Peering inside a Pulse Oximeter

My longstanding streak of not being infected by COVID-19 (knowingly, at least…there’s always the asymptomatic possibility) came to an end earlier this year, alas, doubly-unfortunately timed to coincide with the July 4^th holiday weekend:

I’m guessing I caught one of the latest FLiRT variants, which are reportedly adept at evading vaccines (I’m fully boosted through the fall 2023 sequence). Thankfully, my discomfort was modest, at its worst lasting only a few days, and I was testing negative again within a week:

although several weeks later I still sometimes feel like I’ve got razor blades stuck in my throat.

One upside, for lack of a better word, to my health setback is that it finally prompted me to put into motion a longstanding plan to do a few pandemic-themed teardowns. Today’s victim, for example, is a pulse oximeter which I’d actually bought from an eBay seller (listed as a “FDA Finger tip Pulse Oximeter Blood Oxygen meter O2 SpO2 Heart Rate Monitor US”) a year prior to COVID-19’s surge, in late April 2019, for $11.49 as a sleep apnea monitoring aid. A year later, on the other hand…well, I’ll just quote from a writeup published by Yale Medicine in May 2020:

According to Consumer Reports, prices for pulse oximeters range from $25 to $100, if you can find one, as shortages have been reported.

This unit, a Volmate VOL60A, recently began acting wonky, sometimes not delivering definitive results at all and other times displaying data that I knew undershot reality. So, since prices have retracted to normalcy ($5 with free shipping, in this particular case, believe it or not), I’ve replaced it. Therein today’s dissection, which I’ll as-usual kick off with a series of box shots:

Let’s dive inside. The plastic tray houses our patient alongside a nifty protective case:

Underneath the tray is some literature:

The user manual is surprisingly (at least to me) quite info-thorough and informative, but I can’t find it online (the manufacturer seems to no longer be in business, judging from the “dead” website), so I’ve scanned and converted it to PDF. You can access it here. 

And there’s one more sliver of paper under the case (which also contains a lanyard):

Here’s the guest of honor, as usual alongside a 0.75″ (19.1 mm) diameter U.S. penny for size comparison purposes (the VOL60A has dimensions of 62 x 35 x 31 mm and weighs 60 g including batteries):

Before cracking the unit open, and speaking of batteries, I thought I’d pop a couple of AAAs in it so you can see it in action. Here’s the sequence-of-two powerup display cadence, initiated by a press of the grey button at the bottom:

Unless a finger is preinserted in the pulse oximeter prior to powerup, the display (and broader device) will go back to sleep after a couple of seconds. Conversely, with a finger already in place:

As you can see, it measures both oxygen saturation (SpO₂), displayed at the top, and pulse rate below. Good news: my actual oxygen saturation is not as low as the displayed 75%, which had it been true would have me in the hospital if not (shortly thereafter) the morgue. Bad news: my actual resting pulse rate is not as low as 28 bpm, which if true would mean I was very fit (not to mention at lower elevation than my usual 7,500’ residence location)…or conversely, I suppose, might also have me in the hospital if not (shortly thereafter) the morgue. Like I said, this unit is now acting wonky, sometimes (like this time) displaying data that I know undershoots reality.

Let’s next flip it over on its back:

The removable battery “door” is obvious. But what I want to focus in on are the labels, particularly the diminutive bright yellow one:

Here’s what it says:

LED Wavelengths

I showcase this label because it conveniently gives me an excuse to briefly detour for a quick tutorial on how pulse oximeters work. This particular unit is an example of the most common technique, known as transmissive pulse oximetry. In this approach, quoting Wikipedia:

One side of a thin part of the patient’s body, usually a fingertip or earlobe, is illuminated, and the photodetector is on the other side…other convenient sites include an infant’s foot or an unconscious patient’s cheek or tongue.

The “illumination” mentioned in the quote is dual frequency in nature, as the label suggests:

More from Wikipedia:

Absorption of light at these wavelengths differs significantly between blood loaded with oxygen and blood lacking oxygen. Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through. Deoxygenated hemoglobin allows more infrared light to pass through and absorbs more red light. The LEDs sequence through their cycle of one on, then the other, then both off about thirty times per second which allows the photodiode to respond to the red and infrared light separately and also adjust for the ambient light baseline.

Here’s what the dual-LED emitter structure looks like in action in the VOL60A; perhaps obviously, the IR transmitter isn’t visible to the naked eye (and my smartphone’s camera also unsurprisingly apparently has an IR filter ahead of the image sensor):

Note that in this design implementation, the LEDs are on the bottom half of the pulse oximeter, with their illumination shining upward through the fingertip and exiting via the fingernail to the photodetector above it. This is different than the conceptual image shown earlier from Wikipedia, which locates the LEDs at the top and the photodetector at the bottom (and ironically matches the locations shown in the conceptual image in the VOL60A user manual!).

Note, too, that the Wikipedia diagram shows a common photodetector for both LED transmitters. I’ll shortly show you the photodetector in this design, which I believe has an identical structure. That said, other conceptual diagrams, such as the one shown here:

have two photodetectors (called “sensors” in this case), one for each LED (IR and red).

In the interest of wordcount efficiency, I won’t dive deep into the background theory and implementation arithmetic that enable the pulse oximeter to ascertain both oxygen saturation and pulse rate. If you’d like to follow in my research footsteps, Google searches on terms and phrases such as pulse oximeter, pulse oximetry and pulse oximeter operation will likely prove fruitful. In addition to the earlier mentioned Wikipedia entry, two other resources I can also specifically recommend come from the University of Iowa and How Equipment Works.

What I will say a few more words about involves the inherent variability of a pulse oximeter’s results and the root causes of this inconsistency, as well as what might have gone awry with my particular unit. These root-cause variables include amount and density of both fat, muscle, skin and bone in the finger, any callouses or scarring of the fingertip, whether the user is unduly cold at the time of device operation, and the amount and composition of any fingernail polish. While, as Wikipedia notes:

Taking advantage of the pulsate flow of arterial blood, it [the pulse oximeter] measures the change in absorbance over the course of a cardiac cycle, allowing it to determine the absorbance due to arterial blood alone, excluding unchanging absorbance [due to the above variables].

Those sample-to-sample unchanging variables can still affect the baseline measurement assumptions, therefore the broader finger-to-finger, user-to-user, and test-to-test results.

And in my particular case, while I don’t think anything went wonky with the arithmetic done on the sensed data, the data itself is suspect in my mind. Note, for example, that oxygenated blood assessment is disproportionately reliant on successful passage of red visible spectrum light. If the red LED has gone dim for some reason, if its transmission frequency has wandered from its original 660 nm center point, and/or if the photosensor is no longer as sensitive to red light as it once was, the pulse oximeter would then deliver lower-than-accurate oxygen saturation results.

Tutorial over, let’s get back to tearing down. Here are left- and right-side views, both with the front and back halves of the device “closed”:

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

What I’m about to say might shock my fellow electrical engineers reading these words, but frankly one of the most intriguing aspects of this design (maybe the most) is mechanical in nature; the robust hinge-and-spring structure at the top, supporting both linear expansion and pivot rotation, that dynamically adapts to both finger insertion and removal and various finger dimensions while still firmly clinging to the finger during measurement cycles. You can see more of its capabilities in these top views; note, too, the flex cable interconnecting the two halves:

And, last but not least, here’s a bottom-end perspective of the device:

Accessing the backside battery compartment reveals two tempting screw candidates:

You know what comes next, right?

A couple of retaining tabs also still need to be “popped”:

And voila, our first disassembly step is complete:

As you’ll see, I’ve already begun to displace the slim PCB in the center from its surroundings:

Let’s next finish the job:

This closeup showcases the two transmission LEDs, one red and the other IR and with the cluster protected from the elements by a clear plastic rectangular structure, that shine through the back-half “window” shown in the previous shot and onto the user’s fingertip underside:

Chronologically jumping ahead briefly, here’s a post-teardown re-enactment of what it looks like temporarily back in place (and this time not illuminated):

And here’s another view of that flex PCB, which (perhaps obviously) routes both power and the LEDs’ output signals to (presumably) processing circuitry in the pulse oximeter’s front half:

Speaking of which, let’s try getting inside that front half next. In previous photos, you may have already noticed two holes at the top of the device, along with one toward the top on each side. They’re for, I believe, passive ventilation purposes, to remove heat generated by internal circuitry. But there are two more, this time with visible screw heads within them, potentially providing a pathway to the front-half insides:

Yep, you guessed it:

Again, the spudger comes through in helping complete the task:

The display dominates the landscape on this half of the PCB, along with the switch at bottom:

But I bet you already saw the two screws at the bottom, on either side of the switch, right?

With them removed, we can lift the PCB away from the chassis, exposing its back for inspection:

The large IC at the top (bottom of the PCB when installed) is the STMicroelectronics-supplied system “brains”. Specifically, it’s a STM32F100C8T6B Arm Cortex-M3-based microcontroller also containing 32 KBytes of integrated flash memory. And below it, in the center, is the three-lead photosensor, surrounded by translucent plastic seemingly for both protective and lens-focusing functions. In the previous photo, you’ll see the plastic “window” in the chassis that it normally mates with. And, in closing, here’s another after-the-fact re-assembly reenactment:

Note, too, the “felt” lining this upper-half time, presumably to preclude nail polish damage? Your thoughts on this or anything else in this piece are as-always welcome in the comments!

—Brian Dipert is the Editor-in-Chief of the Edge AI and Vision Alliance, and a Senior Analyst at BDTI and Editor-in-Chief of InsideDSP, the company’s online newsletter.

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• Teardown: Inside the art of pulse oximetry

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