Peering inside a Pulse Oximeter

My longstanding streak of not being contaminated by COVID-19 (knowingly, not less than…there’s all the time the asymptomatic risk) 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 unfavorable once more inside every week:

though a number of weeks later I nonetheless typically really feel like I’ve obtained 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. At present’s sufferer, for instance, is a pulse oximeter which I’d truly purchased from an eBay vendor (listed as a “FDA Finger tip Pulse Oximeter Blood Oxygen meter O2 SpO2 Coronary heart Price Monitor US”) a yr prior to COVID-19’s surge, in late April 2019, for $11.49 as a sleep apnea monitoring support. A yr later, alternatively…nicely, I’ll simply quote from a writeup printed by Yale Medication in Might 2020:

In accordance with Client Experiences, costs for pulse oximeters vary from $25 to $100, if you could find one, as shortages have been reported.

This unit, a Volmate VOL60A, just lately started appearing wonky, typically not delivering definitive outcomes in any respect and different occasions displaying knowledge 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 immediately’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 (not less than to me) fairly info-thorough and informative, however I can’t discover it on-line (the producer appears to not be in enterprise, judging from the “useless” web site), so I’ve scanned and transformed it to PDF. You’ll be able to entry it right here. 

And there’s yet one more sliver of paper beneath the case (which additionally incorporates a lanyard):

Right here’s the visitor of honor, as traditional 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 believed I’d pop a few AAAs in it so you may 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:

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

As you may see, it measures each oxygen saturation (SpO₂), displayed on the high, and pulse price beneath. 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. Dangerous information: my precise resting pulse price is just not 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 traditional 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 appearing wonky, typically (like this time) displaying knowledge that I do know undershoots actuality.

Let’s subsequent flip it over on its again:

The detachable battery “door” is clear. However what I wish to focus in on are the labels, notably the diminutive brilliant 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, referred to as transmissive pulse oximetry. On this strategy, 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 embody 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 answer the crimson and infrared mild individually and likewise regulate 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 digital camera additionally unsurprisingly apparently has an IR filter forward of the picture sensor):

Observe that on this design implementation, the LEDs are on the underside half of the heart beat oximeter, with their illumination shining upward by means of the fingertip and exiting through the fingernail to the photodetector above it. That is completely different than the conceptual picture proven earlier from Wikipedia, which locates the LEDs on the high and the photodetector on the backside (and sarcastically matches the places proven within the conceptual picture within the VOL60A consumer handbook!).

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

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

Within the curiosity of wordcount effectivity, I gained’t dive deep into the background principle and implementation arithmetic that allow the heart beat oximeter to establish each oxygen saturation and pulse price. Should you’d prefer to observe in my analysis footsteps, Google searches on phrases and phrases corresponding to pulse oximeter, pulse oximetry and pulse oximeter operation will doubtless show fruitful. Along with the earlier talked about Wikipedia entry, two different assets I also can particularly suggest come from the College of Iowa and How Tools Works.

What I will say a number of extra phrases about entails the inherent variability of a pulse oximeter’s outcomes and the foundation causes of this inconsistency, in addition to what might need gone awry with my specific unit. These root-cause variables embody 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 gadget operation, and the quantity and composition of any fingernail polish. Whereas, as Wikipedia notes:

Making the most of the pulsate circulation 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 attributable to 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, due to this fact 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 carried out on the sensed knowledge, the information itself is suspect in my thoughts. Observe, 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 purpose, if its transmission frequency has wandered from its unique 660 nm middle level, and/or if the photosensor is not as delicate to crimson mild because it as soon as was, the heart beat 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 gadget “closed”:

and “open”, i.e., expanded as could 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 vital intriguing facets of this design (possibly the most) is mechanical in nature; the sturdy hinge-and-spring construction on the high, supporting each linear enlargement and pivot rotation, that dynamically adapts to each finger insertion and elimination and numerous finger dimensions whereas nonetheless firmly clinging to the finger throughout measurement cycles. You’ll be able to see extra of its capabilities in these high views; observe, too, the flex cable interconnecting the 2 halves:

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

Accessing the bottom battery compartment reveals two tempting screw candidates:

You recognize what comes subsequent, proper?

A few retaining tabs additionally nonetheless have to 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 shielded from 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 briefly 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 indicators to (presumably) processing circuitry within the pulse oximeter’s entrance half:

Talking of which, let’s attempt getting inside that entrance half subsequent. In earlier images, you might have already seen two holes on the high of the gadget, 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, probably 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 are able to elevate the PCB away from the chassis, exposing its again for inspection:

The big IC on the high (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 beneath it, within the middle, is the three-lead photosensor, surrounded by translucent plastic seemingly for each protecting and lens-focusing features. Within the earlier picture, 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:

Observe, too, the “felt” lining this upper-half time, presumably to preclude nail polish injury? 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 e-newsletter.

 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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