Peering inside a Pulse Oximeter

My longstanding streak of not being contaminated by COVID-19 (knowingly, no less than…there’s at all times the asymptomatic risk) got here to an finish earlier this 12 months, alas, doubly-unfortunately timed to coincide with the July 4^th vacation weekend:

I’m guessing I caught one of the newest FLiRT variants, that are reportedly adept at evading vaccines (I’m absolutely boosted via the autumn 2023 sequence). Fortunately, my discomfort was modest, at its worst lasting just a few days, and I used to be testing damaging once more inside every week:

though a number of weeks later I nonetheless typically really feel like I’ve received 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 the moment’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 Charge Monitor US”) a 12 months prior to COVID-19’s surge, in late April 2019, for $11.49 as a sleep apnea monitoring support. A 12 months later, alternatively…nicely, I’ll simply quote from a writeup printed by Yale Drugs in Could 2020:

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

This unit, a Volmate VOL60A, lately started appearing 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 explicit case, imagine it or not), I’ve changed it. Therein as we speak’s dissection, which I’ll as-usual kick off with a sequence of field photographs:

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 (no less than 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 one more sliver of paper beneath the case (which additionally comprises a lanyard):

Right here’s the visitor of honor, as regular 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 high, and pulse charge 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 charge 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 regular 7,500’ residence location)…or conversely, I suppose, may 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 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 need 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 provides me an excuse to briefly detour for a fast tutorial on how pulse oximeters work. This explicit unit is an instance of the commonest method, often known as transmissive pulse oximetry. On this method, quoting Wikipedia:

One facet of a skinny a part of the affected person’s physique, normally a fingertip or earlobe, is illuminated, and the photodetector is on the opposite facet…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 gentle and permits extra crimson gentle to move via. Deoxygenated hemoglobin permits extra infrared gentle to move via and absorbs extra crimson gentle. The LEDs sequence via 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 gentle individually and in addition alter for the ambient gentle baseline.

Right here’s what the dual-LED emitter construction appears 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):

Observe that on this design implementation, the LEDs are on the underside half of the heart beat oximeter, with their illumination shining upward via 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 high and the photodetector on the backside (and sarcastically matches the areas 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 similar construction. That mentioned, different conceptual diagrams, comparable 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 gained’t dive deep into the background principle and implementation arithmetic that allow the heart beat oximeter to establish each oxygen saturation and pulse charge. In case you’d wish to comply with in my analysis footsteps, Google searches on phrases and phrases comparable to pulse oximeter, pulse oximetry and pulse oximeter operation will seemingly show fruitful. Along with the earlier talked about Wikipedia entry, two different assets I may also particularly suggest 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 explicit 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 machine operation, and the quantity and composition of any fingernail polish. Whereas, as Wikipedia notes:

Making the most of the pulsate move 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 on account 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 explicit case, whereas I don’t assume something went wonky with the arithmetic executed on the sensed information, 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 gentle. If the crimson LED has gone dim for some purpose, 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 gentle 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 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 may shock my fellow electrical engineers studying these phrases, however frankly one of the crucial intriguing points of this design (possibly the most) is mechanical in nature; the strong hinge-and-spring construction on the high, supporting each linear enlargement and pivot rotation, that dynamically adapts to each finger insertion and removing and numerous finger dimensions whereas nonetheless firmly clinging to the finger throughout measurement cycles. You possibly can 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 machine:

Accessing the bottom battery compartment reveals two tempting screw candidates:

You recognize 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 shielded from the weather by a transparent plastic rectangular construction, that shine via 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 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 attempt getting inside that entrance half subsequent. In earlier images, you could have already observed two holes on the high of the machine, together with one towards the highest on both sides. They’re for, I imagine, passive air flow functions, to take away warmth generated by inside 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 via 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 facet of the swap, proper?

With them eliminated, we are able to raise 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 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:

Observe, 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 newest programmable SoCs
• Teardown: Contained in the artwork of pulse oximetry

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