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Measure 100% of CO2 inside a chamber

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Dave_PT:
Hi hello all.

I need to do something very simple, but at the same time it has become complex.

My task is to measure the CO2 concentration inside a pressurized autoclave up to a maximum of 2 bar.
The concentration should reach 100% (1000000ppm) because the gas will be injected through an industrial cylinder.
The aim is to expose a material to 100% CO2.

The material in test can absorb some CO2, and this difference (nobody knows, but its maybe 2/3%) needs to be measured.

The material can absorb some CO2, and this difference needs to be measured.
After some search, I've found some expensive sensors that do the job, but I need something that gives a rough value with the lowest price possible (of course).

So I found these sensors: MD62 from WINSEN (chinese brand) - ~25€ (aliexpress).



DATASHEET

The sensor, from what I can see through the datasheet, can do the job.
But the datasheet is too simple and without any practical information about the driver circuit, etc ...

Now the questions:
* Do you have any idea (other sensor, technique, etc.) to solve my problem of measure until 100% of CO2?
* Have someone use this sensor? Any feedback about it, please?
* Suggestions?


NOTES:
Inside the autoclave I will have some other sensors, like temperature, humidity, pressure (MS5803-05BA) and to complete, need the CO2 sensor.
The interior of the chamber can reach 90% of humidity and can be heated to 70 ° C.


Thank you all!

ejeffrey:
I'm not clear what you are actually trying to do.

First off, you won't reach close to 100% CO2 unless you either A) pump the chamber out (vacuum) first, then backfill with CO2, or B) Flow CO2 through it for a while to purge any air present.  If you are trying to measure CO2 absorption by looking at the decrease in CO2 pressure, a continuous flow is going to make that quite difficult.

The sensor you posted is a wheatstone bridge.  So you apply a voltage to the "input" terminals and sense the output voltage with an instrumentation amplifier.  You should be able to find plenty of example circuits for measuring a wheatstone bridge online or in any electronics textbook.

The sensor in question appears to be a thermal conductivity sensor.  So it measures the difference in thermal conductivity of air vs. CO2. But pressure and other cases may also change the reading.

Ian.M:
It will be very difficult to distinguish between 99% CO2 and 100% CO2 using a CO2 sensor. 

Also that sensor does *NOT* offer CO2 detection with total discrimination against other gasses. e.g. 1% of a hydrocarbon gas can cause a 30mV output change in the opposite direction to that caused by CO2, counteracting the reading one would normally get for a 60% CO2 concentration.  Furthermore the datasheet says prolonged exposure to a high 'gas' concentration will permanently affect the sensor (but doesn't specify whether or not the gas in question is CO2).

IMHO you'd be better off attempting to measure whatever non-CO2 gasses your material outgasses (assuming you know their expected composition), or maybe the residual oxygen concentration as a proxy for the proportion of atmospheric air in the gas mix.

tpowell1830:
If the material absorbs the gas immediately, this probably won't work. However, if you are raising pressure to 2 BAR in order to infuse part, there is hope. (And as my dad used to say "Hope in one hand and spit in the other and see which one fills up first.")

Step 1. Vacuum chamber down to as near vacuum as you can get.
Step 2. Isolate chamber with valve, turn off vacuum pump.
Step 3. Inject the CO2 into chamber, until you reach a predetermined pressure (2 BAR?).
Step 4. Wait until pressure stops dropping (material has absorbed all it can).
Step 5. Measure final pressure, determine with math how much CO2 is lost into material using volume of gas (derived from delta drop in pressure, cubic volume of chamber (you figure out math part)) as a percentage.
Step 6. Exhaust the pressurized gas from chamber.
Step 7. Remove infused part from chamber, lose all infused CO2 (eventually, depending on permeability and other factors).

Just my 2 cents...

Dave_PT:

--- Quote from: ejeffrey on March 28, 2019, 08:04:36 pm ---B) Flow CO2 through it for a while to purge any air present.

--- End quote ---
This is how it is done.
Sorry for misunderstanding.

The pressure sensor are only to give feedback about the pressure inside the chamber.


--- Quote from: ejeffrey on March 28, 2019, 08:04:36 pm ---The sensor you posted is a wheatstone bridge.  So you apply a voltage to the "input" terminals and sense the output voltage with an instrumentation amplifier.  You should be able to find plenty of example circuits for measuring a wheatstone bridge online or in any electronics textbook.

--- End quote ---

After reading the datasheet I did not realize that it worked like a wheatstone bridge. The image does not help  ;D


@Ian.M
Thank you, you have made very important observations  :-+


@tpowell1830
Thank you. Based on your 7 steps, I will ask some questions about it.
I think that the pressure inside the chamber need to be the same. The industrial cylinder have a mechanical regulator that maintains the pressure in the defined value.

Maybe, what I need is a sensor that measures the quantity of gas that enter in the chamber ...
With this and the chamber volume, I can get a percentage of absorption.


I will come back with more news about this topic.

Thank you  :-+ :-+

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