In my experience with the tunnel type the compass will still move but very little like maybe a swing of 20 degrees or so. I also use a magnetism app to check before and after demagnetizing.

 

The cheap blue demagnetizer has done well for me for nearly a decade. I remove the watch bracelet and end links and demagnetize all of them separately.

 

I bought a vintage one at a watch show. Works for me. Similar to mine

 

I’ve had a few that were magnetized, so I compromised and bought the next step up from the $11 blue one. It was $24 and it’s worked great, with zero deflection on the compass afterwards.

 

I’ve had a few that were magnetized, so I compromised and bought the next step up from the $11 blue one. It was $24 and it’s worked great, with zero deflection on the compass afterwards.
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I have this same one and it woks great, the only issue I have is when I need to demagnetize watch parts before cleaning them, super pain in the butt!
Really considering the Elma Antimag, but it ain’t cheap.

 

Elimag and Magna-flux units sometimes show up on eBay for reasonable prices. These are superior to the little unit.
The best units are made by Horotec (Magtest), which detects amount and location of magnetism then runs a programed demag cycle to elimiate it.
Vigor also makes an excellent instantaneous machine...which is essentially similar in operation to the older Elimag and Magna-flux machinesa (and current Elma machine).

The only issue with the older machines is that non-working machines are often represented as working. These have numerous (ususally) electrolytic capacitors...these are easy enough to replace (catastrophic failure can be quite exciting)...often these have ungrounded cords...I usually replace with grounded.

Regards, BG