A fitness watch can find how hydrated you are? Explained!
Hey friends, Happy Wednesday!
Answer to the question I posed last week
Last week, I asked about some cases where there can be inaccuracies in the measurement of the oxygenation level using a fitness watch. And how they can be rectified. You should be resting your hand for a few minutes before taking the reading for the blood oxygenation level to report a correct value. Also, you can take the reading multiple times over the course of a day to be sure about the value you get.
This week, we’ll look at how your fitness watch can tell if you’re hydrated or not. The following is a possible model and not the one that is used in every fitness watch out there but the general principle remains the same! If you'd prefer to read from my website, you can use the link attached towards the end of the newsletter.
I aim to write my newsletter issues in a way one can follow them while traveling on a bus, having a coffee, waiting for your food, etc. Let’s jump in!
Hydration level measurement in a fitness watch
Hydration tracking is performed non-invasively through perspiration (sweat for example) that is produced by the user.
A basic science recap before we proceed….
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.
Electrode: An electric conductor that carries current into the non-metallic part of a circuit like some liquids, gases, etc.
Underlying 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: 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. 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
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. Low level of hydration implies —> Higher concentration of electrolytes in perspiration —> Higher conductance. High level of hydration implies —> Low level of concentration of electrolytes in perspiration —> Lower conductance.
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.
Question of the week
For a change, there are not going to be any technical questions this week! 😄
What are your holiday plans for Christmas and New year? Would love to hear from you!
Gadget of the week
Color-changing car: The BMW iX Flow is a color-changing car that has a 12-V electronic coating that works similarly to a Kindle display and consumes very less energy to sustain. It uses a flexible material and the product has a lot more potential in the field of advertisements and informational displays. You could turn your black car white on a sunny day!
Blog posts
My blog post S1E11 on how hydration levels are measured by fitness watch is out! Read it here, happy learning!
Read how fitness watch can measure your hydration levels
I’m thinking of writing about how humidifiers and hearing aids work for next week’s newsletter issue (Not as a blog post). Also, let me know if you’d like to read about a specific gadget/device. Thank you for reading.
Have a nice rest of the week, and take care!
Until next Wednesday,
Chendur
