[PSUBS-MAILIST] Optical Oxygen Sensors

[Sean T. Stevenson via Personal_Submersibles]

A true calibration should encompass the range limits of the sensor, because interpolation between calibration points can still be considered valid data, whereas when extrapolating beyond them, you can't accurately characterize the error. With a 0%-100% sensor, this is easy, as you can calibrate at 0 (inert gas), 20.9 (atmospheric air) and 100% (oxygen). To calibrate a 0%-25% sensor, you either need to source a highly accurate reference standard gas at 25%, or calibrate at 0 and 20.9 only and extrapolate beyond, which would not, strictly speaking, be a traceable / reportable value.

Sean

-------- Original Message --------
On Aug. 19, 2019, 07:14, Jon Wallace via Personal_Submersibles wrote:

> Hi Alan,
>
> I still think the major advantages of the optical sensor is the digital output and it's expected life of 5 years.  But I have a hard time getting past the 25% scale because there's a lot of "head room" between 25-100% and you just don't know where you are in that range.  My concern is not from a biological perspective since short term high levels of O2 are not an issue at 1-ATM, but from an environment one...high levels of 02 start representing a real fire danger.
>
> Calibration in free air is easiest and why handheld (mobile) sensors are convenient.  I'm not sure I would trust just opening the hatch to get fresh air in the vessel, unless you forced fresh air inside with a fan or something.  Getting into an open air environment would be best.
>
> Jon
>
> On Monday, August 19, 2019, 05:58:02 AM EDT, Alan via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>
> Jon,
> am having second thoughts about the optical O2 sensor.
> I was given data that said calibration was needed after a while because of a drift
> caused by temperature fluctuations.
> Although calibration would only need to be done every year or two, I would
> need to write a program for that. And I would need to write a program for
> calibration of galvanic O2 sensors if I went with them. So although the optical
> sensors are factory calibrated, this is not going to be that much of an advantage
> to me. They would also be more difficult to source than the standard sensor.
> The optical sensors only have the 0-25% O2 range but I can't imagine going over
> 25%. The optical sensor has better accuracy, as all the information I have seen
> on the galvanic sensors say they have an accuracy of +/- 2% on full scale.
> I wonder what procedures Psubbers take with regards to checking calibration
> of their O2 sensors. I am thinking a best practice would be to leave the hatch
> open before a dive & check that the O2 reading corresponds to the standard
> level of O2 in air.
> Cheers Alan
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