Pulse oximeter with a photodiode – Truth or false?

Pulse oximeter with a photodiode – Truth or false?

Written by Deepak Bhagat, In Technology, Published On
September 8, 2022

The sports equipment has experienced quite an overhaul in the last few years. For a long time, the watch worn around the wrist does not provide the most basic of parameters and can take a variety of extremely sophisticated measures. One of the primary measurements used in training to develop forms is the heart rate measurement.

For years the method for counting heartbeats has been to determine the intensity of electric impulses that are generated by the heart muscle when it contracts which is done using a strap that is worn around the chest. This method, although precise, it isn’t very practical. Therefore, equipment makers have met the needs of their customers and created a novel method to measure heart rate without wearing belts. You just have to wear a wristwatch with an optical sensor for heart rate to determine the heart rate. The system is, in addition to its simplicity, has a number of disadvantages that the user should be aware of if they are planning to utilize this system to ensure the accuracy of exercise.

What exactly is an optical heart rate monitor work?

Monitoring heart rate through your wrist relies on the idea that measures the quantity of light that is reflected by blood vessels. Green diodes lighten the skin, while the photodiode (usually situated within the center of the sensor) determines the amount of light that is reflected and the variation in that light signifies the rhythm in our heart. Therefore, it’s enough to determine the frequency of fluctuations in the quantity of light reflected to calculate the heartbeat frequency. Of course, any changes that occur in light reflected are very minor. Additionally, it is nice to know more how does a photodiode work.

Unfortunately, measurement using optical technology comes with one major drawback It is extremely difficult to alter it.

Optical heart rate measurement – how to improve the quality of the measurement?

The precision of the heart rate that is measured by the wrist is affected by a variety of elements. They range from the ones we are able to influence to those which unfortunately, we are unable to not control. One thing we are able to decide on is the position and quality of the clasp on our watch.

As much soft tissue is below the sensor the better as there are more veins to take measurements. For those with thin wrists, there’s less of this tissue therefore we will have lower precision in measurement. The watch must be strapped at a level so much that an optical device monitoring heart rate doesn’t sit on the bone on the wrist. If we move our hands in the upward direction (as while riding on an MTB bicycle) and the palm of our hand presses against the back of the watch’s protective envelope and the watch is not able to keep it in place, this indicates that the strap is too low.

Furthermore, when the watch is strapped too tight the blood won’t be allowed to flow freely through the veins. If you fasten it too loosely it can allow light to pass through the lens, which can alter the intensity of light absorbed by the photodiode and alter the signals. The equipment must be secured with enough sturdiness that it isn’t able to move around the wrist and that light isn’t able to penetrate the envelope. It should also be fastened at a moderately low level so that it doesn’t disrupt the circulation of blood within the tissues.

The last thing that could be accomplished at the perspective of the user is to eliminate hairs from where the sensor is positioned against the skin to improve the appearance of a watch fitted that has an optical sensor for heart rates and reveal the light produced by those backlighting LEDs.

The next step is to consider things that are beyond our control. One of these can be the level of pigment present in the skin or in the tattoos we’ve made. The skin’s pigment is more absorbent of light which means the amount of light reflected is reduced, which leads to a less precise measurement.

Also, we don’t take any steps to increase the accuracy of the measurement while performing activities that require us to place our hands over the line of our hearts. When you are pulling upwards on the bar, for instance, the hands are facing upwards which means that blood does not circulate as effectively because of gravity. The high frequency of vibrations when riding a bike or MTB bike could result in the optical heart rate gauge moving away from the wrist’s surface. The continuous bending of the wrist during riding a bicycle can cause the blood vessels to narrow and blood cannot flow freely.

Measurement precision

The wrist measurement is most effective when your heart rate doesn’t fluctuate rapidly. When you are running at the same speed on flat terrain the average heart rate is similar to that measured from your wristband (deviation of the maximum heart rate of 2 beats/minute). Troubles can arise when sudden fluctuations of heartbeats during interval training or during mountain running. The deviations in measurement can be significant, which results in errors in the mean heart rate.

Therefore, we suggest using wrist measurements in activities that require a pulse that doesn’t change rapidly like long runs or cycling for recreation instead of wearing an interval belt.

What is the reason we require wrist measurements?

Most importantly, it’s because of the ease of use. We don’t require any extra equipment to monitor our pulse. The belts that measure heart rate could result in skin abrasions and flare-ups. This is a personal problem – for some, the problem is present, while for others, they don’t. It is also difficult to imagine wearing this type of strap all day every day. A heart rate monitor that is optical is much more practical. It is not necessary to dress or not worry about anything, nothing is a burden on us, and nothing is snagged also. Furthermore, the measurement is done constantly, and on the base of the pulse, it is possible to estimate more precisely, such as the state of recovery of the body or the energy expenditure.

Summary of the optical pulse meter

The measurement of heart rate using optical technology is a great solution to measure heart rate around the clock. Without a belt, we can determine the resting heart rate is as well as what our pulse is when we are exercising at the same load. However, we should be aware that this isn’t the most accurate method to measure, the accuracy of which is affected by many variables, many of which are out of our control. It’s also not a matter of “weak” sensors, but of the measurement method itself, which can be easy to alter.

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