The Apple Watch Series 6 is the best smartwatch on the market. (Don't take my word for it; plenty of reviewers feel the same way.) It has a new processor. It charges a little faster than previous models. Sleep tracking is native. The always-on standby screen is a somewhat brighter than the Series 5.

And it includes -- wait for it -- a blood oxygen sensor.

Which doesn't work very well, at least for me.

If you aren't familiar, pulse oximetry -- health care professionals call it "pulse ox" -- measures the percentage of blood hemoglobin carrying oxygen. Typically pulse ox is measured by placing a device on the end of the index finger; the device passes a beam of red light through your finger and the result is calculated by measuring how much light gets absorbed as it passes through.

Generally speaking, a normal pulse ox reading is at least 95 percent. (As I write this, mine is 97 percent.) People with conditions like chronic lung disease or sleep apnea can have levels of approximately 90 percent. But -- again, generally speaking, so consult a health care professional for advice specific to you -- a pulse ox reading of less than 95 percent can be cause for concern.

That's especially true since, according to health care organizations like the Minnesota Department of Health, "many patients with COVID-19 disease have low oxygen levels even when they are feeling well. Low oxygen levels can be an early warning sign that medical intervention is needed."

All of which should make the Apple Watch 6's blood oxygen capability an extremely handy feature. According to Apple:

Your blood oxygen level is a key indicator of your overall wellness. It can help you understand how well your body is absorbing oxygen, and the amount of oxygen delivered to your body. The remarkable new sensor and app in Apple Watch Series 6 allow you to take on-demand readings of your blood oxygen as well as background readings, day and night.

Sounds good.

Except it's not particularly accurate.

It's inaccurate ...

If you want to check your pulse ox, one way is to do so manually. Move your watch a little farther up your arm than normal (especially if you have bony wrists like me), snug it up tightly, and rest your arm on a horizontal surface for 15 or 20 seconds.

Sometimes you'll get a reading. Sometimes you won't. When you do, it's likely to be only directionally accurate. The 20 times I got a reading, 18 showed my pulse ox levels to be between 2 and 3 percent lower than what a finger monitor recorded. While that doesn't sound like much, if your pulse ox tends to be 96 and your Watch indicates 93, you may be unnecessarily concerned.

The Apple Watch 6 can also monitor your pulse ox in the background. For me, that was even less accurate, probably because it wasn't positioned correctly on my wrist. To be fair, that also often happens with my watch's heart-rate monitor, especially if I'm working out and doing pushups or yoga or anything with my hands on the floor; in those cases, my watch usually can't detect my heartbeat, much less measure it accurately.

Apple recognizes that. In a statement to the Wall Street Journal, Apple said:

The Blood Oxygen feature has been rigorously tested across a wide spectrum of users and across all skin tones. For a small percentage of users, various factors may make it difficult to get a blood oxygen measurement including motion, watch placement on the wrist, skin temperature and skin perfusion.

Apple also says the blood oxygen monitor is a "wellness" feature not intended for medical diagnostic use.

So why include a problematic feature on a new watch release?

... but Still a Smart Move

I've used heart-rate monitors -- the kind that strap around your chest -- for training purposes for years. They're really accurate. So when Apple released a watch with a heart-rate monitor, I was skeptical.

Surely it couldn't be as accurate as a "real" heart-rate monitor.

And it wasn't. Which kind of irritated me. But then I got used to it. After all, I'm not a professional athlete. I don't need to know my heart rate down to the precise beat. Plus or minus five beats per minute is fine -- and at this point, my watch is usually within a beat or two per minute of what a heart-rate monitor registers.

Sure, sometimes it can't detect my heart rate, most frustratingly during HIIT workouts, when I'm flirting with my max heart rate. 

But I've become OK with that, and see my Apple Watch as a useful heart-rate tool to the extent that only occasionally do I wear another heart-rate monitor. 

So: Do I think the pulse ox readings from an Apple Watch 7 are extremely accurate? Rightly or wrongly, I don't.

But having that capability, glitchy or not, will start the slow drip process of making me think my watch has even greater health and fitness monitoring capabilities. I'll still check out my pulse ox, even I think it's off. I'll use my watch even more for checking out personal health data.

Even just for curiosity's sake.

In the meantime, Apple will improve the sensors and the software, and the pulse ox readings will become more accurate. 

Sure, I may never totally trust my watch. The data it provides -- whether heart rate or pulse ox or ECG or whatever features Apple introduces in the future -- may never serve as more than an indication that, in certain circumstances, you and I should probably see a health care professional for more accurate results.

After all, it's a wellness and fitness tool, not a medical tool.

But periodically introducing new wellness features may cause me, and plenty of other people, to think more about their health and wellness, and grow to see their watches as indispensable. (I kind of already do.)

Which, for a company that prizes long-term customer value as much as Apple, is the real point.