Actually even the ones from walmart are inaccurate. If you want an accurate hydrometer they sell one that can be calibrated and it's an analog one. I unfortunately don't have a link to it off hand and I'm on my cell phone so I can't find it. I'll repost with the link once I can get to it.
Hey Neal, don't suppose you could give us that link? I need a hygrometer and the online reviews for the ones I'm finding are just so-so. Thanks!