If you know which wearable gadget you need, then thanks to our catalog with selection by parameters, you can, for example, filter the current assortment by the presence of certain sensors. We also recommend reading the material "How to choose smart watch and fitness bracelet: basic criteria and useful tips".

To begin with, it is worth determining which sensors are actually found in modern wearable devices. Everything is very individual here, because individual manufacturers love experiments (or marketing - promoting already known functions, like know-how). Therefore, it is worth listing the most popular and encountered:

  • pedometer;
  • heart rate monitor;
  • pulse oximeter ;
  • GPS;
  • thermometer;
  • tonometer ;
  • ECG sensor;
  • compass;
  • altimeter;
  • barometer.

About how most of them work, as well as how watches and trackers "give out" other indicators of our body - sleep quality, stress level, calorie consumption, etc. - we'll talk in more detail.

Many sensors are hidden inside the case, but some of them are clearly visible.

How do smart watches and trackers count steps?

Mandatory for almost any wearable smart gadget today is pedometer. It is even in children's models, like Garmin Vivofit JR 3. Accelerometer is responsible for counting steps, which determines the acceleration of the gadget and its movement in space. Previously, these were devices with weight and spring, but today they are miniature chips with set of conductors.

It is easier to understand how the sensor works on mechanical device, although the general principles remain the same for modern electronics. So, during acceleration, that is, the beginning of the movement (whether it’s wave of the hand when walking or turning the body), the spring is compressed or stretched under the influence of counterweight. And if you take three such devices, you can evaluate movements in height, length and width. The accelerometer is now found in many mobile devices, including smartphones and tablets, where it is primarily responsible for the orientation of the image on the screen.

Often, gyroscope is also present in wearable gadgets to improve the accuracy of counting steps. This sensor takes into account the spatial movements of the device (in three axes), including various rotations of the hand. Combination of an accelerometer and gyroscope is able to minimize false positives, such as when person actively gestures or stirs porridge in pot. These two sensors can, for example, detect fall of person and send an SOS signal.

Step counting is one of the basic functions of smart watch.

Since the watch is worn on the wrist, it is difficult to achieve 100% accurate count of the steps taken. Extra sensors can be added while driving car (including on rough roads - when shaking), while shaving or taking shower. They can also miss something - when walking very slowly or walking with a baby carriage, when the hand remains practically motionless.

Popular smart watches

How do smart watches and trackers measure heart rate?

Heart rate monitor using photoplethysmography technology helps to measure heart rate in wrist gadgets. It is also called an optical sensor, since its operation is different from classic heart rate monitors.

So, let's say a chest heart rate monitor has electrodes that read impulses with each contraction of the heart muscle. The received data is then transferred to compatible device (for example, smartphone).

Optical sensor functions differently: it gives light to the skin of the wrist, which passes through the tissue (to the vessels), and then the photocell captures the reflected flow. Fact is that with each contraction of the heart, blood flow is saturated, in other words, the vessels become more filled and red. Optical sensors use green diodes, which are as contrasting as possible with respect to blood, which means they are perfectly absorbed. Each change in the level of light reflection from the vessels is recorded by photocell, which allows you to determine the heart rate.

When person is at rest, the pulse sensor on the wrist works with minimal errors. When walking and various physical activities, inaccuracies increase. But in general, for everyday use and amateur sports (with the exception of high-intensity loads), the optical sensor data is quite enough. Poor fixation of the watch or bracelet on the arm, sunburn, tattoos, dirt on the skin - all these factors can cause flaws in the calculations.

Calculation of heart rate provides an optical heart rate monitor.

So, the green blinking LEDs on the back of the watch or bracelet are the work of photoplethysmography, that is, the optical sensor. By the way, if earlier heart rate was measured non-stop by wearable gadgets only during training, today most models provide round-the-clock heart rate monitoring by default (with the ability to be turned off by the user).

Smart watches and fitness trackers with heart rate monitor

How do smart watches and trackers determine saturation?

Pulse oximeter, also known as an SpO2 sensor, is responsible for determining the level of blood oxygen saturation in wearable gadgets. Principle of its operation is very similar to an optical heart rate monitor. There is also light emitter and photodetector that captures the reflected flux. Only in this case it is not green, but red.

By the way, red diodes are used in professional medical equipment for measuring heart rate, but they are as accurate as possible only at rest. That's why the watch uses green elements to measure the pulse, and when determining the level of oxygen saturation in the blood, the user usually requires to be in stationary state. The light reflected from the artery enters the photodetector, providing the necessary data for analysis. At the same time, the saturation indicator itself as percentage is calculated by the software (that is, the algorithm is slightly more complicated than just with pulse).

In general, finger pulse oximeters have similar principle of operation, which allows owners of smart watches and trackers to rely on acceptable accuracy in the analysis of blood oxygen saturation. But errors are still not excluded, and wearable gadgets are not medical devices. Result of measuring SpO2 can be distorted up or down by weak fixation of the strap on the wrist or cold (the extremities are usually the first to freeze).

After the coronavirus pandemic, the watch got pulse oximeter.

Red flashing LED on the back of the watch is the pulse oximetry function. It can be performed on demand or in continuous monitoring mode. In the latter case, the watch starts the measurement process in accordance with its algorithms, for example, during sleep. This feature expands the overall range of analyzed data about the user's activity and health.

How smart watches and trackers determine the location?

For navigation capabilities, both your own GPS module and the corresponding smartphone receiver can be used. In the latter case, accurate information about the route and distance of the race or race can only be counted on when the mobile phone was also taken to training. Most smart watches and top-end trackers have their own GPS.

In fact, this is not aensor, but receiver of signals from satellites. It is on this that the work of the global positioning system is based. To determine the exact coordinates of the user (his watch or tracker), you need to get information from at least four satellites. But the more of them (usually the maximum is 12), the higher the accuracy of determining the coordinates.

Navigation options may not be limited to geolocation, but include access to alternative satellite systems, track guidance, or direct display of maps on the watch screen. And the module can be supplemented with the aGPS function, which speeds up the search for satellites.

GPS in watches works the same way as in other modern gadgets.

Using GPS improves the accuracy of your training data, including distance and speed, altitude, and more. At the same time, errors directly in the geopositioning itself can reach several tens of meters. But for sports purposes, this is not big problem.

Separately about the accuracy of the main sensors

Above four sensors can be considered the main set for modern smart watches. Accuracy of their work directly depends not only on the manufacturer (and his reputation), but on the actions of the owner. If you set yourself the goal of "deceiving" the gadget, then it is not so difficult to do it.

Most often, to improve the accuracy of the results of health and activity indicators, it is recommended to properly fix the device on the arm (tightly, but so that the strap does not squeeze the skin), wash the wrist regularly, and wipe the body of the gadget. Since pulse and saturation are measured by optical sensors, it must be understood that air temperature directly affects blood circulation, so heat or cold can make their own undesirable corrections.

In general, fitness tracker and watch is not medical diagnostic device. You can use them for general health monitoring and the training process, but they will not replace doctors for you.

GPS smart watches and fitness bands

What other sensors are used in watches and trackers and how do they work?

In modern wearable gadgets, there are other sensors:

  • Tonometer. To measure blood pressure, either separate sensor or an optical heart rate monitor is used. Vascular saturation data is read and then the software interprets it. I must say that the final readings are not very accurate, against the background of conventional tonometer. Firstly, measurements on the wrist are more difficult: the arteries are close to the skin. Secondly, adjustments are required for the individual characteristics of the user, for example, age. This is achieved through pre-calibrations with simultaneous measurements on watches and professional equipment. Without this, deviations from real numbers will be very large.
  • ECG sensor. This is set of electrodes that are designed to measure the electrical activity of the heart. In full-fledged medical equipment, there are up to 10 of them. In smart watches, there are no more than two. For example, on the Apple Watch, one is next to the heart rate monitor and the other is on the Digital Crown. Same is true for Samsung smart watches. Result is not full-fledged electrocardiogram, but only data collection in one lead. In theory, such diagnosis helps to identify potential problems, but in practice there can be many false results and erroneous visits to the doctor.
ECG data from smartwatches is not very accurate. At least for now.
  • Thermometer. This is temperature sensor, it determines the corresponding indicators on the surface of the skin in the wrist area. Slight increase here is unlikely to be detected, but strong heat is quite. Also, similar sensor can be located away from the skin - closer to the dial or bezel. Then it serves to measure the ambient temperature. It can be useful, for example, on hike.
  • Geomagnetic sensor (and if it's simpler - compass). It is predominantly digital chip that tells the direction of the world. Useful in tactical and field conditions when there is no GPS or bad weather makes it difficult to find satellites.
  • Altimeter and barometer. First sensor measures the height or difference between several points. Second allows you to determine the atmospheric pressure. Barometer in tandem with an altimeter improves the accuracy of altitude measurements. It can also be used to evaluate weather indicators.
  • Light sensor. Typically used to automatically adjust the brightness of the watch face.
An altimeter and barometer pairing will provide accurate altitude data, which is important when running and not only.

Quite rare sensors include BIA - bioelectrical impedance analysis. It is designed to determine the percentage of fat, the amount of muscle, bone mass, etc. on the basis of an assessment of tissue resistance. Something similar is performed, for example, diagnostic floor scales. Again, there are at least 4 sensors, and there is only one in the clock. Overall measurement accuracy is questionable even for scales, to say nothing about wearable devices.

Introduction of glucometer into smart watches and trackers looks very promising. Such independent devices are able to determine the level of sugar and are very useful for diabetics. Main catch is that they require blood sampling. In the meantime, periodically there is talk about non-invasive measurements in hours. Something similar was even announced at the release of Huawei Watch 4 Pro, but in reality it turned out to be just marketing ploy.

Smart watches and fitness trackers with ECG

But how is stress or calorie consumption calculated, sleep analyzed?

Smart watches or fitness trackers provide lot of data not directly from sensors, but based on the interpretation of their indicators or various calculations. You can treat them differently.

For example, counting calories is very conditional moment, which depends not only on physical activity. The current physical condition, age, gender, weight - all this directly affects how much the body spends energy to maintain life. Therefore, the corresponding data from the watch is miscalculation based on mathematical models embedded in the software. Most indicator shown on the watch screen is very conditional. Error with reality can be more than 50%. But in any case, even in terms of approximate calorie costs, you can compare two of your activities, including in order to conclude where you spent more effort.

Sleep monitoring is usually based on the performance of two sensors - an accelerometer and heart rate monitor. Pulse oximeter can also be used here (if the corresponding option is active not only on request). In the deep sleep phase, the pulse slows down, there are no body movements, and in fast sleep, the heart rate accelerates, the respiratory rate is higher. In general, nighttime analytics works quite well, especially if person does not have serious health problems. For general idea of how well you sleep and recover, there is enough information. More detailed conclusions should be made by somnologists.

Sleep data on smartwatch can be quite useful.

It is also worth dwelling on two more interesting indicators. One concerns overall health – stress levels. Other is physical fitness - VO2max or maximum oxygen uptake.

To find out the level of stress, different manufacturers of wearable gadgets offer their own calculation methods. Similar indicator makes it possible to monitor today Samsung, Huawei, Honor, Garmin, Xiaomi, Fitbit. Main data for its calculation is given by the heart rate monitor. It provides the heart rate itself, as well as its variability, that is, the difference in time between contractions. Respiratory rate is also usually used (by the way, it is also obtained from the pulse sensor - after all, when inhaling, the heart rate rises slightly, and when exhaled, it decreases). Further, their algorithms from different manufacturers can “work”. Final results are easy to compare by day, time of day, etc.: the higher the score, the more stressful your condition is.

VO2max value is also calculated indicator that reflects the current level of physical activity (the higher the number, the better). As rule, data is taken not at rest, but during cardio training. So, for example, when jogging, heart rate and running pace are compared.

Athletes need to keep track of their VO2max.

Most wearable manufacturers, including Garmin, Suunto, Polar, Apple, and Huawei, use Firstbeat's calculation methodology. And she performed well. Without errors, the maximum oxygen consumption can only be determined in the laboratory - using spiroergometry. This is special exercise test that measures ventilation and lung capacity. But Firstbeat VO2max calculations usually give minimal deviations from spiroergometry.

Each brand can refine the analysis systems, offer its own options for displaying data, which are clearer and more convenient for review. In this regard, when choosing smart watch, you need to look not only at the abundance of sensors, but also at how high the reputation of the manufacturer is, what reviews its mobile application has. After all, it is the software that interprets data about health and sports activities.

Popular fitness trackers

As conclusion

Yes, smartwatches and fitness trackers are not precision medical devices. But they can really be useful to many categories of users. For example, for hundreds of thousands of people, regular pedometer is great motivation to maintain certain physical activity, which is already lot.

Smart watches, although not medical, but popular gadget.

For amateur athletes, the combination of built-in sensors and high-quality software provides lot of useful information necessary for progress in the training process. And here the achievements of brands such as Garmin or Polar, which attract professional athletes and coaches to cooperate, should definitely not be underestimated. Yes, data reading is possible with certain error, but proprietary software is capable of analyzing and interpreting data at high level. For lovers of running, cycling or other sports activities, this is quite enough.

What can be said about various medical indicators from smart watches? Of course, they cannot be considered the ultimate truth. But, there are many cases that prove that even this is sometimes enough to save life. Pulse disorders, decrease in the level of oxygen in the blood, information about poor sleep quality or high stress - this is not reason to diagnose yourself, but to think about whether everything is in order. And the most correct solution for problematic indicators, of course, will be to see doctor.

And one moment. Competition in the market of smart watches and fitness trackers is very high. This means that manufacturers are interested in improving the quality and quantity of sensors, improving their software, and innovating. And users will only benefit from this race of brands.