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Pulse Oximeter

Thread Status: Hello , There was no answer in this thread for more than 60 days.
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Mermaid, Musician and Marketer
Nov 12, 2002
I have acquired a fingertip Pulse Oximeter to review for freedivers use and wonder if anyone has some good ideas to test it out.

What kind of drop in O2 should I be expecting after a max static (Dry, its not a waterproof one!) ? It's taking a load of activity in apnea to see much of a drop so far.... even exhale statics don't show much of a drop...

anyone got experience of these things?

Hi Sam,
a fingertip sensor won't be that useful for freediving as it will tend to drop out due to vasocontriction when you start getting more hypoxic. Do a search because there is plenty on the topic on the DB forum. (I think Eric has tried every pulse oximeter known to man :) On my friends machine we found an earlobe sensor to work far better. Perhaps you could still atach it to the earlobe, or maybe make some mods. As for how long it takes to see a drop I have atached a graph of a static I did a few years back to give you an idea. It's not a Linear drop, as you can see in my case it doesn't even drop below resting until nearly 3min. In the first few minutes the O2 percent in the lungs starts dropping but blood O2 is still being replaced. This is up until a point where PaO2 becomes too low for efficient oxygen exchange, then blood O2 starts dropping also.



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I think even with the fingertip sensor there is at least one demo you can do to newbies. That is simply, put it on, and tell them "try to hold your breath untill it goes below 90".

Most of them will not be able to...But the point is, to show people who are afraid of brain damage etc, that you really are not dying of low oxygen even if you hold your breath for a minute or two.

Of course, it highly depends on the person how low it will actually drop, so test it first before demoing :)

From my experience, fingertip sensors pretty much "loose it", below 70-80% range. In water, even sooner than that...But on dry apnea and especially on newbies, you may get better readings because the vasoconstriction might not be that strong
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I haven't ever had a chance to try an oximeter. I would be interested to know just how much O2 I have left after a long static! The only way I can get a rough idea is to look at the colour of my hands. :D

Jome, that's a good demo. Many people seem to think that holding your breath for 30 seconds is 'depriving your brain of oxygen'.

Hearing about pulse oximeters more often here, I was curious about the principle, and how complicate or expensive they are. I was quite surprised how simple the measurement is! At least electrically, it is really trivial - practically just two LED's and a fotosensitive sensor. Interface to a PC (i.e. simple RS232 port), little bit of software for evaluating the data (it needs to read also the pulse to calculate the O2 level properly), and that's about all.

If you are also interested, you can find very nice explanation at Oximetry.org. Another one can be found in [ame="http://en.wikipedia.org/wiki/Oximeter"]Wikipedia[/ame] (together with other related info and links). In Wikipedia, they also mention "The latest generation pulse oximeters use digital signal processing to make accurate measurements in clinical conditions that were otherwise impossible. These include situations of patient motion, low perfusion, bright ambient light, and electrical interference." - I wonder if Eric tested some of them too, and if he got more reliable results. I guess they should be quite well suited for freediving/apnea. This system allows building reflective LED/sensor combos (unlike the classical oximeters that are translucent), so you can attach them practically anywhere - hence you could choose area not too influenced by the vasocontriction. It should be also much easier to build waterproof and pressure-resistant sensors with this method (though it is really not difficult with the classical one either).
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I like Simo's idea - great way of proving to my mum I am not killing my brain!

Naiad - will try and remember to bring it to the pool comp so you can try

Trux - this one retails around £100. It is very neat and small and gives heartrate and O2 levels. No download capability though, which would be nice.
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I was in hospital yesterday, an experience I feel cut up about, and had bp, pulse and O2 levels tested. It was a fingertip sensor and I was 99 or 100. I asked the nurse if she needed the equipment immediately but she didn't. Hooray! I therefore did a couple of statics (having warmed up a bit beforehand) and the second was 3:30, near my PB, and the oximeter read 91 at the end. After the static it didn't take long to pop back up to 100. Interestingly, during the easy phase, about 2 to 2:30 in total, it read 100, which makes sense, I suppose as I had no desire to breath.
Waterysmile said:
... After the static it didn't take long to pop back up to 100. Interestingly, during the easy phase, about 2 to 2:30 in total, it read 100, which makes sense, I suppose as I had no desire to breath.
The desire to breath is no way triggered by a low O2 level, but rather by high level of CO2. That's one of the reasons why hyperventilating is dangerous - the O2 level can drop below safe level without you feeling it at all, since the CO2 is still low.
Well, again - having high O2 level does not mean you did not hyperventilate. Hyperventilation has several effects that can lead to problems. Besides messing up the sensorics, it also influences the binding of O2 to blood (and its releasing in tissue), and the vasoconstriction of both small and big blood vessels (including carotids going to the brain). So in fact, the pulse oximetry can still show very high O2 level after hyperventilating, but your brain may be greatly starved of oxygen, because the O2 binding is too high and the oxygen cannot be released, and because the carotids do not let enough blood in to the brain.
I have friends who are paramedics and they say that a fingertip pulse oximeter have problem to give readings when your hands are cold. I did ask them if they could check my pulse and O2 and after 3 min the O2 still was 98% and my pulse had droped from 65 to 31 (they had connected me to the heartstart EKG macine :)).
If you read the post of Walrus above, you can see that he already warned about using fingertip oximeters for apnea - although not because of vasoconstriction due to the cold environment, but because of vasoconstriction due to the diving reflex that is a common and wanted effect at longer breath holds.
we did a few trials with the fingertip oximeter in between times at the Mavericks pool competition on Sunday. We found that the only way to see any significant real drop was to do exhale statics - where it dropped into the 80s after about a minute but no lower before everyone gave up.

I'm going to take it out to Hurghada and see if the UK team can find a good use for it/break it/make up some stories using it.... more here after that.
We did a few trials with the oximeter out in Hurghada - I got it down to 77% before giving up, a newbie gave up at 94% - anyone else have any figures on their "give up" point as far as SPO2 goes?

we also used it to see Herbert's heartrate drop to 34 bpm......
samdive said:
we also used it to see Herbert's heartrate drop to 34 bpm......

Was this with or without packing?

I've only tried an oximeter a couple of times. I think it went into the 50s, but I don't think my saturation was actually that low, more likely the vasoconstriction just fooled the sensor, and they are inaccurate anyways under 70
jome said:
Was this with or without packing?

I've only tried an oximeter a couple of times. I think it went into the 50s, but I don't think my saturation was actually that low, more likely the vasoconstriction just fooled the sensor, and they are inaccurate anyways under 70

The oximeters intended for medical use is set with sensors that works well to 60-70%.
But as Jome points out, due to the vasoconstriction, the measured sat in the fingers are "inaccurate".
I have seen 55% along with a heartbeat at 53, in a 5'10 STA, measured with fingerclip. Usually a sat in the 50% area is found during breatholds that last 7-8min.
The best and most accurate way to determinate the saturation is by blodgas analyzing.

But it's still intressting to use non invasive ways to measure the oxygen.
This is quite normal phenomenon. Pulse oximeters are not quite accurate in low oxygen levels. I just monitored one interesting pilot study where the test person made dry static packed and and without packing. Packed heart rates were something about 80/mins and without packs same person had heart rate bellow 40/mins. Interesting thing was to monitor this persons blood pressure which was for minutes 60/40 level while packed. These values vere monitored with the pulse oximeter as well as straight from inside of the wrist artery + transcutanic measuring.

- kimmo
Fingertip oximeters are not very well suitable for freedivers - they will show wildly incorrect values due to vasoconstriction once the diving response sets on (or due to vasodilatation after hyperventilation as recently discussed here, too)
Does anybody know whether there exists a kind of pulse oximeter that can be connected to a computer via USB? That would be nice!
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