Do heart rate monitors work underwater?
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Enjoy swimming? Wish to level up your training? As with running or cycling, knowing your heart rate data while training is a game-changer. It will help you understand when you are pushing too hard, and when you aren’t really. It’s easy to lie to yourself until you see the real data. It’s a good way to measure progress. But can heart rate monitors (HRM) work underwater?
Heart rate monitors that work underwater exist. You will find straps specifically designed for swimming, as well as wrist heart rate monitors, which tend to be less accurate. But not all HRM work underwater. And even if they do, not all HRM will display the data while swimming.
Let me give you more information about heart rate monitors for swimming. I will review the differences between straps and wrist HRM in terms of accuracy. I will detail which models work underwater and which one can even display the data while swimming.
Accuracy of chest straps vs wrist heart rate monitors
Wrist heart rate monitors are less accurate than chest straps because they work with optical sensors while others are based on electrodes. It means that wrist HRMs use a flashing light to measure but water can easily affect light refraction and give wrong readings.
Behavior of writs heart rate monitors underwater
Wrist heart rate monitors use light refraction on your skin to measure the change in blood volume at each pulsation. They can then calculate your pulse. This type of HRMs really depends on having the flashing green light tight against your skin, which means that you can’t wear your watch loose, else it won’t be accurate.
Underwater it’s even more important, water tends to change the diffraction of light, so if water manages to get between your skin and the watch, the pulse measurement will be incorrect. So this type of HRM can easily give erroneous measurements underwater, even if you wear your wearable tightly. Indeed, when you swim, your movements can also give opportunities for the water to find its way below the watch. So it’s unlikely you’ll have 100% of a swim session with correct results. But still, they give you a good indication and if you wear it tightly it’s not going to be wrong very often.
Garmin and Polar, who are very renowned for their swim watches, mention on their website that the accuracy of wrist heart rate monitors is less than strap heart rate monitors.
The behavior of strap heart rate monitors underwater
Strap heart rate monitors work in a closed circuit using two sets of electrodes against your skin. Thanks to these electrodes, the strap can detect the electric signals transmitted by your heart, and thus measure your pulse. The data is then recorded or directly transmitted to your watch by Bluetooth or ANT+ depending on the brand and model.
Underwater, this technology is not really affected by the water. The real issue is if the straps move against your skin while you swim. This is something that can happen, especially when pool swimming. Indeed, you will be doing laps, and each time your turn around you push against the wall. This acceleration tends to separate the strap from the skin, water getting in the way, and sometimes it can even roll up. This is why some manufacturers make much bigger straps for pool swimming, to prevent the strap from moving, like with Garmin HRM-Swim. Also, this issue never happens if you have a trisuit, or a swimsuit that covers the strap. So it can be an issue for men training in the pool.
So in terms of accuracy, straps are really good because their mode of functioning is not affected by water. The only risk is that the strap moves a bit while pool swimming but most straps will be wide enough to compensate and prevent this risk. So it’s not really a problem.
Why don’t all HRM work underwater?
Only a handful of wrist and strap HRMs work underwater. Even if they are labeled as waterproof, they aren’t suitable for recording your heart rate underwater. It’s generally a manufacturer’s choice, especially with watches.
You will find several watches on the market which have a wrist HRM but don’t have a swim mode at all or have a swim mode without displaying heart rate data while swimming. To give you an example, the recent Garmin Forerunner 55 has a pool swimming mode but even though the watch has an HRM, it’s not compatible with HR data in water. This is quite common for manufacturers to restrict some functionalities by software on some watches. This is linked to the segment the watch is marketed for, the price of the unit, and the battery life. This is why I am providing you a list of watches for which their wrist HRM works underwater.
If you really wish to have heart rate data, you could also look at HRM straps. Just understand that depending on your watch and on the strap, you might not get live data during your swim. But in all cases, you will get the data displayed on your phone once your activity is over. I will go into more detail on these specificities at the end of this article.
List of Heart Rate Monitors that work underwater
Straps HRM that work underwater
- Garmin HRM-Pro
- Garmin HRM-Swim
- Garmin HRM-Tri
- Suunto Smart Sensor
- Polar H10
- Polar OH1*
- Polar Verity Sense*
*Please note that despite being optical sensors, the Polar OH1 and Verity Sense are quite good underwater. Their design is more advanced than the optical sensors embedded in sports watches. Thus they are also more accurate than the latter.
If you want to know what is the best heart rate monitor for swimming among the list above. Do not hesitate to read my article about this specific topic:
Best Heart Rate Monitor for swimming
Wrist HRM that work underwater
Garmin watches:
- D2 Air
- D2 Air X10
- D2 Mach 1
- Descent Mk2 series
- Enduro series
- Epix (Gen 2) series
- Fenix 6 series
- Fenix 7 series
- Forerunner 55
- Forerunner 245 series
- Forerunner 255 Series
- Forerunner 745
- Forerunner 945/945 LTE
- Forerunner 955/955 Solar
- Garmin Swim 2
- Instinct 2 series
- Instinct Solar series
- Legacy Hero series (Captain Marvel, First Avenger)
- Legacy Saga series (Darth Vader, Rey)
- MARQ collection
- Quatix 6
- Tactix 7 series
- Tactix Delta series
- Venu series
- Venu Sq series
- Vivoactive 4 / 4S
Polar watches:
- Pacer
- Pacer Pro
- Unite
- Ignite
- Ignite 2
- Grit X
- Grit X Pro
- Vantage V
- Vantage V2
- Vantage M
- Vantage M2
Suunto watches:
- Suunto 9 Peak
- Suunto 5 Peak
- Suunto 9 Baro
- Suunto 9
- Suunto 7
Others:
- Coros Apex
Specificities of some heart rate monitors straps
The difficulty with heart rate monitors straps underwater is to send the live data to your watch while you swim. Heart Rate Monitors generally transfer the data via Bluetooth or ANT+. But this won’t work while swimming. It can work with 1-3cm of water but not more.
So manufacturers have two options:
- store the data until you save your activity – you can’t view your HR data while swimming
- store the data and partially transmit a summary or the entire data every time it’s possible to transfer
Garmin has chosen to store data on the HRM-Pro, while on the HRM-Swim and HRM-Tri, it stores the data but gives you partial information while swimming. You will get after each interval (meaning each time you go out of the water for a short time), a summary of your maximum & average heart rate over the last interval.
Suunto has chosen with their Suunto Smart to fully store the data until your finish the activity. Same with Polar’s H10 strap.
In summary:
| Strap HRM | Send partial live data while swimming? | Send full data once activity is over? |
|---|---|---|
| Garmin HRM-Pro | No | Yes |
| Garmin HRM-Swim | Yes | Yes |
| Garmin HRM-Tri | Yes | Yes |
| Suunto Smart Sensor | No | Yes |
| Polar H10 | No | Yes |
| Polar OH1 | No | Yes |
| Polar Verity Sense | No | Yes |
Please note that compared to straps, most sports watches that have a working HRM while swimming will also have the information displayed live on the watch screen. So wrist HRMs are less accurate compared to straps but have the advantage of showing live data while swimming.
What should you use to record your heart rate underwater?
Strap heart rate monitors are more accurate than wrist ones, but for most people the accuracy of wrist HRM is good enough. In my experience with my Garmin 745 swimming, in open water and in pools, I have never seen crazy readings. The heart rate data I see generally match what I expect from the intensity of my training. But I wear it quite tight on the wrist, so I don’t think water can get between the skin and the HRM.
If you are an amateur swimmer, more for pleasure than really making a performance at your next race, you can benefit from the data of your watch without bothering with buying a strap. Of course, this means you must own one of the watches which measure HR data while swimming (please refer to the list I made earlier in this article). If you own a different watch, then it makes sense to buy a strap to see your HR data after your swim.
And of course, if you really want to focus on your training and improve your stats and PB, then it makes sense to have an HRM strap while swimming. Just remember only Garmin HRM-Swim and Garmin HRM-Tri HRM will give you information while you swim. For all the others, you will see the data once your activity is over and saved.
I believe Garmin HRM-Swim is the best HRM available on the market. If you are interested, have a look here for more information and price.
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