How are your Hydration levels measured in a Fitness watch?
Introduction
Ever wondered how significant the impacts on health could be if someone dehydrates? It can impair performance and can cause heat strokes. While overdrinking can result in fatigue, confusion, coma, and even death.
Hence, fitness watches have started inventing methods to track your hydration levels, and the following is one such methodology for the same. While the design could change from model to model, the underlying principle remains the same. Hydration tracking is performed non-invasively through perspiration (sweat for example) that is produced by the user. And here is how it works, let’s go!
Quick Science recap!
Electrolytes: Substances that have a natural positive or negative charge when dissolved in water. Example — Sodium and chloride in our sweat.
Electrical conductance: The ease with which current can flow through a material. And, resistance is the exact opposite.
Capillary effect: In simple words, when the adhesive forces between a liquid and a solid are more than the cohesive forces between the liquid molecules, then the liquid is drawn towards the solid. Sometimes, even against gravity.
Electrode: An electric conductor that carries current into the non-metallic part of a circuit like some liquids, gases, etc.
Principle
Watch measures the electrical properties of perspiration and correlates it with the concentration of the electrolytes in perspiration. The concentration of electrolytes can in turn be used to determine the hydration level of the person wearing the watch.
For example, a high level of the electrical conductance of perspiration means a high concentration of electrolytes, implying a low level of hydration. Similarly, a low level of the electrical conductance of perspiration means a low concentration of electrolytes, implying a high level of hydration.
Hardware Setup
Figure 1: Enlarged view of the arrangement of electrodes and meter in the watch band
Electrodes
Multiple electrodes are placed on the inner surface of your watch band. They are positioned in such a way as to come in contact with the perspiration (e.g., sweat) that is present on your skin. Some models may use the capillary effect to easily measure perspiration without requiring the electrodes to be positioned on the outermost surface of the watch. We can use these electrodes to measure the electrical properties of your perspiration.
Meter
The meter can be mounted anywhere on the watch with an independent connection to each of the electrodes. It can measure an electrical property across any pair of electrodes. And here is how!
The meter applies a fixed current through the pair of electrodes, flowing across the perspiration that is in contact. Then, the voltage difference across the perspiration is measured. Thus with the voltage and current values available, we can compute the resistance through Ohm’s law. Hence we can compute the Conductance, G = 1/Resistance.
Perspiration provided at any portion of the length of the electrodes can be sampled and measured.
Working Mechanism
Figure 2: Workflow of computing the hydration level in a fitness watch. Source: US Patent 11089999
- First, the watch detects the presence of perspiration between electrodes. This detection is based on a minimum threshold value for conductance that should be present. Basically, it should be well above the conductance value observed when there is an absence of fluid, such as perspiration. Moreover, this detection algorithm can be initiated by the fitness watch when you move, for example.
- Then, the electrical conductance is measured as mentioned in the previous section.
- Finally, your hydration level is indicated to you based on the electrical conductance of the perspiration. This is because of the following.
A low level of hydration implies → Higher concentration of electrolytes in perspiration → Higher conductance.
A high level of hydration implies → Low level of concentration of electrolytes in perspiration → Lower conductance.
Conclusion
The mathematical relationship between the electrical conductance and the user-level hydration is precomputed and stored in a lookup table. The corresponding mapping is done by the processor in the watch and is finally displayed on your smartwatch.
And that is how hydration level is measured on your fitness watch.
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