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Might sound like a Sciencesplainer myself but I looked a bit into the subject last year when I needed to buy one for a friend. You can find the TL;DR on Wikipedia[^].
Quote: HOWEVER, it take no account of the optical density of the person's skin, thickness of that skin, and their blood-count. We haven't even entered into scattering as each person's body's a bit different. In other words, it really cannot work unless it is specifically set up (standardized) for a particular user
Quoting from the same source: "By subtracting the minimum transmitted light from the transmitted light in each wavelength, the effects of other tissues are corrected for, generating a continuous signal for pulsatile arterial blood."
A coda triggered also by Griff's post: went on Amazon to look at the oxymeter I bought in 2018 for 35$.... it is now 90$... man, some people a making a killing from this killer pandemic!
Mircea
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Went to your Wikipedia article: It talked about normalization - and that brought a question to my mind: normalization is essentially putting all items on a fixed scale (for example, 0 to 1 is very common, by subtracting the minimum and then dividing the values by the maximum in the range).
Anyway - normalized with respect to what it has - which is (as I had noted in my OP) that you can use it to compare you to various states of yourself and not much more.
Further down the page, in the limitations section, this was born out.
Quote: The metabolism of oxygen can be readily measured by monitoring expired CO2, but saturation figures give no information about blood oxygen content. Most of the oxygen in the blood is carried by hemoglobin; in severe anemia, the blood contains less hemoglobin, which despite being saturated cannot carry as much oxygen.
So - it compares you to you. In actuality, since it does not have a zero value, it cannot even really normalize. How does it know how low you can go? Essentially, this is why none of the medical professionals I've seen use it for any purpose beyond pulse. You'd need a blood-count to know how much hemoglobin is potentially available.
As for the interesting concept of price: The reason I happened to have been looking at this before Griff's post is this item on NewEgg's email offers for the day.[^] Others are on the page, as well, more in line with you original $34 price point. I've seen what you're referring to: a $25 webcam offered for $100 - $200.
| Ravings en masse^ |
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| "The difference between genius and stupidity is that genius has its limits." - Albert Einstein | | "If you are searching for perfection in others, then you seek disappointment. If you seek perfection in yourself, then you will find failure." - Balboos HaGadol Mar 2010 |
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Some people are just criminals, robbing the gullible: go to FleaBay, search for "HDMI cable", and put the sort order to "most expensive first". Were you really expecting people to pay that much? 
"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony
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Geez!
All this science - I don’t understand.
This is just my job ( five days a week)!
If you can't laugh at yourself - ask me and I will do it for you.
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I use an SpO2 and SmO2 sensor in my training and the physics behind Near Infrared Spectroscopy (NIRS) Sensors is well understood. If you want to read about it check out Moxy's explanation. It's a bit brief but they had a virtual conference a couple of weeks ago where Roger at Moxy went deep into the details of it all. I'm still waiting for the on-demand videos to be posted but if you're at all interested I'll post them when they appear.
One interesting bit is they absolutely do take into account differences in skin, adipose layer etc and use ray tracing to model the path of the light to form a lookup table that can be used to interpret the light scattering. A simple and clever solution that also handles whether the skin or wet or dry (we get sweaty during tests!) while providing the ability to sample multiple times a second.
cheers
Chris Maunder
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I spent literally several years doing spectroscopy via transmitted light. Things you must absolutely have for absolute values: blanks. Blank are essentially unredacted samples of the materials with which you compare your experimental results. You use a sample of a known concentration to check it's absorbance (a value linear with concentration). You also have a sample, the 'blank' which has all of he background media (in which the samples were dissolved, for example) so you know their contribution to the spectrum. The result is that you can analyze the spectroscopic results independent of artifacts. It was common, in many instruments, to have duel beams, one with the sample and one with the medium: per-selecting out the background.
A good example: when I did infrared spectroscopy I had not only the absorption of the sample but also the contribution from the water in the air. One had a spectrum, in the same apparatus/container with no sample. This yielded, of course, the spectrum of the air. This could then be subtracted from the original with the result that the spectrum was now just that of the sample.
So - what do you get when you have a single beam instrument on a variable medium? You can make measurement and compare them to one another. You can then get relative values. You can (a lookup table) then have a mechanism to measure the state of that particular system. If you had concurrent actual blood work to analyze and label each reading you can even give them absolute values (which you do not have, otherwise). Without that, your real reading scale has as it's limits "exhausted" and "rested". That can be useful - but it all works only for a single user because it is a single system and a single lookup table. Using that table for someone else's finger won't work . . .
. . . they need their own table. Hence, that is why the MD's realized it's not useful for blood oxygen levels. They measure you once, when you come by. How accurate a reading can they get with a single value?
So - you can certainly use it to check yourself out. Your mental lookup table that maps it's readings to how you feel and what you've been doing can give you some information. It's a device that must be personalized (general case).
A single-beam one-shot measurement of a sample in an environment that is itself only characterized in the most general sense will only yield the most general (and of limited accuracy) data.
Fat? Thin? Black? White? Healthy? Anemic? Living at high altitude? How does it know - and if you could tell it yourself, how much of the preceding do you actually know?
| Ravings en masse^ |
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| "The difference between genius and stupidity is that genius has its limits." - Albert Einstein | | "If you are searching for perfection in others, then you seek disappointment. If you seek perfection in yourself, then you will find failure." - Balboos HaGadol Mar 2010 |
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W∴ Balboos, GHB wrote: I spent literally several years doing spectroscopy via transmitted light
Awesome. We need to meet up, get drunk, and you can talk me through it start to finish. That would be awesome.
With O2 sensors on a person it's always been remarked that it's the relative numbers that matter. Even shifting the sensor half a cm totally changes the readings during a workout, so good luck getting consistent absolute values between workouts.
For me it's the trends over time in a workout (is my SmO2 recovering each interval, have I hit my utilisation limit? HAve I opened up my capillaries enough to start the workout?) and for SpO2 I'd be assuming that the sensors will give you a good enough answer to "Am I oxygenated or should I call the hospital" that they are worth the time.
cheers
Chris Maunder
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Chris Maunder wrote: Awesome. We need to meet up, get drunk, and you can talk me through it start to finish. That would be awesome.
If that seems exciting, consider that in graduate school I separated Uranium isotopes using a pulsed CO2 laser in a tube 30 cm long - about 10x enrichment factor than the current state of the art.
There's also "induced surface ensembles on transition metal surfaces" - computer modeled (Monty-Carlo) and then proven experimentally.
Basically, perhaps you meant "very drunk".
I've never been a bar-drinking type of guy. More like opening a bottle of single malt or getting some six-packs of good IPA in a comfy spot in someone's home - even kitchen tables are good. Wine is too effective at putting me to sleep, but I'll admit that a five liter box can take you a long way into the evening.
But back to the subject at hand. I'm glad you brought in that it is a relative measurement - and selfishly, it's to confirm I wasn't just blowing smoke out my ass on the way spectroscopy works. Tools are good for what they can do and as long as you know their limits they're perfectly fine.
| Ravings en masse^ |
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| "The difference between genius and stupidity is that genius has its limits." - Albert Einstein | | "If you are searching for perfection in others, then you seek disappointment. If you seek perfection in yourself, then you will find failure." - Balboos HaGadol Mar 2010 |
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W∴ Balboos, GHB wrote: More like opening a bottle of single malt or getting some six-packs of good IPA in a comfy spot in someone's home - even kitchen tables are good
Me too, absolutely.
cheers
Chris Maunder
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Well done Paul - for your efforts you can have a go on my MV when ewe is better 
"We can't stop here - this is bat country" - Hunter S Thompson - RIP
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I will not hold you to that!
"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony
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You are welcome mush
"We can't stop here - this is bat country" - Hunter S Thompson - RIP
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OriginalGriff wrote: This is going to take a while.
Yup.
I didn't hear you complain about being confused anymore. Just take it slow and avoid people who have the common flu; you don't want that right now.
Bastard Programmer from Hell 
If you can't read my code, try converting it here[^]
"If you just follow the bacon Eddy, wherever it leads you, then you won't have to think about politics." -- Some Bell.
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I'm not confused anymore, provided I don't try to think too hard. That uses extra oxygen, so it's as bad - or worse - than physical exercise. Apparently, overall for the brain it's a total of 1 ~ 2% extra, which is trivial, but in local cortical regions it's 5 ~ 10%, and that's enough to tip me into deficit in places.
So heavy thinking, real concentration, is right out as it almost immediately becomes counter productive ... 
"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony
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As bad as it sounds, it's still a step up from last week. Hope your taste is at least improving a bit.
Lots of relaxation, lots of protein. Buy an extra steak. You have to recover, after all 
Bastard Programmer from Hell
If you can't read my code, try converting it here[^]
"If you just follow the bacon Eddy, wherever it leads you, then you won't have to think about politics." -- Some Bell.
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Taste is still weird, but improving - steak is a ways away: I need a bigger stomach first. Tonight's supper was 7 sous vide King Prawns in Marie rose sauce * with two small slices of Olive Bread, and even that was filling, possibly too much food. Tasty, but "Big". I'm working on it - Steak au poire, with triple-cooked chips (fries for our American cousins) and roasted vine tomatoes is looming big on my menu when I can eat a reasonable size! At the moment, I'd struggle loads with a quarterpounder ...
* I.e. a deconstructed Prawn Cocktail without the rabbit food that might be hard to swallow.
"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony
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OriginalGriff wrote: Steak au poire, with triple-cooked chips (fries for our American cousins)...
I understand chips vs fries but what's this triple cooked thing? Problems with leftovers?
I'm retired. There's a nap for that...
- Harvey
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No, no, no, no, no!
Triple cooked chips are incredibly wasteful: you might get four chips from single reasonably sized potato!
You cook them three times to basically dry them out, they start off big, fat, an inch thick, and end up eight or ten hours later half that, but with a golden outer that is like glass and an inner so light, flavour packed, and fluffy that they are a positive joy to eat.
Very, very brief instructions:
0) Use the right potato: Maris Piper is a good choice, but you definitely want a floury type, not waxy.
1) Peel, cut, and rinse the chips in cold water for a couple of minutes, tossing them as you go to ensure each surface is washed. Cloudy water indicates the chips are still starchy.
2) Simmer in boiling water until the surface starts to break up - a little further than you'd go for Roasties, but not so far they fall apart completely when you pick them up. Forom hear be really slow and careful when moving them.
3) Drain, separate onto a cooling rack, let cool completely.
4) Fridge, 5 hours min. This dehydrates the outside, when they come out, then will be a lot smaller.
5) Fry at 130C 4 minutes, gently moving then basket around every minute or so. Don;t crowd the basket - do them in batches if your deep fat fryer isn't big enough. Mine is: it's called Stephen, and holds 8L of cooking oil, or better Beef Dripping (but that's a PITA to clean out and store for next time).
6) Carefully back to the drying rack, and complete cooling again. Back in the fridge for at least two hours! They will not take on colour at this point.
7) When they come out, they should be a load smaller than when they started ...
8) Heat the deep fat fryer to 180C. Cook for the final time - up to 8 minutes, but keep an eye on them, you are looking for colour here, as well as "heated through".
9) Drain on kitchen paper (I use a a bowl lined with a double layer) and serve hot!
It's a lot of palaver, but the results are worth it! (For special occasions, it's just too much effort for "everyday". For that, cheat and buy McCain Gastro Chips[^] which you slam in the oven and turn over half way through. Not quite as good, but a load less effort and pretty spectacular results if nobody knows any better.)
Have a look here: Heston Blumenthal Triple Cooked Chips[^]
"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony
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Sounds like an expensive restaurant
Bastard Programmer from Hell
If you can't read my code, try converting it here[^]
"If you just follow the bacon Eddy, wherever it leads you, then you won't have to think about politics." -- Some Bell.
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It's less "skill" than "loads of time".
But it's worth the effort for special occasions like Herself's birthday and so on. The result really is spectacularly good, for a chip.
Like the difference between a "Chip Shop Chip"1 and "McVomits Fries"2.
Actually ... no, forget that comparison.
1: Or the "soggy bag of pus" as it's also known.
2: Not known to contain any potato or other recognisable ingredient.
"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony
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First: I'm really, really happy you're OK.
Second: please keep up the reports. This sort of first hand info is way more interesting / relevant / useful / informative than the stuff the news are blaring. I know you're only a data point but I'm living vicariously through your experience here.
You're still using gloves: is that to not frighten the natives, or because you can't be sure you're still not viral shedding, or because you can't be sure you won't get it (or something else, given your weakened state) again?
cheers
Chris Maunder
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Quote: “You're still using gloves: is that to not frighten the natives, or because you can't be sure you're still not viral shedding, or because you can't be sure you won't get it (or something else, given your weakened state) again?”
Nah it’s cos he lives in wales and it’s f***ing freezing.
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cheers
Chris Maunder
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Mask to protect them from me. Technically, officially, without-a-test-I-can't-be-surer I'm not infected, infectious, or in any way capable of giving this to others. But ...
Gloves for two reasons: to protect me from them, and to help maintain "zones".
Hot Zone: shops, everywhere other people are, everything they touch.
Warm Zone: inside the cabin of my car - not Hot, not Safe.
Safe Zone: home.
The idea is I transit from Safe to Warm, and need no additional protection - no mask, no gloves. Then I get to the shops, and everything is Hot - starting with the trolley handle (if only because the cleaning station is generally inside the store, and the trolleys are parked outside) and continuing with the produce. So Hot protocol applies: gloves, mask, touch nothing you don't have to, and then use only your right (gloved) hand. Use the cleaning station, (right hand again), and shopping list / contactless payment is left hand operation only to preserve it's "warm at best" status. Produce goes in trolley using right hand only, bags are touched only by left hand (since they came from Safe Zone). Contactless payment using phone in left hand.
Shopping goes in boot - all of it, it's Hot goods and the cabin is Warm.
Mask goes in boot, gloves are binned at shops.
Transit from Warm to Safe with Hot means handwashing, dried on a new set of teatowels that are then bagged and disinfected using this wash aid: Dettol fabric antibac[^]* more gloves and Isopropyl Alcohol (normally used for cleaning the 3D printer, but 99.99% IA kills everything very quickly, and leaves no residue on food packaging). Bags, mask can be recycled after 3 days if not separately decontaminated.
That was the regime before we got it, so I've continued it after just in case. All the research I've done - and I've had a lot of motivation to do this pretty well - says that you can't catch it twice, then "double infections" were probably bad data / dead virus / poor testing conditions. But ... this is a new virus, and it mutates, as they all do. The common cold is a disease caused by up to 200 different viruses: rhinovirus, coronavirus, influenza virus, adenoviruses orthopneumovirus, enteroviruses, human parainfluenza viruses, and human metapneumovirus and people catch that every damn year! If I can prevent a mutated version getting us, then when Herself returns to work, I can help prevent us infecting the residents with another version.
And despite this, we got it ... from Herself's work where insufficient PPE was provided despite changing from street clothes to work and back again, gloves, aprons, full face screen, ... and one breathable mask that had to be hand washed every day ... one tiny chink in the armour is all it needs, one trace that someone else left by touch on a door handle several hours ago ...
* Normally, I use this for Herself's uniforms when they have D&V in the home since they can't be boil-washed and we haven'y had that get us since she started working there over ten years ago. But it says on the bottle "Kills 99.99% of viruses and bacteria, laboratory tested on Influenza H1 N1; RSV; Coronavirus; Herpes Simplex Type 1" which doesn't mention Covid-19 but it probably helps: 15 minute soak kills the lot in theory, so 30 minutes soak, followed by as hot a wash as the teatowels can stand.
"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony
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