How accurate are the heart rate displays on cardio machines?

I was running on a new treadmill at my local gym yesterday, and I had really exerted myself and my breathing started getting a little out of control (I'm asthmatic, so I'm careful not to overdo). I slowed to a walk, and as I did so, I put my hands on the heart rate monitors, and ... according to the display, my heart rate was at only 112, but I know from monitoring it at other times, at a similar intensity, it has shown 180+. Huh?

I've noticed before that my heart rate fluctuates considerably depending on the day, but never looked into it. Yesterday, though, it made me realize that I really have no idea what heart rate I reach at my peak, and what I maintain.

So, I got home and started Googling, and I wasn't too surprised to see that I'm not alone in experiencing some wonky readings. And while a variety of factors can influence one's heart rate -- diet, drugs (like caffeine), and hydration -- they shouldn't cause such a fluctuation.

If you want to actually monitor your heart rate, you'll need to get a personal heart rate monitor. Fortunately, there are loads of options in tons of price ranges, so you should be able to find something that works for you. I'm hoping to find one for myself, too, so if you have a recommendation, leave it in the comments!