We’ve been testing the Nix sensor during sessions in the Precision Performance Lab at our HQ in Dorset.
I've collated our initial thoughts and findings from using the Nix in earnest. And we'd love to hear from you if you need help interpreting any data you've collected or if you want to share your experiences of using the device, you can book a free one-to-one Video Consultation with a member of our Athlete Support Team.
The key takeaways
- The Nix Biosensor is a wearable device designed to estimate electrolyte and fluid loss during exercise
- It aims to optimise performance and recovery by helping you understand your individual sweat rate and electrolyte losses across different conditions
- We tested the Nix in both cool and hot conditions to evaluate its accuracy in estimating sweat rate and sweat sodium concentration
- On average, the device underestimated sweat rate by 0.44 litres per hour (L/h) compared to pre- and post-exercise body mass changes
- The average sweat rate we observed with measured values was 1.19L/h versus the Nix reported average of 0.75L/h for the same exercise sessions
- It underestimated sweat sodium concentration by an average of 212 milligrams of sodium per litre of fluid (mg/L) compared to readings from the Precision Fuel & Hydration (PF&H) Sweat Analyzer
- In its current form, the Nix provides potentially valuable insights into sweat electrolyte composition. However, it struggles to accurately measure sweat rate at levels exceeding 1L/h, which means it can significantly underestimate fluid loss during exercise
- The Nix connects to your phone, providing estimated hydration advice on when, what, and how much to drink in real-time
- It also offers a post-session summary, helping you personalise your hydration strategy based on your estimated fluid and electrolyte losses
Last updated: April 2025
The Nix Essentials
What does it do?
The Nix reports the following sweat composition and hydration data:
- Whole-body (total) sweat loss (millilitres)
- Whole-body (total) sweat rate (millilitres per hour)
- Net electrolyte loss (milligrams per hour)
- Net sodium loss (milligrams per hour)
- Sweat composition (milligrams per millilitre)
The Nix app presents this data in a clear and user-friendly format, generating a personalised ‘Sweat Profile’ based on an average of all your completed sessions. This profile helps you understand your hydration and electrolyte needs, with a visual chart recommending products that best align with your sweat composition to guide your rehydration strategy.
However, while the app provides useful hydration guidance, it currently lacks specific post-exercise recovery recommendations - an addition that would enhance its utility in the future.
How much does it cost?
As of April 2025, the Nix device is more competitively priced than some of its competitors at £108 (USD$132). But, it’s worth noting that the Nix requires repeat purchases of single-use patches, whereas a device like the FlowBio S1 is reusable.
Additional single-use patches are available for £21 or (USD$26) per pack of four, meaning each test costs approximately £5 (USD$6) after purchasing the sensor.
Beyond the cost of replacement patches, there are no additional charges to use the device (e.g. ongoing membership fees).
The Nix package includes:
- 1 x Reusable/ rechargeable sweat sensor ‘pod’
- 4 x Single-use patches
- 1 x Hard-shell charging case
- 1 x USB-C charging cable
How do you wear the Nix Biosensor?
To apply the device:
- Attach the patch to clean, dry skin on the outer bicep
- Secure the Nix pod onto the patch
When removing the device, end your session in the app first to prevent contamination from sweat in other areas, ensuring accurate data collection.
How does the device work?
The Nix Biosensor estimates, rather than directly measures, your sweat rate and fluid losses using real-time sweat analysis and proprietary algorithms.
To estimate whole-body fluid loss, the biosensor collects sweat from a specific area (the outer bicep) and measures its composition and flow rate. This data is then processed using proprietary equations that account for exercise intensity and environmental conditions to generate an overall fluid loss estimate.
For electrolyte concentration, the device uses embedded sensors to detect the concentration of electrolytes in your sweat and also provides the data for the most important electrolyte when it comes to exercise-related hydration: sodium.
The underside of the Nix patch features two flow channels that allow sweat to pass through, where two pairs of electrodes measure the ionic potential (electrical impedance) of the sweat. The device collects measurements every second from both sets of electrodes (Impedance 1 & Impedance 2), creating a time series of data at each location along the flow path.
A small reservoir on the underside of the patch allows sweat to accumulate before flowing across the electrodes. The device then performs flow-rate analysis, using the two electrode pairs to measure sweat movement across a known distance within the flow channel. This local measurement from the outer bicep is extrapolated to estimate whole-body sweat rate using Nix’s proprietary algorithms.
Hardware maintenance and session set-up:
To provide a reading, the device requires approximately 4.5 microlitres of sweat, and it may take up to 25 minutes for the sensor to begin measuring because we typically don’t start sweating immediately when we begin exercising, especially if the intensity is low or conditions are cold.
Once sufficient sweat is detected, the clip-on pod attaches to the outside of the patch, transmitting data continuously by connecting three small metal pins on the pod with the pin pads on the patch.
Software nitty gritty:
The Nix Hydration Biosensor connects via Bluetooth to the Nix Solo app, which is compatible with iOS devices and Android devices running version 13 or higher. The app provides real-time data on fluid and electrolyte losses, delivering personalised hydration notifications to your phone, Apple Watch (Series 3 and newer), or compatible Garmin devices.
During workouts, users can log their fluid intake, select from preloaded electrolyte options and energy drinks, or you can add custom beverages. Post-exercise, the app provides detailed results for the metrics mentioned above, but include sweat electrolyte composition, total sweat loss, hourly sweat rate, and total electrolyte loss, along with tailored hydration product recommendations. Data typically appears in the app within a few minutes of completing a session.
The app currently supports indoor and outdoor cycling and running, and doesn’t require you to record an activity with another platform like Garmin or Wahoo; you’ll just need to start the activity within the Nix app.
It currently integrates with TrainingPeaks, enabling users to upload post-workout hydration data for in-depth analysis alongside other performance metrics. This integration helps athletes and coaches make informed adjustments to both training and hydration plans, although it may be less useful for those seeking real-time insights directly within TrainingPeaks. Future updates are expected to support Strava, Wahoo, and Zwift.
The Nix Biosensor provides real-time hydration data directly to your phone, watch, or bike computer, allowing you to monitor and adjust your hydration strategy mid-activity. The biosensor battery lasts approximately 36 hours and charges via USB-C. Data is stored on the pod until a Bluetooth connection is established, allowing for post-activity syncing.
A key feature of the Nix Solo app is the Nix Index, an environmental score ranging from 0 to 100 that predicts how weather conditions may impact sweat rate when you’re outdoors. This score is calculated using data reported from nearby weather stations: air temperature, relative humidity, wind speed, solar load, dew point, and altitude, based on your phone’s GPS location.
A higher Nix Index suggests greater heat stress and an increased likelihood of sweating. For indoor activities, the app assigns fixed environmental values: 21.1ºC (70°F) 50% relative humidity, 0% solar load, and 0km/h wind speed - resulting in a consistent Nix Index of 59, categorised as moderate heat stress.
What are the unique aspects of this device?
The Nix Biosensor stands out with its compact, reusable pod, featuring a USB-C rechargeable battery and Bluetooth connectivity for real-time sweat analysis. While its accuracy in predicting total sweat loss may be limited, the device provides valuable real-time data on sweat electrolyte concentration and fluid loss through the Nix app.
The unique Nix Index offers personalised hydration insights, helping users make informed decisions during exercise. Compared to other devices like the FLOWBIO S1 and HDrop, Nix’s user-friendly interface and immediate feedback on hydration metrics set it apart, providing actionable data without relying solely on generalised predictive models.
Another key advantage is its ability to store data offline and sync post-exercise, ensuring uninterrupted data collection even when a Bluetooth connection isn’t available. Compared to other tracking devices, the real-time, customisable notification system provides more actionable advice during activities, making it particularly useful for endurance athletes managing hydration strategies in varying conditions.
Customer Support
Nix Biosensors offers customer support via email and provides a detailed FAQ section on their website. Throughout our testing, the Nix team has been highly responsive and supportive, providing regular communication and assistance when needed.
How accurate is the Nix?
Eleven members of the PF&H team have used the device during a variety of bike and run sessions over 6 months.
Of the 70 completed tests, 8 consisted of low-moderate-intensity running, which lasted ~62 minutes. The remaining 62 were all completed while cycling, mostly at a low-moderate intensity, with some consisting of intervals or short efforts. The session lengths ranged from 50 minutes to 2 hours.
Data was collected in a range of conditions, with an average temperature of 27ºC (81ºF) and 58% relative humidity. This included a low of 12ºC (54ºF) with 85% relative humidity, and a high of 42ºC (108ºF) with 15% relative humidity. More specifically, 30 of the tests were completed in conditions lower than 30ºC (86ºF).
Key findings:
- The Nix underestimated sweat sodium concentration (i.e. how much sodium you lose in your sweat) by an average of 198mg/L compared to sweat collected during exercise and measured using our PF&H Sweat Analyser
- We collected sweat from the inside of the upper forearm, as we normally do during our Sweat Test with a macroduct (sweat collection device), but this time during exercise
- The area where the macroduct was placed was wiped with gauze and then another time with gauze and deionised water to prepare the skin surface. The macroduct was then placed immediately onto the skin surface and sampled through our PF&H Sweat Analyzer to assess sweat sodium concentration
- The macroduct was removed prior to it overflowing
- Additionally, the Nix underreported sweat rate (i.e. how much fluid you lose) by an average of 0.44L/h compared to sweat rate measured using pre- and post-exercise body mass. And in an extreme case, sweat rate was underestimated by 1.13L/h.
| Measured (PF&H) | Estimated (Nix) | Mean difference | |
|---|---|---|---|
| Sweat sodium concentration (mg/L) | 980±473 | 768±228 | 198mg/L |
| Sweat rate (L/h) | 1.19±0.46 | 0.75±0.30 | -0.44L/h |
Data are reported above as the average ± standard deviation.
Sweat Sodium Concentration:
Our convenient, at-rest, pilocarpine-based Sweat Test maximally stimulates sweating to determine your sweat sodium concentration. Sweat sodium concentration increases with sweat rate because sweat glands have less time to reclaim electrolytes at higher sweat rates as the sweat is moving faster through the sweat duct. As a result, the test result may be higher than what you’d see at lower sweat rates, such as during low intensity exercise, or in exercise in cool conditions. This makes it a useful predictor of sodium losses during intense exercise.
The amount of sodium you lose through sweat is largely determined by genetics. However, it's normal to experience small fluctuations in sweat sodium concentration due to factors like changes in sweat rate during and between exercise sessions. While these variations can significantly impact your race strategy, the most important factor is ensuring you're within the correct range (i.e. low, moderate, high, or very high sodium loss) for effective sodium replacement.
These fluctuations are often linked to changes in whole-body sweat rate, influenced by temporary shifts in exercise intensity, environmental conditions during a session, or differences between sessions, making direct comparisons challenging.
Our data suggests that Nix may underestimate your sweat sodium concentration, possibly due to underestimating your sweat rate. If you don’t have access to a reference sweat sodium test, such as a PF&H Sweat Test Center, or if you’re looking to gauge your during-exercise sodium loss over multiple uses, the Nix can still provide useful insights to get an idea of where you sit on the continuum of salty sweaters, for refining your hydration strategy.
If you notice a significant change in your reported sweat sodium concentration with Nix, we recommend repeating the session multiple times to confirm consistency. Sweat sodium levels typically remain stable unless the whole-body sweat rate differs significantly between sessions (e.g. comparing very low to very high sweat rates).
Sweat Rate:
Guidelines on acceptable dehydration levels before they negatively affect performance and health have evolved over the years. Previously, the American College of Sports Medicine (ACSM) recommended replacing 100% of sweat losses to maintain performance. However, we now know this isn't necessary and can even be dangerous. This outdated guidance contributed to cases of hyponatremia (low blood sodium levels caused by excessive water intake), leading the ACSM to revise its stance.
Sweat rate varies significantly within and between individuals. Research shows that factors like environmental conditions, clothing, body composition, and level of heat adaptation (heat acclimation or acclimatisation status), influence sweat rate.
Data from Nix can help you understand your fluid loss across different intensities and environmental conditions, guiding a more effective hydration strategy.
To evaluate Nix Biosensor data, we measured sweat rate using nude body mass changes pre- and post-exercise, accounting for any fluid and fuel (e.g. energy gels, energy chews) intake , following our how to measure your sweat rate protocol.
In our data, the Nix consistently underestimated sweat rate by an average of 0.44L/h. We found that the Nix underreported sweat volume loss for all athletes in our group, especially at higher measured sweat rates. Similar to other sweat sensors we’ve tested, the Nix tended to be marginally better at reporting sweat rates in those with lower sweat rates; but since many of the team had sweat rates above 1L/h, the Nix struggled. Unfortunately, it’s beyond 1L/h where adequately replacing your losses becomes increasingly relevant as you’re likely exercising in hotter and humid conditions, where dehydration can harm your performance and health.
Underestimating your sweat rate by nearly half a litre per hour on average could have a pretty big impact on your hydration strategy, especially during longer races where your net sweat losses will build up.
If you use the Nix to measure your sweat rate, it’s worth conducting your own sweat rate measurements to build confidence and understanding of the Nix’s data, especially if you’re someone who experiences a high sweat rate regularly.
How to Use the Nix
The Nix Biosensor provides valuable insights into sweat rate, electrolyte composition, and sodium concentration, making it a useful tool when laboratory-grade sweat analysis or highly accurate sweat rate measurements are unavailable or impractical. While it does not yet replace gold-standard methods for athletes, the Nix team continues to refine and improve its accuracy.
Our data suggests that Nix consistently underestimates sweat rate, indicating that its algorithm still has room for improvement. For now, measuring sweat rate via pre- and post-exercise body mass changes remains a simple, cost-effective, and highly accurate method, making it the preferred choice for serious athletes.
If you choose to use Nix, we recommend testing it multiple times to establish your own sweat sodium concentration profile. The ‘Sweat Profile’ feature helps average results, refining your electrolyte replacement strategy, especially given the variability compared to PF&H’s Sweat Analyzer.
In high-intensity sessions, particularly in hot and humid conditions where sweat rates exceed 1.0L/h and dehydration risk increases, having accurate sweat rate and sodium concentration data is crucial. This is where devices like Nix could become increasingly valuable for endurance athletes looking to fine-tune their hydration strategy.
Getting the Most Out of the Nix Biosensor
To get the most out of the Nix Biosensor, we recommend:
- Complete at least 10 tests using the device.
- Ensure these tests vary in intensity, duration, and type of exercise session.
- Record your own sweat rate data for each session.
- Compare your results with the Sweat Profile feature to refine your hydration strategy.
What’s next?
We’ll continue to stress-test the Nix in hot and humid conditions at our Precision Performance Lab and update this blog when more data becomes available, especially if there are any compelling changes to the device’s accuracy, or if there are major software and hardware updates.
