Campbell Scientific EC155 CO2 and H2O Closed-Path Gas Analyzer and EC100 Electronics with Optional CSAT3A 3D Sonic Anemometer - Manual (Page 42)

EC155 CO
2
and H
2
O Closed-Path Gas Analyzer
1. Connect the zero-and-span gas to the Sample inlet, and disconnect the
pump, leaving the Pump connection open. The zero-and-span gas will
be pushed forward through the EC155 sample cell and exhausted out
the Pump fitting. In this case the Zero/Span connection may be left
plugged.
2. Connect the zero-and-span gas to the Zero/Span inlet, and disconnect
the intake tube from the Sample connection. Disconnect the sample
pump and plug the Pump connection. The zero-and-span gas will be
pushed backwards through the EC155 sample cell and exhausted
through the Sample fitting.
f. Click on the Zero/Span button on the main screen of ECMon. A real-time
graph at the bottom of the window will appear that displays concentrations
of CO
2
and H
2
O (see FIGURE 9-6). Next allow CO
2
span gas to flow
through the sample cell. The exact flow rate is not important since sample
cell pressure is being measured; however, a flow rate should be high
enough to flush the tubing and sample cell in a reasonable time. If the
tubing from the CO
2
span tank to the EC155 is kept short, the CO
2
will
equilibrate in several seconds even at relatively low flow rate (< 0.5 LPM).
However if the tubing is long (e.g. if the EC155 is left in place at the top of
the tower) it may take a few minutes to flush the tube, and a higher flow
rate (> 1 LPM) may be useful to reduce the equilibration time.
Once gas begins to flow through the sample cell, watch the graph on
ECMon for the measurement readings to stabilize. Once stable, write
down the reported CO
2
concentration.
Optimally the concentration of span CO
2
should be near the
concentration of CO
2
being measured in the field. Also, the user
is advised to use CO
2
mixtures in dry ambient air for the CO
2
span
gas. The use of reference CO
2
gas mixtures in pure nitrogen will
lead to errors due to a carrier gas effect on pressure-broadening of
the CO
2
absorption lines since oxygen gas has a smaller line-
broadening coefficient than nitrogen.
g. Stop the flow of CO
2
span gas, and provide H
2
O span gas to the analyzer.
A dew point generator is often used for this. Allow a relatively high flow
rate for the first several minutes to more quickly stabilize the system, and
then decrease the flow to 0.2 to 0.4 L/min before making the measurement.
Higher flow rates should not be used when taking the measurement
because back-pressure on the dew point generator will cause errors. Write
down the reported H
2
O concentration.
As H
2
O may adsorb to surfaces inside the tubing and sample cell,
allow plenty of time for the system to reach equilibrium.
h. Stop the flow of H
2
O span gas, and allow zero air (no CO
2
or H
2
O) to flow
through the analyzer. Dry nitrogen is often used as zero air. The exact
flow rate is not important since sample cell pressure is being measured,
however, a flow rate should be high enough to flush the tubing and sample
cell within a reasonable time period. Wait for the measurement readings
to stabilize and write down the reported values for CO
2
and H
2
O
concentrations.
NOTE
NOTE
32