We see smartwatches and fitness trackers coming out with advanced health monitoring and fitness tracking features. But have you ever stopped to question how they do it? In this article, we’ve covered eight smartwatch sensors seen in the latest devices and how they work.
One of the most common smartwatch sensors is the pulse sensor. Ever wonder why a smartwatch or fitness tracker has a green light that flashes? Smartwatches and fitness trackers test heart rate by measuring blood circulation near your wrist and light it up with LEDs. The colour green was chosen because it is well absorbed by our red blood, allowing optical sensors to accurately gauge blood flow and heartbeat.
GPS (Global Positioning System)
GPS is old technology, but its use in fitness trackers is relatively recent because processors are becoming more powerful.
The global positioning system comprises a network of 29 satellites in orbit. At any given time, an individual should be within four satellites’ range to determine a precise location.
The GPS receiver receives a low-power, high-frequency radio signal from the satellites. With data from enough satellites, the time it takes for a signal to hit your smartwatch can be converted to your distance from the satellite, which can then be interpreted into precise coordinates. GPS chips are getting great at managing battery consumption, but GPS is still relatively power-hungry compared to other sensors.
GPS, as opposed to basic step counting, enables runners, hikers, and cyclists to quickly chart their workout and assess the environment where they are.
- Check out the best GPS smartwatches here.
A smartwatch can use two sensors to monitor blood pressure using Pulse Transit Time (PTT). The electrocardiogram (ECG) sensor comes first, followed by the optical Heart Rate (PPG) sensor. Your smartwatch measures Pulse Transit Time (the time taken for a pulse to pass from your heart to your wrist) using these two sensors. With elevated blood pressure, the pulse travels faster; when blood pressure is low, the pulse travels slower. Calibration with a validated blood pressure monitor is required to obtain an exact blood pressure value.
When the user puts their finger on the ECG sensor, the blood pressure calculation begins. A one-lead ECG signal is registered, and each R-peak big spike on the ECG indicates a heart contraction – this is roughly when a pulse exits the heart.
The pulse then moves down the arm until it reaches the PPG sensor on the underside of the device, resulting in a pulse wave in the PPG waveform. The time between the R-peak of the ECG and the pulse wave of the PPG waveform is measured as the pulse transit time, which is how long it takes for a pulse to pass from the heart to the rest of the body.
- Check out the best blood pressure watches for 2022 here.
Pulse oximeters, known as SpO2 sensors on fitness trackers, monitor blood oxygen levels or oxygen saturation in the blood. Finger-based pulse oximeters are gaining prominence on smartwatches. Either medical-grade or wearable oximeters need light to work.
A pulse oximeter usually has two LEDs with separate light wavelengths — one red and one infrared. This is due to the difference in light absorption between blood with high blood oxygen levels with low levels of oxygen. More infrared light is absorbed by oxygenated blood, less so by deoxygenated blood. This enables pulse oximeters to detect oxygen levels rapidly and non-invasively, as well as to measure how efficiently oxygen is delivered to the extremities.
The new Mi Band 6 is an excellent budget fitness tracker with SpO2 capabilities.
- Check out how we think it compares to the Mi Band 5 here.
Heart rate sensors are used in smartwatches like the Apple Watch 6 and 7, Samsung Galaxy Watch 4, and countless other trackers. They provide continuous updates on our heart rates, and we can deduce stress from this.
Other heart rate exercise measurement methods have been introduced in recent years by watch manufacturers, which can unlock new health and wellness insights. Heart rate variability is the one most related to the increase of stress monitoring.
HRV (heart rate variability) is a calculation of the time difference between heartbeats. Unlike measuring beats per minute, HRV readings concentrate on the small variations of the heart.
But what can trigger such variations? Age, body posture, time of day, and current health status are all factors. However, so are emotional, physical, and behavioural experiences.
It’s generally thought that having a high heart rate variability is a positive thing. Stress is often associated with a low HRV reading. Diabetes, heart disease, and high cholesterol are also medical conditions that are generally associated with people that have a reduced heart rate variability.
- The Fitbit Sense was the first from the brand to demonstrate a more holistic view of health and wellbeing; check it out here.
These smartwatch sensors are still relatively new. Fall detection is accomplished in three stages:
Sensors detect and record information
Accelerometers track the user’s movements. Modern smartwatches use three-axis accelerometers.
Detection of falls
By detecting unexpected changes in body movements, smartwatches can predict whether a person has slipped suddenly. The technology will assess an individual’s body posture, physical activity, and the smoothness of movement acceleration. If the system decides that these factors are in the danger zone and that a fall has happened, it will trigger an emergency fall warning and contact the user’s assigned SOS contact.
A smartwatch with fall detection will sense a fall when appropriately used and immediately contact emergency responders (if the device has wifi, LTE or is connected to a nearby smartphone). If the smartwatch can make calls, the emergency services will assess the situation using the device’s mic and speaker.
- This feature was made famous by the Apple Watch Series.
An ECG (electrocardiogram), tests the electrical function of the heart.
An electrical pulse is transmitted through the heart with each beat. It therefore contracts and pumps blood to the remainder of the body. An ECG measures this electrical wave to help assess the fitness of the user’s muscle.
This is accomplished by calculating the amount of electrical activity in the heart and the interval between heartbeats. This will assist in determining if the heart’s rhythm is regular, sluggish, rapid, or erratic. It can also detect when those areas of the heart are too big or overburdened.
Read more about Smartwatches that measure ECG.
ECG and an EKG – Is there a distinction between them?
Just one difference between ECG and EKG is how the abbreviation is spelt. ECG is an abbreviated form for electrocardiogram or electrocardiograph, both of which are English terms, while EKG is an abbreviation for elektrokardiogramm; the German spelling. Otherwise, the sensors are functionally identical.
Skin temperature monitoring on smartwatches such as the Fitbit Sense is identical to SpO2 in that it does not include a calculation on demand but rather indicates when you’ve deviated from the norm in a regular graph. This is accomplished by the use of a skin temperature sensor underneath the watch. Needless to say, it’s not going to take the place of your thermometer anytime soon. So the Sense can create a baseline, you’ll need to record sleep for a minimum of three nights.
We hope that’s helped you understand the powerhouse of technology that lies within that small computer that lives on your wrist!
While the Fitbit Sense was revolutionary in offering this feature, some budget fitness trackers also offer this advanced feature for less.
Ultimately, the more trackers, the better an idea you’ll have of your overall health and wellbeing. Just be mindful to check the battery life on these devices. The price you pay for a jam-packed piece of hardware isn’t just the financial cost but the time spent attached to a charger.
- Check out which smartwatches have the best battery life here.
Last Updated on December 31, 2021
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