrometers (have a look online for the cap of salt/few drops of water method, which will lead to the hygrometer being around 75% or so, but all of mine were all well over 80%. these TFA ones are completely off the mark for anything vaguely accurate.

I have some other ones which I bought for less which seemed to be more accurate, so if you are considering these particular hygrometers I'd recommend looking elsewhere for a hygrometer.