Flowbio Sensor

The Flowbio Sensor claims to be 'the world's most accurate sweat sensor', providing post-ride estimates of your water and sodium loss. It's a wearable device that has improved my awareness of hydration needs, and the user-friendly app offers useful insights. However, it's currently only available for iOS, though it's going to be available for Android in April, and the price means it's not for everyone.

> Buy now: Flowbio Sensor for £379 from Flowbio

These days, you can measure pretty much any metric while riding – heart rate, power, cadence, glycogen levels, core body temperature... Yet, despite the importance of hydration, achieving optimum hydration is often a guessing game for most of us. Unlike other metrics, real-time hydration monitoring has been relatively untapped until now, and the Flowbio is one of the first devices to provide this data.

2025 Flowbio S1 - boxed 1.jpg (credit: road.cc)

Inside the box, you'll find the sensor, a simple four-step quick start guide and a small, seemingly mysterious empty bottle with a needle-like attachment.

While it might look like it's for something else, rest assured, using the Flowbio is a non-invasive process. The bottle is actually for cleaning the sensor with water from time to time to help with the accuracy of the readings.

How does it work?

The sensor collects sweat through a small channel that passes over a conductivity sensor on the underside of the device. It then estimates your overall sweat rate by combining local sweat measurements with skin temperature. The device uses algorithms that factor in environmental conditions and session intensity, taking into account your heart rate or power data.

While I have no way of verifying the accuracy of the data, I recorded consistent data throughout a week-long training camp in Lanzarote, with similar trends observed.

> Have I finally found out why I keep getting dropped? Why hydration is more important than you think for cycling performance

The sensor can either be clipped onto a heart rate chest strap or you can use the arm strap that it's meant to come with now (ours didn't); it's then worn on the back, or the arm. I wore it on a chest strap on my upper back. It stayed in place well, but the sensor lacks a bit of finesse and took some getting used to. Wearing it on the chest strap and with a sports bra meant the placement over my ribs on my back occasionally made it uncomfortable.

I'm hoping to receive an arm strap soon, and will update this review with how well that works and how comfortable it feels.

2024 Flowbio S1 hydration sensor (credit: road.cc)

In use

On the unit itself there is a single button, used for pairing, and there are small LEDs that blink different colours to tell you when it's on, connected and tracking. You can start and stop sessions using this button or by using the app on your phone, and you then don't need your phone on you to record while you're riding.

If you're wearing it on your back you can't see these light codes but I tended to stop and start recording through the app on my phone before leaving my house. If you start recording too early, the algorithm crops the activity to the length of the ride, which I found to be effective.

In terms of setting up, it's pretty straightforward and pairing through the Flowbio App is simple. The app is currently only available for iOS devices, but it's coming to Android soon. You can then pair with your fitness tracking platform of choice such as Wahoo, Zwift, Coros, Garmin and TrainingPeaks.

2025 Flowbio S1 - boxed 2.jpg (credit: road.cc)

Post-ride, you'll need your phone to sync the data via Bluetooth, where the algorithm then works out the sweat rate and sodium loss. This failed for me a couple of times but the sensor stores the sessions until they have synced.

At the moment, the Flowbio only provides post-ride data, but there are plans to offer real-time guidance. After a ride, you'll receive an estimate of your total sweat loss, sodium loss and a recommendation of how much fluid and sodium to replenish. Additionally, you'll get a breakdown of your sweat composition, including total sweat rate and sodium concentration. Feedback of data is simple, with key actionable advice on what you need to replace.

2025 Flowbio S1 Completed session, planned session and dashboard.png (credit: road.cc)

For example, during a 4.5-hour ride in around 20°C, I lost 1.94L of sweat and 2,062mg of sodium, which works out to 1.06g of sodium per litre of sweat, for those who are interested. You're then given a recommendation for how much you need to replace, but since the sensor doesn't track what you've consumed during the ride, you'll need to monitor and subtract your intake to figure out what you still need to replenish.

You can also input the details for your next ride, and it will recommend how much fluid and sodium to consume based on the ride's temperature, distance, speed and time.

While a lot of the time I'm happy to ride on feel, I found this system easy to understand and helpful in adjusting my hydration habits.

Flowbio claims a battery life of 100 hours and I found this to be pretty accurate. It charges via a USB-C cable but this isn't provided. For the price, I would expect it to come with a charging cable.

The sensor itself is IP55 rated, offering some protection from dust and water.

What are the benefits?

The Flowbio Sensor provides personalised insights that would traditionally require access to a lab or sports scientist. It doesn't interrupt training, and it's used in the same environments where athletes train and compete.

It helps fine-tune hydration and rehydration, especially before races or multi-day events, and is useful for planning your race day bottles.

Are the pros using them?

The Flowbio hydration sensor is designed for cyclists, triathletes and runners for both training and racing, and is being used by elite athletes such as WorldTour riders from EF Education-EasyPost and the Brownlee brother triathletes.

Value

At £379, the Flowbio is a significant investment – more expensive than the Core sensor I previously reviewed (and I thought that was expensive!). However, the data may be useful to a wider range of people, and with the potential for real-time hydration tracking during rides, I think the Flowbio could offer better value.

Conclusion

Overall, the Flowbio Sensor performed well and the app is easy to use, providing actionable data to help with rehydration and hydration planning for future rides. Given its price, I think it's most valuable for those with specific training goals or who are participating in multi-day events where optimal fuelling is essential. For casual riders, the detailed hydration data may not be as useful, as drinking to thirst and following general guidelines, like a bottle an hour, might be sufficient.

> Buy now: Flowbio Sensor for £379 from Flowbio

Verdict

Offers useful hydration insights in a user-friendly app, but it's pricey and currently iOS only