Low Volt IR Co2 Sensors and Diving

Windwalker

Just curious, I was looking at Co2 Sensors, and I ran across several IR measuring Co2 Sensors with less then 1 sec response times, like this ICX Photonics manufactures IR gas sensors and emitters for Industrial Safety, IAQ and HVAC applications. and was wondering why something like this couldn't be used in rebreathers? They suck low power (35mW), and either return ppm or %.

The only problem with the sensor is it's pressure sensitive so the sensor would have to be sealed in an IR permeable case.

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brockbr

That is a mighty fine question - I am wondering too.

snicks

It is written in the datasheet:

Operational RH
(non condensing) 0-95%

The magic words are "non condensing". When I look into my loop after a dive I see condensation.

The detector may be fine, but it can only be part of a system that solves the water-problem.

Greets
Dirk

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Muddweller

I was thinking that its only necessary to have dry gas around the sensor area, no need to do the whole loop, so would it not be possible to put hydroscopic membranes at front and back of the sensor chamber ?

Must admit I don't know a great deal about air flow through these but o2 sensors dont seem to have a problem or is that just down to the size of the o2 molecules ???

Just a thought.

Karl

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snicks

O2 sensors operate on a chemical reaction with the oxygen. Water is a problem because it can dilute the electrolyte and hence disturb the reaction.

An NDIR sensor works by measuring light passing through a gas and measuring the absorbance (simplified). Unfortunately the sensor cannot discriminate between absorbance and extinction, meaning that if light does not reach the detector the system assumes absorption. If water condenses on optical parts inside the sensor your measurement is compromised. A waterdrop might act as a focussing lense (you get negative absorption) or it may direct light away from the detector (no light - infinite absorption).

There is a reason, why no commercial sensors are available at the moment. Apart from low requests from our small market, it really is tricky. And tricky means expensive. I hope that Tom Rose comes up a solution that works, and then sells the proven principle to many manufacturers for a fat lot of money, so we get cheap reliable sensors and other people think that inventing something like this is worth their time

In short: Working principles on O2 and CO2 sensors have as much in common as an combustion-engine and a fusion-reactor.

Greets
Dirk

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Edvin M

Not that tricky to get around, there is a couple of ways to easily remove and/or eliminate condensation. I think the two best would be: 1. Heat the sensor, if the sensor is hot there will be no condensation 2. Blow diluent over it and let it blow away any water Oxygen could also be measured the same way but you need to measure the absorbation of a different wavelength.

snicks

Using a Nernst-cell would be the logical solution. Infinite lifetime, high accuracy, immune to water or other chemicals...

Unfortunately only runs at 500°C!

When we are talking about rebreather compatible we are also talking about low power consumption. Heating the whole sensor does not strike me as a solution.

Blowing diluent over the sensor... that should work (I think the Mk5 works this way), but I dont know what flow you will need. Anyway, needs carefull design and is not easy at all (I dont want 2l of dil blown into my loop every 5 secs). Measure O2 optically? I dont know which wavelength you will need. If the vibrational modes of the O2 molecule are in the UV you have the problem of a suitable light source.

Besides, I said it is tricky, not impossible

Greets
Dirk

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