[PSUBS-MAILIST] Optical Oxygen Sensors

Mon Aug 19 23:16:08 EDT 2019

Thanks Cliff,
I just noticed the second half of your email below the product picture :(
I like that O2 sensor you linked to & have noted the 10-15mv specs so I
can write code & do the electronics for it, but buy them when I have finished
the rest of the sub. If I bought them first they may expire before I finish.
I am wanting to do similar to you except use 3 X O2 sensors. I am going to
compare the readings & go with the average of the nearest 2, as in a rebreather 
system.
One reason is it is a 1 person sub & I want to be able to send inexperienced
people down in it who may panic if alarms went off for a sudden sensor
failure & also may not recognise what was going on. I also want to signal which 
sensor is going out of range & monitor it.
Alan

> On 20/08/2019, at 1:59 AM, Cliff Redus via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
> 
> 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
> 
> Max-250 | Maxtec
> Replacement oxygen sensor for Maxtec OM-25, Fluke VT Mobile,
> 
> 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 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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