I went looking and while others were really good choices too; when it comes to accuracy Polar heart rate sensors and straps are highly rated across the board, compatible with pretty much everything, and are durable.I was particularly impressed with the in-depth research and testing they do with their equipment, neutral comparisons that they make with other comparable HR sensors as well as certified medical heart rate monitoring.I started leaning toward the Polar H10 , which are the 5th generation of Polar's Heart Rate tech, which started in 1977.According to Polar's testing against medical-grade heart rate equipment and other heart rate sensors - the ECG H10 sensor detects HR within 2 ms accuracy at 92.9% for running, 99.3% for cycling, 95.3% for weight training, 95.6% for all activities combined.Amazing accuracy, and very much as good or better than the informal tests I have done of previous HR sensors and straps that I have had - my tests being comparing them to other equipment as well as manually using the old tried and true 'two fingers on the neck or wrist and a watch' method. This sensor IS quite wide and extends across a large part of the strap, this entire section is the HR sensor area.The H10 can do two Bluetooth conenctions, with ANT+ both will connect to multiple ANT+ channels and there is a proprietary GymLink connection also - which connects to some fitness devices.The H10 can be used with any device underwater.The H10 has an internal memory that can save one training session on its own and can be accessed with one of the Polar apps like Polar's Beat app.The H10 works with pretty much everything and anything that we use for fitness nowadays. Anything that receives Bluetooth (Bluetooth 4.0 and Bluetooth BLE) or ANT+ and the Gymlink 5 kHz signals. It will output Bluetooth and ANT+ at the same time and even two Bluetooth devices at the same time.This of course includes watches by nearly all manufacturers as well as nearly all bike computers (not including non-smart old-style bike speedometers/odometers without ANT+ and Bluetooth), and any smartphone. This list is extensive.The HR sensor is not rechargeable but uses the common 2025 battery, Polar says that it lasts 400 hours. One thing you may want to do to extend battery life (recommended by Polar themselves and individuals who have used it over time) is to pop up one of the sensor pod's connectors out of the strap so that it goes to sleep, I have tested it many hours after I have stopped an activity and taken off the chest strap, and sometimes it still is transmitting. So this is something to keep in mind.The battery is easy to change, via what looks at first glance to be a quarter-turn battery hatch that you turn with a coin to open but it actually is quite different, as it snaps open with a little tab. The battery door is of course sealed with an o-ring and everything seems very tight and secure.The total weight with the strap is 60 grams with the sensor pod itself being 12 grams by itself.The soft strap is very easy to put on, wet it with a little water so the sensor can get good contact with your chest skin (you can wait for a little sweat to do it for you but then the sensor may not read well until that happens). You position the sensor monitor pod itself in the front of your chest, which puts the long wide sensor strap in a very good position all across your chest. This sensor area is quite extensively wide to give you that nice accuracy.You could even position this fairly inaccurately and it may still probably work - whether you positioned it by accident or deliberately as perhaps the strap does not fit you well for whatever reason. Some people do have problems with chest straps for many reasons, so I think those who do might find this works better than other chest straps with smaller sensor areas. It even works if you get it too low or too high on your chest, within a reasonable range.There are little rubbery dots on the strap itself to keep it in place. Does this actually make it work better? I don't know, but it seems like a great idea. I have never had one move on me so I can't say whether this makes any difference to me personally, but perhaps to others who have issues with HR chest straps staying in place, it might be just the thing to help.The latch snaps into place easily with one end snapping directly into the other end, to take it off you push a button on the latch and it disengages easily. The button is slightly recessed so you never accidentally pop it off (at least I have not as yet) and also won't get squeezed even by a tight jersey or compression shirt.The strap is very adjustable, with a conventional adjustable loop system to tighten or loosen it. There are two size options for the strap when you order - XS/S and M/XXL. The smaller one covers chests 20-30 inches (58-71 cm) in diameter while the larger one goes from 30-45 inches (67-95 cm).While riding or running or lifting or anything else I can't even feel it unless I consciously make myself aware of it.The Polar H10 sensor monitor is easily cleaned with a little dish soap and water, and a quick wipe to dry it. The soft strap is also very easy to clean with soap and water, or you can put it in a washer - though I recommend using a small washer bag to keep it from snarling and getting bent around other things, and maybe air-dry it instead of using a dryer Make sure you take the sensor off first of course.The soft strap now comes in not only the standard black but a burgundy with a design on it.The H10 is fully waterproof and can be used underwater for swimming and other things, up to 30 meters deep.And you don't need your watch or phone with you for swimming or anything else, at least for one session as the internal memory can store one workout. To set this up you use the Polar Beat app so you do have to connect it to your phone first, set it up and then you can use the strap without the phone present or nearby. When you are finished you reconnect and download the sensor data to the Polar Beat app, which can upload it to a few other sites or allow a download.The Polar Beat app also can be used to keep the firmware updated, check battery power, etc. And there are a number of workouts and such that you can use it for. The Polar Beat app can also estimate your Vo2 Max, you find this in the app under Upgrades (not sure why - as it's a free option) and then under Fitness Test. It estimated mine at 49, somewhat above what other fitness services estimates mine at but very close.Also, the Polar Flow app is another app with training and syncing to other services.And of course any other device you sync it to will have it's own abilities when connected to the Polar H10.You can even test your HRV (Heart Rate Variability), which is a good test of your health and recovery.You could use this as a 24-hour monitor with the right app, I suppose, but the H10 is really meant for exercise. But if you wanted to test your HR and HRV over an extended period of time you could. I think it would be comfortable sleeping, depending on the person, but it's not really made for that and it's possible that the sensor area might dry out over such a long period of inactivity.Lastly, I was going to include some comparison charts between the H10 and other HR monitors that I have but, outside of a few dropouts by optical heart rate sensors because of arm movement - which is to be expected, they all performed within pretty much identically. I had to really go data point by data point comparisons to see much difference.So in conclusion I would very highly recommend the Polar H10 - you are not going to be disappointed in accuracy, comfort, usability, connectivity, and compatibility.UPDATE Spring 2023 - For the last few months the output has become increasingly erratic with low heart rate readings. I have tried to wash the strap as much as I could and added extra moisture to the sensor pads, and even electrode gel. Both things seemed to help for a while, yet the strap became more and more intermittent over time, despite washing and using the electrode gel. It had to be thoroughly washed in the washing machine after each use for it to have any chance of working halfway decent. Once I started biking outside a few times it went in the opposite direction and started sometimes showing super-high HR readings.I was going to send it back to Polar for warranty but it seemed obvious that it was the strap and not the sensor itself, and instead of paying the shipping cost to send it back for warranty I decided to just buy a cheap replacement strap.That did the trick and it works perfectly and reliably again. The replacement maybe isn't as nice as Polar's but the important thing is that it works, I will see what the durability is for it.