066
Daily Dose of Curious
HOW IT WORKS
It is also why a running reading wanders. Arm movement, a loose grip and dry skin all add noise to a signal of about a millivolt, so the steadiest readings come from walking, or from a chest strap the console can receive instead.
They pick up the tiny electrical signal of your heartbeat through your skin. With a hand on each side, the console can measure the voltage difference across your chest, find the regular spike each beat makes, and time the gaps between them.
What actually happens
Each hand is an electrode
The metal plates on the left and right grips each make electrical contact with one arm.
The difference is amplified
The heartbeat sets up a voltage of around a millivolt between the two arms. A differential amplifier boosts that difference and rejects what both hands share.
Each beat is picked out
Filtering removes mains hum and muscle noise, leaving the sharp spike that each heartbeat produces.
The gaps are timed
The time between spikes is converted to beats per minute and usually averaged over several beats before it is shown.
The clever bit
It needs both hands because it measures the difference between your arms, with your heart in between. One hand on one grip gives it nothing to compare, which is why the reading only appears once you are holding on to both.
The words
- Electrode
- A conductor that makes electrical contact with the skin.
- Differential amplifier
- An amplifier that boosts the difference between two inputs and ignores what they share.
Where this comes from
- Electrocardiography: Einthoven's Lead I, the potential difference between the arms
- Differential amplification and filtering of biopotential signals
This one is not an opinion. It is how the thing works, and these are what that rests on.
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