[PSUBS-MAILIST] Optical Oxygen Sensors

Mon Aug 19 09:59:39 EDT 2019

 Alan, I am with Jon on the span for the O2 sensor.  Granted under NOP a 0-25% span for O2 percent is fine but 0-100% span is handy to detect if you have HP O2 bleed into the cabin.  On one dive weekend a few years ago,   when I got ready to dive the next day I powered up the boat and a high O2 alarm sounded in the boat.  After checking I found that I had left the HP O2 blocking valve on my O2 tank open over night with the hatch secured.  With my system it is possible to have a very low flow bleed into the cabin. The cabin atmosphere had risen to 27% O2.  I got out my handy leaf blower and aired out the cabin and proceeded to dive.
I use the Max-250 | Maxtec

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Max-250 | Maxtec

Replacement oxygen sensor for Maxtec OM-25, Fluke VT Mobile,
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with a custom PCB that converts this 10-15mV signal into a 0-5V signal I use for analog input on my PLC.  I find you have to replace this element every 3-4 years.  It comes factory calibrated.  For field calibration, I added some ladder logic to PLC that lets me add a bias error correction to sensor by comparing what the sensor is measuring to the air it is measuring in the cabin when the hatch is open.  I find this works quite well.  Normal air has 20.95% O2, if I look at my life support display on my boat and find it much off from this, I hit the calibration button and difference between what it is reading and 20.95% is stored as a bias error correction calibration constant.
The accuracy of Max-250 sensor is +/- 1% full scale so the benefit of going to a O2 sensor with a 0-25% span even with with a +/- 2% would be a more accurate measurement because of the span.  Having said that, I have had no issues with 0-100%, +/-1% full scale sensor.
The nice part about having the output from the O2 sensor go through the PLC is that I can have the PLC sound an alarm and lock out diving the boat if the O2 measurement is high or low.
For my boat I sound a high O2 alert if O2 is > 23% and a low O2 alert if the O2 is <18%.  These alarm levels have worked pretty well.
Cliff

    On Monday, August 19, 2019, 08:15:04 AM CDT, Jon Wallace via Personal_Submersibles <personal_submersibles at psubs.org> 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 driftcaused 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 forcalibration of galvanic O2 sensors if I went with them. So although the opticalsensors are factory calibrated, this is not going to be that much of an advantageto 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 over25%. 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 calibrationof their O2 sensors. I am thinking a best practice would be to leave the hatchopen before a dive & check that the O2 reading corresponds to the standardlevel of O2 in air.Cheers Alan

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