CO2 levels stay level in an hermetically closed ecosystem for weeks by cycling day and night while passing all air through a marine tank growing phytoplankton.  When the system is first put together, the CO2 levels spiral until the plants and bacteria cycles get established.  This biosphere has a diversity of 15 different plants growing in an enclosed space.  The trick is to balance the plants out depending on their function in the ecosystem.  (including worms and insects)

Enclosed systems in the past generally consisted of one plant in a bottle.   They are easy to start.  Most of the time they are bogs.  Below is a link to one of them.

http://www.dailymail.co.uk/sciencetech/article-2267504/The-sealed-bottle-garden-thriving-40-years-fresh-air-water.html

Below is a description how the data is collected.  See other posting for how the Biosphere IV is made.

Data Results:

The following chart shows a typical day in the biosphere.  The left half of the chart is with lights on.  The right side is lights off for 12 hours.  The RH is in yellow.  You can see the scales on both sides of the graph.  Twice a day there is a dehumidifier defrosting cycle.  One in the day and one at night.  The blue line is CO2 in ppm.  It's a bit high; however, animals live for weeks at this level. 

Temperature and DP are always the same.  The DP is a calculated number by the measuring device. Here is a simplified version of the formula.  In most cases, air pressure is considered.

Tdew point = T - ((100 - RH)/5.)

And Relative Humidify is also dependent on air pressure and temperature.  It is not Absolute Humidity, but Relative.  

When given temperature and dewpoint, the vapor pressure (plugging Td in place of T into Clausius-Clapeyron equation) and the saturation vapor pressure (plugging T into Clausius-Clapeyron equation) can be determined. The RH = E/Es*100%. 

Clausius-Clapeyron equation


LN(Es/6.11) = (L/Rv )(1/273 - 1/T)

Es = Saturation vapor pressure

L = Latent heat of vaporization = 2.453 × 10^6 J/kg

Rv = Gas constant for moist air = 461 J/kg

T = Temperature in Kelvins

 

Equipement:

• Monitor indoor air quality and carbon dioxide levels in greenhouses 

• Displays temperature, relative humidity and CO2 simultaneously 

• CO2: 0 to 2,000 ppm, Accuracy: ±5% 

• Temperature: 14 to 140ºF (-10º to 60ºC), Accuracy: ±0.9ºF (±0.5ºC) 

• RH: 0.0% to 99.9%, Accuracy: ±3%

There is an RS232 output on the side of the unit.  The RS232 data is 9600 baud at 5 volt levels that needs to be pulled up to 3V3 or 5V using a 300 ohm resistor tied high.  The data is inverted from the standard RS232 standard. "0" =   -15v  equal +5v. "1" =  +15v = 0V.  This means there is no need to use the MAXUM chip MAX232 converter.  It can be directly connected to a Bluetooth module.  (HC-05 or HC-06 will work).  However, don't forget the pull-up resistor to high data output.  There is no connection for sending data to the unit, just out.

 

The RS232 outputs the current data in the format described below.  

CO2              Temp      RH          Dew Point

C594ppm    T22.6C    H89.6%    d20.7C    w21.3C59

Data comes out every 2 to 3 seconds.  It is then captured using a terminal software called Putty.  

Equipment Setup:

It requires opening the CO2 meter up and inserting a HC-06 board.  The power for the Bluetooth device is connected to the power adapter plug-in.  The unit can run off batteries, but it dies after a couple of days.  It's best to run off an adapter.

After the HC-06 is powered by the adapter, connect the connections directly to the "out" of the RS232 1/8 inch mono-audio receptacle.  It appears there are a lot of tricks put into the CO2 meter discouraging what I did.  They want you to buy the USB to RS232 adapter and software.  A wire needs to be connected between the ground of the RS232 connector to the mounting ring of the 1/8 inch receptacle.  It's embedded partly in the plastic housing.  When the jack is in place, it connects the GND to that ring by default.  It will not work without that connection in place.

 

Discussion:

The CO2 trends seem to follow the night and day.  At night plants make CO2 to grow; during the day they take CO2 and produce O2.  O2 is around 21% all the time.  And all air cycles through marine tank throughout the day.  And it appears the CO2 follows the RH changes.  This is really noticeable when a biosphere is first put together.  The CO2 will go as high as 1500ppm and the RH is 100%.  After a week or two, the CO2 comes down and stabilizes around a range of 380 to 600 ppm at a RH of 65 to 80%.  The Earth's average CO2 is around 450 ppm.  Indoor CO2 levels are around 600ppm.

Using three tanks, one for day, another for night and the last for marine phytoplankton CO2 sinking, it's possible to keep a stable CO2 levels.