More than a decade ago, I visited my local doctor’s office, suffering from either kidney stone or back-spasm pain (I don’t recall which; at the time, it could have been either, or both, for that matter). As usual, the assistant logged my height and weight on the hallway scale, then my blood pressure in the examination room. I recall her measuring the latter, then re-measuring it, then hurriedly leaving the room with a worried look on her face and an “I’ll be back in a minute” comment. Turns out, my systolic blood pressure reading was near 200; she and the doctor had been conferring on whether to rush me to the nearest hospital in an ambulance.

Fortunately, a painkiller dropped my blood pressure below the danger point (spikes are a common body response to transient acute pain) in a timely manner, but the situation more broadly revealed that my pain-free ongoing blood pressure was still at the stage 2 hypertension level. My response was three-fold:

• Dietary changes, specifically to reduce sodium intake (my cholesterol levels were fine)
• Medication, specifically ongoing daily losartan potassium
• And regular blood pressure measurement using at-home equipment

Traditional measurement techniques

Before continuing, here’s a quick definition of the two data points involved in blood pressure:

• Systolic blood pressure is the first (top/upper) number. It measures the pressure your blood is pushing against the walls of your arteries when the heart beats.
• Diastolic blood pressure is the second (bottom/lower) number. It measures the pressure your blood is pushing against your artery walls while the heart muscle rests between beats.

How is blood pressure traditionally measured at the doctor’s office or a hospital, specifically via a device called a sphygmomanometer in conjunction with a stethoscope? Thanks for asking:

Your doctor will typically use the following instruments in combination to measure your blood pressure:

• a cuff that can be inflated with air,
• a pressure meter (manometer) for measuring the air pressure inside the cuff, and
• a stethoscope for listening to the sound the blood makes as it flows through the brachial artery (the major artery found in your upper arm).

 To measure blood pressure, the cuff is placed around the bare and extended upper arm, and inflated until no blood can flow through the brachial artery. Then the air is slowly let out of the cuff. As soon as blood starts flowing into the arm, it can be heard as a pounding sound through the stethoscope. The sound is produced by the rushing of the blood and the vibration of the vessel walls. The systolic pressure can be read from the meter once the first sounds are heard. The diastolic blood pressure is read once the pounding sound stops.

Home monitoring devices

What about at home? Here, there’s no separate stethoscope—or another person trained in listening to it and discerning what’s heard, for that matter—involved. And no, there isn’t a microphone integrated in the cuff to listen to the brachial artery, coupled with digital signal processing to analyze the microphone outputs, either (admittedly, that was Mr. Engineer here’s initial theory, until a realization of the bill-of-materials cost involved to implement the concept compelled me to do research on alternative approaches). This Reddit thread, specifically the following post within it, was notably helpful:

Pressure transducer within the machine. The pressure transducer can feel the pressure within the cuff. The air pressure in the cuff is the same at the end of the line in the machine.

So, like a manual BP cuff, the computer pumps air into the cuff until it feels a pulse. The pressure transducer actually senses the change in cuff pressure as the heartbeat.

That pulse is only looked at a little, get a relative beats per minute from the cuff. Now that the cuff can sense the pulse, keep pumping air until the pulse stops being sensed. That’s systolic. Now slowly and gently release air until you feel the pulse again. Check it against the rate number you had earlier. If it’s close, keep releasing air until you lose the sense. The last pressure that you had the pulse is the diastolic.

 It grabs the two numbers very similarly to how you do it with your ears and a stethoscope. But, it is able to measure the pressure directly and look at the pressure many times per second, instead of your eyes and ears listening to the pulse and watching the gauge.

That’s where the specific algorithm inside the computer takes over. They’re all black magic as to exactly how they interpret pulse. Peaks from baseline, rise and fall, rising wave, falling wave, lots of ways to count pulses on a line. But all of them can give you a heart rate from just a blood pressure cuff.

Another Redditor explained the process a bit differently in that same thread, specifically in terms of exactly when the systolic value is ascertained:

OK, imagine your arm is a like a balloon and your heartbeat is a drummer inside. The cuff squeezes the balloon tight, no drumming gets out. As it slowly lets air out, the first quiet drumbeat you “hear” is your systolic. When the drumming gets too lazy to rattle the balloon, that’s your diastolic. The machine just listens for those drum‑beats via pressure wobbles in the cuff, no extra pulse sensor needed!

I came across a couple of nuances in a teardown of a different machine than the one we’ll be looking at today. First off, particularly note the following bolded-by-me emphasis phrase:

The system seems to be quite simple – a DC motor drives a pump (PUMP-924A) to inflate the cuff. The port to the cuff is actually a tee, with the other port heading towards a solenoid valve that is venting to atmosphere by default. When the unit starts, it does a bit of a leak-check which inflates the cuff to a small value (20mmHg) and sits there for a bit to also ensure that the user isn’t moving about, and detect if the cuff is too tight or too loose. From there, it seems to inflate at a controlled pressure rate, which requires running the motor at variable speed depending on the tightness of the cuff and the pressure in the cuff.

Note, too, the following functional deviation of the device showcased at “Dr. Gough’s Tech Zone” (by Dr. Gough Lui, with the most excellent tagline “Reversing the mindless enslavement of humans by technology”) from the previous definition I’d quoted, which had described measuring systolic and diastolic pressure on the cuff-deflation phase of the entire process:

As a system that measures on the inflation stroke, it’s quicker but I do have my hesitations about its accuracy.

Wrist cuff-monitoring pros and cons

When I decided to start regularly measuring my own blood pressure at home, I initially grabbed a wrist-located cuff-based monitor I’d had sitting around for a while, through multiple residence transitions (therefore explaining—versus frequent usage, which admittedly would have been a deception if I’d tried to convince you of it—the condition of the packaging), Samsung’s BW-325S (the republished version of the press release I found online includes a 2006 copyright date):

I quickly discovered, however, that its results’ consistency (when consecutive readings were taken experimentally only a few minutes apart, to clarify; day-to-day deviations would have been expected) was lacking. Some of this was likely due to imperfect arm-and-hand positioning on my part. And, since I was single at the time, I didn’t have a partner around to help me put it on; an upper-arm cuff-based device, conversely, left both hands free for placement purposes. That said, my research also suggests that upper-arm cuff-located devices are also inherently more reliable than wrist cuff alternatives (or alternative approaches that measure pulse rate via photoplethysmography, computer vision facial analysis, or other techniques, for that matter)

I’ve now transitioned to using an Omron BP786N upper-arm cuff device, which also includes Bluetooth connectivity for smartphone data-logging and -archiving purposes.

Dissecting the Samsung BW-325S

Having retired my wrist cuff device, I’ll be tearing it down today to satisfy my own curiosity (and hopefully at least some of yours’ as well). Afterwards, assuming I’m able to reassemble it in a fully functional condition, I’ll probably go ahead and donate it, in the spirit of “ballpark accuracy is better than nothing at all.” That said, I’ll include a note for the recipient suggesting periodic redundant checks with another device, whether at home, at a pharmacy or a medical clinic.

Opening and emptying the box reveals some literature:

along with our patient, initially housed within a rugged plastic case convenient for travel (and as usual, accompanied by a 0.75″ (19.1 mm) diameter U.S. penny for size comparison purposes).

Open Sesame:

I briefly popped in a couple of AAA batteries to show you what the display looks like near-fully digit-populated on measurement startup:

More generally, here are some perspectives of the device from various vantage points, and with the cuff both coiled and extended:

There are two screw heads visible on both the right side, whose sticker is also info-rich:

And the left, specifically inside the hard-to-access battery compartment (another admitted reason why I decided to retire the device):

You know what comes next, right?

Easy peasy:

Complete with a focus shift:

The inside of the top half of the case is comparatively unmemorable, unless you’re into the undersides of front-panel buttons:

That’s more like it:

Look closely (lower left corner, specifically) and you’ll see what looks like evidence that one of the screws that supposedly holds the PCB in place has been missing since the device left the factory:

Turns out, however, that this particular “hole” doesn’t go all the way through; it’s just a raised disc formed in the plastic, to fit inside the PCB hole (thereby holding the PCB in place, horizontally at least). Why, versus a proper hole and associated screw? I dunno (BOM cost reduction?). Nevertheless, let’s remove the other (more accurately: only) screw:

Now we can flip the assembly over:

And rotate it 90° to expose the innards to full view.

We want to pump you up

The pump, valve, and associated tubing are located underneath the PCB:

Directly below the battery compartment is another (white-color) hole, into which fits the pressure transducer attached to the PCB underside:

“Dr. Gough” notes in the teardown of his unit that “The pressure sensor appears to be a differential part with the other side facing inside the case for atmospheric pressure perhaps.”

Speaking of “the other side,” there’s an entire other side of the PCB that we haven’t seen yet. Doing so requires first carefully peeling the adhesive-attached display away:

Revealing, along with some passives, the main control/processing/display IC marked as follows:

86CX23
HL8890
076SATC22 [followed by an unrecognized company logo]

Its supplier, identity, and details remain (definitively, at least) unknown to me, unfortunately, despite plenty of online research (and for what it’s worth, others are baffled as well). Some distributor-published references indicate that the original developer is Sonix, but although that company is involved in semiconductors, its website suggests that it focuses exclusively on fabrication, packaging, and test technologies and equipment. Others have found this same chip in blood pressure monitoring devices from a Taiwan-based personal medical equipment company called Health & Life (referencing the HL in the product code), which makes me wonder if Samsung just relabeled and sold a blood pressure monitor originally designed and built by Health & Life (to wit, in retrospect, note the “Healthy Living” branding all over the device and its packaging), or if Samsung just bought up Health & Life’s excess IC inventory. Insights, readers?

The identity of the other IC in this photo (to the right of the 86CX23-HL) was thankfully easier to ascertain and matched my in-advance suspicion of its function. After cleaning away the glue with isopropyl alcohol and my fingernail, I faintly discerned the following three-line marking:

ATMEL716
24C08AN
C277 D

It’s an Atmel (now Microchip Technology) 24C08 8 Kbit I²C-compatible 2-wire serial EEPROM, presumably used to store logged user data in a nonvolatile fashion that survives system battery expiration, removal, and replacement steps.

All that’s left is to reverse my steps and put everything back together carefully. Reinsert a couple of batteries, press the front panel switch, and…

Huzzah! It lives to measure another person another day! Conceptually, at least …worry not, dear readers, that 180 millimeters of mercury (mmHg) systolic measurement is not accurate. Wrapping up at this point, I await your thoughts 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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