Underwater Heart Rate Recording

Updated January 2018

A little bit about HR and Swimming

Acquiring accurate heart rate data when swimming is complicated. Polar have always had a heart rate strap that transmits LIVE heart rate data to a wrist unit when swimming. This is because Polar use an analogue signal for sending heart rate when in the pool. The DIGITAL signal used by Garmin (ANT+) and Suunto (Bluetooth / Suunto ANT) will NOT transmit underwater. This has meant the companies who have solely switched to digital heart rate signalling platform have had problems recording heart rate while swimming.

Off the Wrist Heart Rate sensors

With the advent of wrist based optical heart rate sensors it is possible to acquire heart rate while swimming WITHOUT a chest strap. HOWEVER - it will only work effectively if the watch is UNDER a wetsuit, to prevent water passing between the skin & sensors, disrupting the HR reading. Any of the strapless heart rate monitors here should be OK when swimming, however, it's something most brands don't guarantee.  Please ensure the watch you are looking at is 100% water resistant, suitable for POOL SWIMMING

1) Polar and Swimming - live Analogue broadcast

Polar have two offering for swimmers. 1) Analogue broadcast and 2) Internal Storage. Their traditional T31 coded analogue heart rate strap which works with popular wrist units like the FT1, FT4, FT7 etc and the Polar H7 sensor uses BOTH Bluetooth 4.0 and Polar analogue coded. That's how a multisport watch like the Polar V800 (below) can support live heart rate when running/ cycling (via Bluetooth) and then analogue heart rate broadcast / detection when in SWIM mode. The Polar A300 also supports heart rate in the Pool but NOT the M400, this ONLY receives HR data in Bluetooth.

Polar V800 Triathlon bundle for swimmers and multisport athletes

2) Internal Memory storage - Polar H10

For simply recording heart rate while swimming in the pool or underwater for extended periods of time, this is the BEST OPTION.

The Polar H10, like the Garmin HRM-swim and Suunto Smart belts below, STORES the HR data to an internal memory. It has enough memory space for 65 hours of training data (in just one file). In the Polar Beat APP confirm you want to "SAVE HR to H10". Press start when you begin swimming, leaving you phone somewhere safe and dry. At the end of the swim press STOP on the Polar Beat app. All heart rate data from the swim is saved to the Polar Beat app and shared to your Polar Flow account. For more information please visit our review of the Polar H10 here

Polar H10 heart rate sensor can be used when swimming

Suunto and Swimming - HR data storage solution

Suunto have their Smart HRM belt. The Suunto Smart belt STORES the data locally and then sends it to the watch (Ambit3 Sport & Spartan Sport) at the end of the swim, when you are free of the water. The result at the end of a session is a detailed time versus heart rate graph for the entire swim session.

Suunto Smart belt with underwater heart rate storage

Garmin take the Storage route

Garmin have also opted for the locally stored HR data options when swimming. To achieve this they have released two new Heart Rate monitor belts, the HRM-Tri and the HRM-Swim. The TRI version is the BLACK strapped model, the SWIM is the BLUE strapped version.

The HRM-Tri and HRM-Swim belts are available with several Garmin Multisport packages like this Forerunner 920XT Tri-Bundle, Forerunner 735XT Tri Bundle and Forerunner 935 Tri Bundle.

Garmin HRM-swim and HRM-tri belts for swimming

What's the difference?

Both main difference between the Garmin HRM-tri (black) and HRM-swim (blue) is the SWIM version comes with a WIDER chest strap that has been produced with a "sticky" coating on the back of the strap to prevent chest strap slippage. This is something that neither the Polar H7 or Suunto Smart offer. Swimming fast, especially during a hard push off, the Suunto and Polar straps can slip down the chest. For females wearing swimsuits or men in tri-suits this has not been a problem, but for bare chested males it can be a problem. The new BLUE strap that comes with the HRM-swim resolves this issue with it's NON-SLIP backing.

Running Dynamics2?

If you want advanced running dynamics data sent to your watch (920XT, Fenix3 / 5, Forerunner 620 etc) then you need to buy the HRM-tri version. The HRM-swim version does NOT support Running Dynamics2.

Can I RUN with the HRM-swim version?

Yes and no. It will work, and it will send HR data to your watch in the regular ANT+ fashion, but the non-slip sticky solution on the back of the HRM-swim strap is not designed to be used out of the water.

How does it work underwater?

The HRM-tri and HRM-swim both work the same way. The module will transmit in regular ANT+ when OUT of the water. When in the water it works in a similar way to the Suunto Smart, as a heart rate data recorder. When the user exits the water the data logger connects to the wrist unit and "dumps" the heart rate data to the watch. Clever.

You can now get heart rate when swimming with Garmin HRM

Garmin Forerunner Tri Bundles

It will come as no surprise that Garmin have several BUNDLEs specifically for triathletes that includes the HRM-tri version of this new data logging chest strap. The TRI-BUNDLE offerings include the Forerunner wrist unit and comes bundled with a QUICK RELEASE wrist strap / bike mount solution.

Garmin Forerunner 935 Tri Bundle

For swimmers and triathletes - the Forerunner 935 Tri Bundle

Garmin Forerunner 920XT Tri Bundle

Garmin Forerunner 920XT Triathlon Bundle

Garmin Forerunner 735XT Tri Bundle

The Forerunner 735XT Triathlon Bundle with HRM-swim and HRM-run

Why not watch the video ...

Garmin have put together this nice sort video that tells it like it is.


Specification / Battery Life

  • The HRM-tri has a predicted battery life of 10 months.
  • The HRM-swim has a predicted battery life of 18 months
  • Both store up to 20 hours of HR data
  • HRM-tri also provides Advanced Running dynamics2, including cadence, vertical oscillation and ground contact time.**

** Running Dynamics2 requires a compatible device eg, Forerunner® 920XT, Forerunner® 620, fēnix® 3, fēnix® 2, epix™

Andrew Wyatt

Andrew Wyatt

Andrew Wyatt from Texas has been the best-selling author in 2014. He was the most appreciated writer in 2015. Different organizations have honored him with different awards and memberships as well. He is a tech shop owner and completed graduation in Electrical Engineering.

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