Peering inside a Pulse Oximeter

My longstanding streak of not being contaminated by COVID-19 (knowingly, at the least…there’s all the time the asymptomatic chance) got here to an finish earlier this yr, alas, doubly-unfortunately timed to coincide with the July 4^th vacation weekend:

I’m guessing I caught one of the most recent FLiRT variants, that are reportedly adept at evading vaccines (I’m absolutely boosted by means of the autumn 2023 sequence). Fortunately, my discomfort was modest, at its worst lasting only some days, and I used to be testing destructive once more inside every week:

though a number of weeks later I nonetheless typically really feel like I’ve bought razor blades caught in my throat.

One upside, for lack of a greater phrase, to my well being setback is that it lastly prompted me to place into movement a longstanding plan to do a number of pandemic-themed teardowns. Immediately’s sufferer, for instance, is a pulse oximeter which I’d really purchased from an eBay vendor (listed as a “FDA Finger tip Pulse Oximeter Blood Oxygen meter O2 SpO2 Coronary heart Fee Monitor US”) a yr prior to COVID-19’s surge, in late April 2019, for $11.49 as a sleep apnea monitoring assist. A yr later, then again…properly, I’ll simply quote from a writeup printed by Yale Drugs in Could 2020:

Based on Shopper Studies, costs for pulse oximeters vary from $25 to $100, if you will discover one, as shortages have been reported.

This unit, a Volmate VOL60A, just lately started performing wonky, typically not delivering definitive outcomes in any respect and different occasions displaying information that I knew undershot actuality. So, since costs have retracted to normalcy ($5 with free delivery, in this specific case, imagine it or not), I’ve changed it. Therein at this time’s dissection, which I’ll as-usual kick off with a collection of field pictures:

Let’s dive inside. The plastic tray homes our affected person alongside a nifty protecting case:

Beneath the tray is a few literature:

The consumer handbook is surprisingly (at the least to me) fairly info-thorough and informative, however I can’t discover it on-line (the producer appears to now not be in enterprise, judging from the “useless” web site), so I’ve scanned and transformed it to PDF. You possibly can entry it right here. 

And there’s yet another sliver of paper below the case (which additionally comprises a lanyard):

Right here’s the visitor of honor, as normal alongside a 0.75″ (19.1 mm) diameter U.S. penny for measurement comparability functions (the VOL60A has dimensions of 62 x 35 x 31 mm and weighs 60 g together with batteries):

Earlier than cracking the unit open, and talking of batteries, I assumed I’d pop a few AAAs in it so you’ll be able to see it in motion. Right here’s the sequence-of-two powerup show cadence, initiated by a press of the gray button on the backside:

Except a finger is preinserted within the pulse oximeter previous to powerup, the show (and broader machine) will return to sleep after a few seconds. Conversely, with a finger already in place:

As you’ll be able to see, it measures each oxygen saturation (SpO₂), displayed on the prime, and pulse price under. Excellent news: my precise oxygen saturation is not as little as the displayed 75%, which had it been true would have me within the hospital if not (shortly thereafter) the morgue. Unhealthy information: my precise resting pulse price shouldn’t be as little as 28 bpm, which if true would imply I used to be very match (to not point out at decrease elevation than my normal 7,500’ residence location)…or conversely, I suppose, would possibly additionally have me within the hospital if not (shortly thereafter) the morgue. Like I mentioned, this unit is now performing wonky, typically (like this time) displaying information that I do know undershoots actuality.

Let’s subsequent flip it over on its again:

The detachable battery “door” is apparent. However what I wish to focus in on are the labels, notably the diminutive vivid yellow one:

Right here’s what it says:

LED Wavelengths

I showcase this label as a result of it conveniently offers me an excuse to briefly detour for a fast tutorial on how pulse oximeters work. This specific unit is an instance of the commonest approach, generally known as transmissive pulse oximetry. On this method, quoting Wikipedia:

One aspect of a skinny a part of the affected person’s physique, often a fingertip or earlobe, is illuminated, and the photodetector is on the opposite aspect…different handy websites embrace an toddler’s foot or an unconscious affected person’s cheek or tongue.

The “illumination” talked about within the quote is twin frequency in nature, because the label suggests:

Extra from Wikipedia:

Absorption of sunshine at these wavelengths differs considerably between blood loaded with oxygen and blood missing oxygen. Oxygenated hemoglobin absorbs extra infrared mild and permits extra crimson mild to move by means of. Deoxygenated hemoglobin permits extra infrared mild to move by means of and absorbs extra crimson mild. The LEDs sequence by means of their cycle of 1 on, then the opposite, then each off about thirty occasions per second which permits the photodiode to reply to the crimson and infrared mild individually and likewise alter for the ambient mild baseline.

Right here’s what the dual-LED emitter construction appears to be like like in motion within the VOL60A; maybe clearly, the IR transmitter isn’t seen to the bare eye (and my smartphone’s digicam additionally unsurprisingly apparently has an IR filter forward of the picture sensor):

Word that on this design implementation, the LEDs are on the underside half of the heartbeat oximeter, with their illumination shining upward by means of the fingertip and exiting through the fingernail to the photodetector above it. That is totally different than the conceptual picture proven earlier from Wikipedia, which locates the LEDs on the prime and the photodetector on the backside (and satirically matches the areas proven within the conceptual picture within the VOL60A consumer handbook!).

Word, too, that the Wikipedia diagram reveals a typical photodetector for each LED transmitters. I’ll shortly present you the photodetector on this design, which I imagine has an similar construction. That mentioned, different conceptual diagrams, corresponding to the one proven right here:

have two photodetectors (referred to as “sensors” on this case), one for every LED (IR and crimson).

Within the curiosity of wordcount effectivity, I received’t dive deep into the background concept and implementation arithmetic that allow the heartbeat oximeter to establish each oxygen saturation and pulse price. If you happen to’d wish to observe in my analysis footsteps, Google searches on phrases and phrases corresponding to pulse oximeter, pulse oximetry and pulse oximeter operation will probably show fruitful. Along with the earlier talked about Wikipedia entry, two different sources I may also particularly advocate come from the College of Iowa and How Gear Works.

What I will say a number of extra phrases about includes the inherent variability of a pulse oximeter’s outcomes and the foundation causes of this inconsistency, in addition to what may need gone awry with my specific unit. These root-cause variables embrace quantity and density of each fats, muscle, pores and skin and bone within the finger, any callouses or scarring of the fingertip, whether or not the consumer is unduly chilly on the time of machine operation, and the quantity and composition of any fingernail polish. Whereas, as Wikipedia notes:

Making the most of the pulsate movement of arterial blood, it [the pulse oximeter] measures the change in absorbance over the course of a cardiac cycle, permitting it to find out the absorbance because of arterial blood alone, excluding unchanging absorbance [due to the above variables].

These sample-to-sample unchanging variables can nonetheless have an effect on the baseline measurement assumptions, subsequently the broader finger-to-finger, user-to-user, and test-to-test outcomes.

And in my specific case, whereas I don’t assume something went wonky with the arithmetic completed on the sensed information, the info itself is suspect in my thoughts. Word, for instance, that oxygenated blood evaluation is disproportionately reliant on profitable passage of crimson seen spectrum mild. If the crimson LED has gone dim for some motive, if its transmission frequency has wandered from its authentic 660 nm middle level, and/or if the photosensor is now not as delicate to crimson mild because it as soon as was, the heartbeat oximeter would then ship lower-than-accurate oxygen saturation outcomes.

Tutorial over, let’s get again to ripping down. Listed below are left- and right-side views, each with the back and front halves of the machine “closed”:

and “open”, i.e., expanded as can be the case when the finger is inserted in-between them:

What I’m about to say would possibly shock my fellow electrical engineers studying these phrases, however frankly one of the crucial intriguing elements of this design (perhaps the most) is mechanical in nature; the sturdy hinge-and-spring construction on the prime, supporting each linear enlargement and pivot rotation, that dynamically adapts to each finger insertion and elimination and varied finger dimensions whereas nonetheless firmly clinging to the finger throughout measurement cycles. You possibly can see extra of its capabilities in these prime views; be aware, too, the flex cable interconnecting the 2 halves:

And, final however not least, right here’s a bottom-end perspective of the machine:

Accessing the bottom battery compartment reveals two tempting screw candidates:

You realize what comes subsequent, proper?

A few retaining tabs additionally nonetheless should be “popped”:

And voila, our first disassembly step is full:

As you’ll see, I’ve already begun to displace the slim PCB within the middle from its environment:

Let’s subsequent end the job:

This closeup showcases the 2 transmission LEDs, one crimson and the opposite IR and with the cluster protected against the weather by a transparent plastic rectangular construction, that shine by means of the back-half “window” proven within the earlier shot and onto the consumer’s fingertip underside:

Chronologically leaping forward briefly, right here’s a post-teardown re-enactment of what it appears to be like like quickly again in place (and this time not illuminated):

And right here’s one other view of that flex PCB, which (maybe clearly) routes each energy and the LEDs’ output alerts to (presumably) processing circuitry within the pulse oximeter’s entrance half:

Talking of which, let’s strive getting inside that entrance half subsequent. In earlier pictures, you will have already seen two holes on the prime of the machine, together with one towards the highest on either side. They’re for, I imagine, passive air flow functions, to take away warmth generated by inner circuitry. However there are two extra, this time with seen screw heads inside them, doubtlessly offering a pathway to the front-half insides:

Yep, you guessed it:

Once more, the spudger comes by means of in serving to full the duty:

The show dominates the panorama on this half of the PCB, together with the swap at backside:

However I guess you already noticed the 2 screws on the backside, on both aspect of the swap, proper?

With them eliminated, we will raise the PCB away from the chassis, exposing its again for inspection:

The massive IC on the prime (backside of the PCB when put in) is the STMicroelectronics-supplied system “brains”. Particularly, it’s a STM32F100C8T6B Arm Cortex-M3-based microcontroller additionally containing 32 KBytes of built-in flash reminiscence. And under it, within the middle, is the three-lead photosensor, surrounded by translucent plastic seemingly for each protecting and lens-focusing features. Within the earlier photograph, you’ll see the plastic “window” within the chassis that it usually mates with. And, in closing, right here’s one other after-the-fact re-assembly reenactment:

Word, too, the “felt” lining this upper-half time, presumably to preclude nail polish harm? Your ideas on this or anything on this piece are as-always welcome within the feedback!

—Brian Dipert is the Editor-in-Chief of the Edge AI and Imaginative and prescient Alliance, and a Senior Analyst at BDTI and Editor-in-Chief of InsideDSP, the corporate’s on-line publication.

 Associated Content material

• Studying and dealing within the period of COVID-19
• Easy pulse oximetry for wearable monitor
• Pulse oximetry fundamentals and MCUs
• Sign processing and calibration enhance blood measurements
• Pulse oximetry advantages from the most recent programmable SoCs
• Teardown: Contained in the artwork of pulse oximetry

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