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The book Lost Moon by Jim Lovell and Jeffrey Kluger had this interesting description in Chapter 10 (page 254 in my copy) regarding the CO2 gauges (emphasis added):

Carbon dioxide concentrations in the command module and the LEM were tracked with a non-power-consuming instrument resembling a thermometer, which measured the pressure of the toxic gas in the overall atmosphere.

How did this passive device work, such that it didn't draw any power?

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  • $\begingroup$ The CO2 sensors described in the Experience Reports and News Reference Manual draw power. They were problematic, so perhaps what is being described is some kind of backup device. Maybe a Drager tube. $\endgroup$
    – Organic Marble
    Commented Jun 28, 2021 at 0:03
  • $\begingroup$ Page 375 in the air-to-ground transcripts show that MCC could read the CO2 level, so I'm tempted to conclude the memoir is incorrect. hq.nasa.gov/alsj/a13/AS13_TEC.PDF $\endgroup$
    – Organic Marble
    Commented Jun 28, 2021 at 1:39
  • $\begingroup$ no answers here, but a lot on modern CO2 sensing tech, in order of decreasing informativeness: 1, 2, 3, 4. Solid electrolyte sensors may need very little power, though the MG-811 has a 200 mA heater. The infrared absorption cells need a filament to glow in IR so those will always be power-hungry I asssume. $\endgroup$
    – uhoh
    Commented Jun 28, 2021 at 3:08
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    $\begingroup$ @OrganicMarble The Drager tube you mention seems to be a Dräger tube of the Dräger Safety AG & Co. KGaA in Lübeck. But these tubes are not suitable for a continous carbon dioxide measurement. You need a new tube for every measurement point. $\endgroup$
    – Uwe
    Commented Jun 28, 2021 at 15:44
  • $\begingroup$ @Uwe agreed. They carried a kit of them on Shuttle, but I have not found any reference to their use on Apollo. $\endgroup$
    – Organic Marble
    Commented Jun 28, 2021 at 15:46

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Several NASA-related papers mention partial-pressure CO2 sensors in use. I found a quick mention in a Wiki page about SCUBA equipment (oxygen sensing in their case) which says

This type of sensor operates by measuring the voltage generated by a small electro-galvanic fuel cell.

edit Per JRE's comment - the voltage here is generated by the chemical reaction related to the specie of interest. I have to admit I don't know whether this is possible with CO2, which is less reactive than O2 .

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    $\begingroup$ If you'd like to improve this answer with a reference, ntrs.nasa.gov/api/citations/19720013195/downloads/… has a description and diagram of the LEM CO2 sensor on page 21. But it's powered, so it's not what the question is asking about. $\endgroup$
    – Organic Marble
    Commented Jun 28, 2021 at 14:09
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    $\begingroup$ @Carlwitthoft: "This type of sensor operates by measuring the voltage generated by a small electro-galvanic fuel cell." This does not mean that the sensor was externally powered. It means the oxygen is detected by the fuel cell. You let air into the fuel cell chamber and the oxygen reacts chemically with something in the chamber. The reaction causes an electrical charge to build up, which you can detect with a very sensitive (and passive) voltmeter. $\endgroup$
    – JRE
    Commented Jun 28, 2021 at 14:23
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    $\begingroup$ The sensor with a small electro-galvanic fuel cell is detecting oxygen, but the question is about a carbon dioxide sensor. $\endgroup$
    – Uwe
    Commented Jun 28, 2021 at 15:21
  • $\begingroup$ There are solid electrolyte sensors (see comment) that do develop millivolt potentials that change with CO2 concentration, and those might need power for an amplifier (they're a bit like pH or pKa sensors in that you need to measure a potential with a very, very high input impedance amplifier) and perhaps a heater and something to drive the meter's needle. $\endgroup$
    – uhoh
    Commented Jun 28, 2021 at 22:55

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