Why is the CO2 level lower in my house than outside?

[mess]

TL;DR Summary

I purchased a high accuracy IR co2 sensor. Outside it reads ~400ppm co2 inside I am getting about ~300ppm co2.

How is this possible? I can't think of what could be absorbing Co2 inside my house, when I am producing it at a pretty high rate.

I have my HVAC running on a frequent cycle mode also. I have no plants.

The sensor: https://www.sensirion.com/en/enviro...n-dioxide-sensors/carbon-dioxide-sensors-co2/

Here is the data:

Inside:


��������������

2020/12/05 12:12:22PM	342
2020/12/05 12:12:19PM	346
2020/12/05 12:12:17PM	349
2020/12/05 12:12:14PM	350
2020/12/05 12:12:12PM	353
2020/12/05 12:12:09PM	364
2020/12/05 12:12:07PM	370
2020/12/05 12:12:04PM	372
2020/12/05 12:12:02PM	375
2020/12/05 12:11:59PM	377
2020/12/05 12:11:56PM	378
2020/12/05 12:11:54PM	381
2020/12/05 12:11:51PM	382
2020/12/05 12:11:49PM	384
2020/12/05 12:11:46PM	388

Outside:


������������������

2020/12/05 11:28:28AM	432
2020/12/05 11:28:26AM	433
2020/12/05 11:28:23AM	434
2020/12/05 11:28:21AM	437
2020/12/05 11:28:18AM	438
2020/12/05 11:28:16AM	440
2020/12/05 11:28:13AM	442
2020/12/05 11:28:11AM	443
2020/12/05 11:28:08AM	444
2020/12/05 11:28:06AM	445
2020/12/05 11:28:03AM	444
2020/12/05 11:28:01AM	446
2020/12/05 11:27:58AM	449
2020/12/05 11:27:56AM	450
2020/12/05 11:27:53AM	450
2020/12/05 11:27:50AM	453
2020/12/05 11:27:48AM	463
2020/12/05 11:27:45AM	464
2020/12/05 11:27:43AM	463

 

[Ivan Seeking]

My first guess is that the levels are cycling with the time of day and your levels inside are lagging those outside.

 

[mess]

My first guess is that the levels are cycling with the time of day and your levels inside are lagging those outside.

im not sure what you mean by that :(

 

[Ivan Seeking]

im not sure what you mean by that :(

Take data over a 24 hour period and see how much the levels vary. It may be rising and falling outside, which causes it to rise and fall inside but after a delay.

 

[mess]

Take data over a 24 hour period and see how much the levels vary. It may be rising and falling outside, which causes it to rise and fall inside but after a delay.

The outside concentration can only reduce the inside concentration if the inside concentration was higher.
because it is higher outside, that can only mean there is something constantly absorbing CO2 inside my house or the sensor is defective. I understand there is a correlation between the outside CO2 and inside, but I am trying to understand why it is on average lower.

 

[Ivan Seeking]

The outside concentration can only reduce the inside concentration if the inside concentration was higher.
because it is higher outside, that can only mean there is something constantly absorbing CO2 inside my house or the sensor is defective. I understand there is a correlation between the outside CO2 and inside, but I am trying to understand why it is on average lower.

Take data inside and outside over a 24 hour period and I'll bet it all makes sense. ;)

 

[Stephen Tashi]

Is the ppm rating independent of the temperature of the gases being sampled?

 

[mess]

Is the ppm rating independent of the temperature of the gases being sampled?

It comes with a temp and humidity sensor built into compensate

 

[mess]

Take data inside and outside over a 24 hour period and I'll bet it all makes sense. ;)

Lets say over 24 hours the concentration inside is higher than outside. I still don't understand why the smaller sample size would not match

 

[anorlunda]

The outside concentration can only reduce the inside concentration if the inside concentration was higher.

Suppose the outside concentration varies between 200 and 600 though the day, but the inside concentration responds so slowly that it varies only 300 to 500.

CO2 generated or absorbed inside the house could bias the inside reading. However, it's easy to generate C02, and difficult to absorb it.

 

[Stephen Tashi]

It comes with a temp and humidity sensor built into compensate

Perhaps you can think of a simple test to see if that function actually works.

What's the standard definition of "ppm"? Parts-of-what per million what?

Is it a volume measurement ? - volume of CO2 per volume of space?

Is it a fraction-of-volume measurement? - fraction of volume of CO2 per total volume of sample?

Is it a weight measurement? - weight of CO2 in sample per weight of whole sample?

Is it a count of molecules? - number of CO2 molecules in sample per total number of molecules in the sample

 

[Bystander]

However, it's easy to generate C02, and difficult to absorb it.

You're certain? Construction materials? Sheetrock/drywall effects? You're familiar with the (non-)results from "Biosphere 2"?

 

[BillTre]

I was thinking exposed amounts of water inside the house could absorb the CO₂.
CO₂ is highly soluble in water (like open aquariums or inside pools). A humidity sensor would only sense water in the air for its corrections.

But I like the idea of construction materials better.
Lots of those in houses. Not so much, bodies of water.

Some plastics can absorb water to some extent also, but they have limited capacity and would become saturated.

 

[fresh_42]

My first thought was a broken device. There is definitely more $CO_2$ inside than outside: breathing or smoking are two sources. As I read about the correction automatism of the device, I thought: what does it correct where to? If this automatism is meant to make different sources comparable, then it might well tune down actual values inside.

An easy test would be to compare two measurements directly after an air exchange, and then after the room is back to its normal temperature.

 

[BillTre]

Some things I noticed about the numbers:

1. the time points were fairly tightly clustered in time (not much time for variations to arise, unless due to wind)
2. the numbers seemed to vary quite a bit, but not enough to overlap the two distributions
3. Outside and inside readings both generally trended down over a period of less than a minute

This indicates possible problems/tests WRT the readings:

• Accuracy not down to the tens of PPMs? Find a CO2 standard to test your instrument on (or make one), or get a test based on a different process for comparison.
• There maybe some kind of warm up or equilibration period for the probe in a new condition (some pH meters can be like that); take readings over a longer time period, wait until meter settles down to take more accurate readings. Consult those who made it for details.
  On the other hand, such a trend in one direction could be an electronic problem in the sensor system (in pH and I think redox measuring systems, a leak of a current into the body of water being measured can off-set the readings an electronic sensor like a pH electrode would detect).
• Wind blowing variable air conditions to the sensor (making for a series of different air samples being measured); take a sample of air in a large garbage bag. Take a series of readings of that specific air sample to see whether or not the readings are stable.

 

[cmb]

Summary:: I purchased a high accuracy IR CO2 sensor. Outside it reads ~400 inside I am getting about ~300.

How is this possible? I can't think of what could be absorbing Co2 inside my house, when I am producing it at a pretty high rate.

I have my HVAC running on a frequent cycle mode also. I have no plants.

The sensor: https://www.sensirion.com/en/enviro...n-dioxide-sensors/carbon-dioxide-sensors-co2/

Here is the data:

Inside:


��������������

2020/12/05 12:12:22PM	342
2020/12/05 12:12:19PM	346
2020/12/05 12:12:17PM	349
2020/12/05 12:12:14PM	350
2020/12/05 12:12:12PM	353
2020/12/05 12:12:09PM	364
2020/12/05 12:12:07PM	370
2020/12/05 12:12:04PM	372
2020/12/05 12:12:02PM	375
2020/12/05 12:11:59PM	377
2020/12/05 12:11:56PM	378
2020/12/05 12:11:54PM	381
2020/12/05 12:11:51PM	382
2020/12/05 12:11:49PM	384
2020/12/05 12:11:46PM	388

Outside:


������������������

2020/12/05 11:28:28AM	432
2020/12/05 11:28:26AM	433
2020/12/05 11:28:23AM	434
2020/12/05 11:28:21AM	437
2020/12/05 11:28:18AM	438
2020/12/05 11:28:16AM	440
2020/12/05 11:28:13AM	442
2020/12/05 11:28:11AM	443
2020/12/05 11:28:08AM	444
2020/12/05 11:28:06AM	445
2020/12/05 11:28:03AM	444
2020/12/05 11:28:01AM	446
2020/12/05 11:27:58AM	449
2020/12/05 11:27:56AM	450
2020/12/05 11:27:53AM	450
2020/12/05 11:27:50AM	453
2020/12/05 11:27:48AM	463
2020/12/05 11:27:45AM	464
2020/12/05 11:27:43AM	463

Is it a new house and/or with new concrete?

Do you have many house plants?

 

[mess]

Is it a new house and/or with new concrete?

Do you have many house plants?

house from 1950, no plants

 

[fresh_42]

house from 1950, no plants

weed counts as plants

 

[cmb]

house from 1950, no plants

A mystery then.

These are very low levels of CO2 for accurate measurements. What is the calibration and precision of the instrument, and what is the temperature offset of the gases it is measuring?

 

[mess]

Suppose the outside concentration varies between 200 and 600 though the day, but the inside concentration responds so slowly that it varies only 300 to 500.

CO2 generated or absorbed inside the house could bias the inside reading. However, it's easy to generate C02, and difficult to absorb it.

From my understanding its not possible under standard environmental conditions for the outside air to be under 400ppm CO2 since that's the global level for a few decades now. Therefore the outside air could never influence the inside air to be under 400 ppm.

 

[BillTre]

Have looked more closely at your numbers.
There are two time separated sets of closely clustered points, at two different locations.

The first (based on the time stamp) set (outside) has a smaller downward (wrt time) slope (-0.688 ppm/s)
than the second set (inside; -1.277 ppm/s).
They both slope down.

The period between the readings (2658 s) is too large account for the overall differences inside/outside just based on either of the slopes, but it is in the same temporal direction.

I suspect something instrumental.
Is this a leave it on all the time type meter or a turn it on and use it for spot checks kind of meter?

 

[OmCheeto]

I've never owned nor operated a CO2 device, but the rapidity with which the CO2 levels dropped, and the sequence of the readings, indicates that your batteries might need changing.

Just a guess, of course.

 

[jrmichler]

Do one continuous ABAB test: Inside for one hour, outside for one hour, inside, outside. ABAB tests are good for separating sensor drift from actual reading changes.

The datasheet states a 20 second time constant response to a change in $\rm{CO_2}$ concentration, so the test should be long enough for the sensor to stabilize, plus longer to look for concentration fluctuations.

 

[Frodo]

The data sheet specifies: Accuracy: ± (30 ppm + 3%)

It's a bit ambiguous what that means. If I assume the 3% applies to the measured value and not to the error itself I have, at 400ppm, an accuracy of ± (30ppm plus 12ppm), or ± 42ppm. So, an actual value of 400ppm could display as 358 to 442 ppm.

Also, the measurement range starts at 400ppm so values below that are out of spec. The present global CO2 level is about 412ppm so you are unlikely to get external measurements differing much from that.

Never confuse accuracy with resolution!

Second, I would have thought the outside values to remain fairly stable assuming no close by variable CO2 emitters. What readings do you get far from potential sources?

 

[Rive]

Apart from addressing the seemingly systematic change @OmCheeto found I think you should also check the functionality of the instrument with some obvious CO2 sources.

 

[Barakn]

The o.p. is frustratingly light on details. Assuming the times are the local time zone, the measurements were taken close to solar noon. One important consideration, especially considering the instrument utilizes IR, is whether it was exposed to direct sunlight outside (presumably was shaded inside). I also wonder about contamination from the user's breath or heating effects if it was hand-held.

 

[Frodo]

Evolution and variation of atmospheric carbon dioxide concentration over terrestrial ecosystems as derived from eddy covariance measurements says

Using 355 site-years of CO2 concentration observations at 104 eddy-covariance flux tower sites in Northern Hemisphere, we presented a comprehensive analysis of evolution and variation of atmospheric CO2 concentration over terrestrial ecosystem (ACTE) for the period of 1997–2006. Our results showed that ACTE exhibited a strong seasonal variations, with an average seasonal amplitude (peak-trough difference) of 14.8 ppm, which was approximately threefold that global mean CO2 observed in Mauna Loa in the United States (MLO).

So, they see a peak-to-trough variance of 15ppm across seasons, which equates to ± 7.5ppm. Without access to the paper this suggests the daily or hourly or minute by minute variability is low.

I strongly suspect that your instrument and/or your method of use are faulty.

Edit: I have accessed the paper and the daily variability is much higher than I supposed. The text states that they took "half-hour CO2concentrations" and " spike screening was applied for all dataset. Any data outside ±3 standard deviations within 6 hours (12 points) were regarded as outliers and excluded from the records".

Figure 2 shows significant daily variations.

It seems something interesting happened at the end of 2005 ...

It also says

The difference between intensive human activity areas and natural ecosystems was similar to the increase in CO2 concentration found in Phoenix, USA, where an increase of 111-185 ppm was reported from a pristine rural site to the city center (Idso et al., 1998, 2001).

 

[.Scott]

I don't think there is any problem with these measurements at all. And they do not reflect less indoor CO2 than the outdoor level. In fact, it suggests the opposite.

Before I tell what I believe is happening, let me first say that you need a lot more data. @jrmichler suggested an ABAB measurement pattern. I suggest you collect measurements at a rate of one sample every 30 minutes for 24 hours, then switch between indoors and outdoors. Do this for 4 days so that you have the two examples of the daily pattern each for indoors and outdoors.

Plants consume CO2 in the day and exhale it at night.
Plant "dark respire" all the time - releasing CO2.
They photosynthesize only with light.
https://www.bbc.co.uk/bitesize/guides/zpwmxnb/revision/3

Your samples were taken midday, when the consumption is at its greatest - and thus the CO2 levels are dropping the fastest. You saw them drop while taking the outdoor readings, they continued to drop as you relocated the instrument, and then you saw them continue to drop as you measured them indoors.

Here is a plot of both sets of readings, outdoor (green) and indoor (blue):

Extrapolating, it would appear that had you left the instrument outside, you would have picked up even lower reading that what you got indoors.

I would come to 2 conclusions:
During the time you took these measurement:
1) It was quite sunny; and
2) Your house was ventilated - but not completely open to the outdoors.

 

[FMPeck]

The size of the NDIR sampling chamber is fixed and is open to the atmosphere so that air can move in and out. The number of air molecules in a given volume is affected by temperature and air pressure but not the concentration of CO2. At low pressures or high temperatures, there will be fewer air molecules in the sample chamber, so there will also be fewer CO2 molecules, even though the ppm of CO2 hasn’t changed. Fewer CO2 molecules “fools” the sensor into thinking that the CO2 concentration is lower than it really is. At high pressures or low temperatures, there are more air molecules in the sample chamber and more CO2 molecules, even though the CO2 concentration hasn’t changed. More CO2 molecules “fools” the sensor into thinking that the CO2 concentration is higher than it really is. Therefore a CO2 sensor calibration will only be accurate at one temperature and one air pressure. Odds are the ratio of PV=nRT (ideal gas equation) for inside and outside conditions may serve as a useful correction for your results.

 

[Ray Rebeiro]

I have owned 4 CO2 meters. Three different manufacturers. 2 units from the same manufacturer provide readings that were different by almost 40%. They insisted that I should recalibrate both units. I did and the results did not change. They accepted return of the so called defective unit and refunded the purchase price
although I did not ask for this. I was expecting a functioning unit to be returned. All of the 3 units I now have provide different readings and the differences in the readings are not trivial. The readings respond differently to changes in placement location. The units all cost more than $75 each.
I hope that this helps.

 

[Frodo]

See How we measure background CO2 levels on Mauna Loa for hourly measurements during the day at Mauna Loa which says

Measurement Method

An example of the output from the analyzer for one day is shown in Figure 1 [below]. Two separate intake lines are used for sampling ambient air. The intake lines are from the top of a 38 m tall tower next to the observatory, to avoid any influence on the measurements by human activities at the observatory. Each intake line is measured for five minutes, alternating between line 1 and line 2. Once an hour, the reference gas R0 is measured for five minutes. Once per day, two target tanks are measured for fifteen measurements each (seen in hour 6 and hour 17 in Figure 1). The first few minutes after each gas change are not used to allow time for the previous gas to be completely flushed from the analysis system. The difference of the ambient air measurements from the reference R0 are calculated, and these differences are put into equation 1 to calculate the true ambient mole fraction CO2. By making the measurements relative to the difference from R0, any short term drifts in the analyzer are accounted for.

 

[chemisttree]

Could it be that the AC water condensate is scrubbing CO2 from the air inside your house?

Try measuring CO2 at the AC outlet vent and at the inlet (filter). Note any differences. I assume the meter does automatic temperature correction?

 

[iollmann]

Since you are recording in December, I'm going to suggest that there is a substantial thermal difference between inside and outside. Here I'm assuming you are some place where it gets reasonably chilly in December. If that is the case, we may note that warmer air expands -- fewer molecules / m**3 -- and colder air contracts -- more molecules / m**3. We should expect the infrared transmittance of the air to go up when it is warmer on this basis, because there are fewer molecules to absorb the signal. So, we may indeed see changes in the detector based on temperature with constant ppm of CO2, beyond changes that are simply based on the temperature of the detector itself.

Boyle: amount of substance / Volume = Pressure / (R * Temperature)

As temperature goes up, amount of gas in a fixed volume goes down. Temp is in Kelvin here, so we'd need maybe 30°C to make a 10% difference. The relative concentration of CO2 in the gas should remain constant, so the "apparent concentration" of CO2 in the air would appear to go down a little as the air heats up based on a spectrophotometer like this device for a naive calculation that didn't take this into account.

Now, I'd like to think that a device with a humidity and temperature sensor in it have those things so it can correct for such factors! But, as it claims "Best price / performance", perhaps there wasn't enough engineering $$ spent on that calibration, or this one is defective, or there is a lot of variance between devices, making that sort of calibration difficult without individually calibrating each one which might be too expensive. It seems deeply suspicious to me that the device measures lower concentrations as time progresses over a short period. (Because the IR lamp heats up the detector or the gas? Because the detector has some issue? Because the IR source gets brighter as it warms?) But, I'm not a analytical chemist, so listen to anyone with actual experience with NDIR photometry over me.

There is probably someone in the engineering department of the company that can help you out, but finding that person is going to take some doing. I might see if the detector reaches some steady state if left on for a few hours, and if so, try those measurements instead. And of course, a real scientist might prepare calibrated samples in a closed gas bag and recalibrate the thing himself and see whether he could experimentally account for whatever nonsense the device is putting out and correct for it.

The only other speculation that came to me is that air conditioning might remove some CO2 from the air as it condensed out some of the water from the air. The air recirculates. Maybe it could do it if your home was particularly air tight. <Vigorous hand waving> Of course, in December, this seems unlikely unless you are enjoying summer at this time of year. However, I am really just rampantly speculating at this point and should quit before I get even further behind.

It is certainly observed that bubbling gas through water systems (e.g. ozone treatment of water) pulls dissolved CO2 out of the water driving away some of the hardness. This is something of the opposite of the process:

CO3(2-) + 2 H+ <==> HCO3- + H+ <==> H2CO3 <==> CO2 + H20 (and the CO2 escapes with the gas)

Normally, all these variants exist in solution in some concentration in equilibrium, based on pH. However, when bubbling gas through the system, it pulls out CO2, thereby driving the reaction to the right. This consumes both CO2 and H+ ions. This has the net effect of removing some of the acidity from the water, and because carbonates are often less soluble than bicarbonates, you might observe CaCO2 coming out of solution and coating your pipes.

So, we might imagine that a steady process of condensing water out of the air would remove the CO2 with it. CO2 is far more soluble in water than oxygen or nitrogen. There is also a lot less of it in the air, so if we steadily draw away small amounts of CO2, O2 and N2 with water condensing out of air, the CO2 will be rapidly depleted and the O2+N2 won't change much. Eventually, you might have a fairly pure N2, O2 mixture. The problem is that it would have us believe that the CO2 is not leaking into your house at a comparatively tremendous rate to replace it. Since generally people don't asphyxiate in their homes from CO2 buildup even if they stay inside for days -- you may have recently tried this experiment yourself in the last year -- it seems quite reasonable to expect that the house is in fact not that air tight, and this mechanism really couldn't account for your observations. This former scientist would prefer to go with the explanation that CO2 is in steady state exchange with the outside air. The CO2 ppm inside and outside is going to be very similar and you are just observing miscalibration or other confusion in the device.

...but you know, I haven't actually measured, so confidence is low.

 

[LeeMcK]

Summary:: I purchased a high accuracy IR CO2 sensor. Outside it reads ~400ppm CO2 inside I am getting about ~300ppm co2.

How is this possible? I can't think of what could be absorbing Co2 inside my house, when I am producing it at a pretty high rate.

I have my HVAC running on a frequent cycle mode also. I have no plants.

The sensor: https://www.sensirion.com/en/enviro...n-dioxide-sensors/carbon-dioxide-sensors-co2/

Here is the data:

Inside:


��������������

2020/12/05 12:12:22PM	342
2020/12/05 12:12:19PM	346
2020/12/05 12:12:17PM	349
2020/12/05 12:12:14PM	350
2020/12/05 12:12:12PM	353
2020/12/05 12:12:09PM	364
2020/12/05 12:12:07PM	370
2020/12/05 12:12:04PM	372
2020/12/05 12:12:02PM	375
2020/12/05 12:11:59PM	377
2020/12/05 12:11:56PM	378
2020/12/05 12:11:54PM	381
2020/12/05 12:11:51PM	382
2020/12/05 12:11:49PM	384
2020/12/05 12:11:46PM	388

Outside:


������������������

2020/12/05 11:28:28AM	432
2020/12/05 11:28:26AM	433
2020/12/05 11:28:23AM	434
2020/12/05 11:28:21AM	437
2020/12/05 11:28:18AM	438
2020/12/05 11:28:16AM	440
2020/12/05 11:28:13AM	442
2020/12/05 11:28:11AM	443
2020/12/05 11:28:08AM	444
2020/12/05 11:28:06AM	445
2020/12/05 11:28:03AM	444
2020/12/05 11:28:01AM	446
2020/12/05 11:27:58AM	449
2020/12/05 11:27:56AM	450
2020/12/05 11:27:53AM	450
2020/12/05 11:27:50AM	453
2020/12/05 11:27:48AM	463
2020/12/05 11:27:45AM	464
2020/12/05 11:27:43AM	463

The difference in humidity inside versus humidity outside might be causing the difference in your CO2 values.

I was recently trying to set up a Cozir CO2 sensor outdoors on a 12 foot pole. I had two of these pole devices, For weeks of tests at a time, the data returned was displeasing. I took the sensors off of the poles, and put them in 2" diameter 3/4" high plastic containers. I made a moisture absorber with a 1 pint screw lid canning jar, a small sample pump, with 1/4" copper tubing inlets and outlets hot glued to the plastic containers and the jar. The moisture absorbent is silica gel from a craft supplies store (Joanne crafts).

With the humidity absorbing device inline, the two CO2 sensors (now on my workbench, not outdoors) show steady consistent values. . What I am trying to build is several remote CO2 meters to estimate local CO2 emissions. The sample pump came from Excess Solutions and Halted Electronic Supply, San Jose. CA

 

[Keith_McClary]

https://www.researchgate.net/publication/318919872_Study_on_effect_of_temperature_and_humidity_on_the_CO_2_concentration_measurement