Using Molecular Sieve to Zero Infrared Gas Analyzers for Eddy

Covariance or Atmospheric Profile MeasurementsSteve Sargent, Campbell Scientific, Inc. Logan, Utah, USA

Poster 143

Acknowledgments• Kate Sargent: CAD support for proof-of-concept

design

• Paul Fluckiger: testing cylinders of ultra zero air

and general technical support

• Travis Kendall and Roy Sorenson: in-house

testing of the Zero Air Generator

• Karen G. Wolfe: poster layout and editing

ConclusionsMolecular sieve can safely, easily, and effectively remove CO2 and

H2O from air for zeroing an IRGA. It is available in a variety of

scrubber devices optimized for various applications:

• Remove traces of CO2 and H2O from cylinders of compressed gas

• On-line automated field zero; with or without pump

• Hand-held zero air generator with battery-powered pump

Molecular sieve scrubbers make checking and setting the zero of a

CO2/H2O analyzer easy and convenient, improving data quality and

confidence.

Closed-Path EddyCovarianceThe CPEC200 (Campbell Scientific, Inc.) is a closed-path EC flux

system used for long-term monitoring of atmospherebiosphere

exchanges of CO2, H2O, heat, and momentum. It includes an EC155

CO2/H2O analyzer, CSAT3A sonic anemometer, pump module, and

system enclosure that houses a CR3000 datalogger, and an optional

valve module for on-line automated zero/span.

The CPEC200 Scrub Module is

an accessory to the CPEC200. It

is used to provide a source of

zero air for zeroing the EC155

CO2/H2O analyzer. It uses a

small, low-power (2W)

diaphragm pump that is mounted

in a temperature-controlled and

weathertight fiberglass enclosure.

Ambient air is drawn through

molecular sieve to remove CO2

and water vapor, and push zero

air to the EC155 at

approximately 2 l·min-1 flow.

The pump and its heater are powered only when in use, to avoid any

significant effect on system power consumption.

Only an annual maintenance is required to replace molecular sieve in

one of the three stages.

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Molecular SieveType 13X molecular sieve is an effective desiccant and adsorber of

CO2. The CAS number for 13X molecular sieve is 6321-69-6. It is

available through Campbell Scientific, Inc. as pn 27450.

Scrubbing Compressed Air

Thirteen cylinders of Airgas Ultra Zero air were tested to verify specifications (CO2 < 1

ppm, H2O < 2 ppm) and to demonstrate the use of molecular sieve as a scrubber. A

cylinder of zero air was connected to two inlets of a CPEC200 zero/span module. One

inlet was connected directly, while the other inlet was connected through a BMT-4. The

datalogger program was modified to alternate four times between these two inlets, such

that the air stream spent 2 minutes on each. The CPEC200 measured the CO2 and H2O in

the air streams. The difference was calculated to quantify the CO2 and H2O in the zero

air before scrubbing by the molecular sieve.

All cylinders tested at CO2 < 0.3 ppm; below specifications. Water vapor was between 0

and 8 ppm, but still well below practical requirements for an IRGA intended for ambient

humidity. In all cases the zero air had equal or less CO2 and H2O after passing through

the molecular sieve.

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AbstractInfrared gas analyzers (IRGAs) used to measure carbon dioxide (CO2) or water vapor for

eddy covariance (EC) flux or atmospheric profile applications can drift over time. It is

important to regularly monitor and set their zero (offset) and span (gain). The span of an

IRGA is generally more stable than its zero, but even an error in the zero can lead to

errors in EC flux and storage flux estimates.1 Most commonly, the zero and CO2 span are

checked using cylinders of compressed gasses, and water vapor span is checked with a

dewpoint generator.

Cylinders of compressed gases are difficult to transport to a field site because of their

bulk, weight, and safety issues, and dewpoint generators are generally not designed for

field use. Chemicals such as magnesium perchlorate and sodium hydroxide can be used

to scrub water vapor and carbon dioxide from ambient air to zero an IRGA, but these

chemicals are difficult to transport, use, and dispose of safely. Many IRGAs used in the

field are not checked regularly because of these difficulties.

Molecular sieve can provide a safer, more convenient alternative for zeroing an IRGA in

situ. Three example configurations are discussed: a field-rugged scrub module that

includes a 2 l·min-1 pump for automated on-line zeroing of a closed-path EC system, a

field-rugged scrub bottle with no pump for automated on-line zeroing of an atmospheric

profile system, and a light-weight (1.2 kg), hand-held zero air generator with a battery-

operated, 0.2 l·min-1 pump for manually zeroing an open-path IRGA. The molecular

sieve provided air with CO2 and H2O concentrations equal to or lower than ultra-zero

grade air, and required very little maintenance.

Molecular sieve is commonly used to scrub a tank of

compressed zero air of CO2 and H2O. Given a high-

pressure gas, safe operation is a necessary design

feature. Agilent Technologies’ BMT-4 contains 750

ml of 13X molecular sieve, and is made of heavy-

walled aluminum, making it a good choice to scrub

CO2 and H2O from tanks of compressed air.

Atmospheric ProfilesThe AP200 CO2 and H2O atmospheric profile system

(Campbell Scientific, Inc.) measures atmospheric CO2

and H2O at up to eight positions, usually spaced along

a tower to measure a vertical profile. The AP200 is

commonly used with an EC system to measure the

storage term to give a more complete measurement of

surface exchange.

The AP200 includes automatic zero and CO2 span

capability, with zero air provided either with a cylinder

of compressed zero air, or with the AP200 Scrub Bottle.

The AP200 Scrub Bottle uses molecular sieve to

remove CO2 and H2O from ambient air as it is pulled

through the scrub bottle by the AP200 sample pump.

The AP200 Scrub Bottle has two outlet connections:

one for normal operations; and one that allows the user

to assess the state of the molecular sieve. This test port

samples the scrubbed air from part way through the

column of molecular sieve to indicate when to replace

the sieve.

An AP200 Scrub Bottle was tested in

an accelerated lifetime test (February – October

2014) at the Campbell Scientific test site in

Logan, Utah. The bottle was connected to an

AP200 atmospheric profile system, configured

as follows:

• A step was added to the zero/span cycle

to measure the AP200 Scrub Bottle test port

• The zero/span cycle was configured to run

hourly, rather than daily

• The CO2 span setting was suppressed; a tank

of CO2 span gas was measured but not set-3

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The AP200 Scrub Bottle will easily last a year in normal

operation. The test port gives an indication it is time to

replace the molecular sieve with a large safety margin.

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Portable Zero AirGenerator

An accelerated usage test was performed at Campbell Scientific, Inc., from

January to May, 2016. Open-path IRGAs (EC150 and IRGASON) were

zero and spanned using traditional compressed gases and dewpoint

generator, and then air from the Zero Air Generator was measured. The run

time for the pump was recorded for each test.

A total 54 IRGAs were tested, for a total run time of 15.5 hours. At the end

of the test there was no indication of CO2 or H2O in the scrubbed air stream,

and no indication of battery exhaustion. Other tests (data not shown)

showed the batteries generally run for approximately 40 hours before the

flow begins to decrease.

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No H2O in test port over entire test (239 days, 2533 liters of air)

CO2 span tank was steady, confirming no CO2 in the air used to set the CO2 zero

Reference1 Fratini, G., McDermitt, D. K., and Papale, D. (2014) “Eddy-covariance flux errrors due to biases in gas

concentration measurements: origins, quantification and correction.” Biogeosciences, 11, pp 1037 – 1051.

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CO2 appeared in test port after 132 days (3168 zero/span cycles, 1399 liters of air)

The Zero Air Generator (Campbell Scientific,

Inc. pn 31022) is a convenient tool for zeroing

an IRGA. The Zero Air Generator is very

similar to the AP200 Scrub Bottle but includes

a small diaphragm pump to circulate air

through the IRGA. The flow rate (0.2 l·min-1)

flushes the calibration shroud or sample cell of

the IRGA effectively without excess

pressurization. Power consumption is very low

and requires just one pair of AA batteries for

approximately 40 hours of use. The Zero Air

Generator weighs only 1.2 kg making it

extremely useful in zeroing IRGAs in remote

field sites.

AMS 32nd Conference on Agricultural and Forest Meteorology

CO2 remained below 300 ppm