Introduction to the LI-5405A pCO2 system

Welcome, and thank you for your purchase of the LI-5405A pCO2 system from LI-COR. If you have just taken delivery of your instrument, check the packaging list to be sure that you have received everything that you ordered.

Part number 5405A-7815

The LI-5405A features the pCO2 module, the LI-7815 CO2/H2O Trace Gas Analyzer with reduced flow rate kit, and a computer to control the system and record data. It also includes a built in Aanderaa Dissolved Oxygen Sensor, Turner Designs Chlorophyll Sensor, Sea-Bird SBE 45 Thermosalinograph, and an external Airmar Weather Station.

What's what

Each pCO2 system includes a pCO2 module, a computer, and an external gas analyzer. For water handling, the pCO2 module features a water inlet, water filters, flow meters, two-stage equilibrator, and waste-water discharge. It features a panel for gas standards, ambient air, and the gas analyzer. The pCO2 module handles sample processing, gas switching, and data from sensors. The gas analyzers characterize the gases. The computer controls the system, logs data, and performs initial data analysis.

The pCO2 module

The pCO2 module is the most conspicuous component. It features a water intake and discharge, connections for gases, and the valves and tubing that handle samples and gases.

Figure 1‑1. The front of the pCO2 system gives access to valves and components that are used during measurements.
  • Air Vents: Normally closed, these vents allow air into the system when draining water after measurements.
  • Water Filters: Two filters separate large particles from the water sample. Only one filter is used at a time, and you can switch from one filter to another without stopping measurements by turning both filter selection valves simultaneously.
  • Filter Selection Valves: Two valves allow you to switch from one intake water filter to the other when both valves are turned simultaneously.
  • Cyclops-7F: A Turner Designs Cyclops-7F chlorophyll fluorescence sensor measures chlorophyll in the sample.
  • Water Intake Valve: The water valve can be adjusted to achieve the ideal flow rate for the system.
  • Water Intake: The yellow connector at the water intake should be installed on the inlet. The water inlet accepts a 3/4" US garden hose fitting. The system supports water supply flow rate of 3.5 liters per minute (lpm), with a range of 2.5 to 5 liters per minute (lpm). The water should be supplied with pressure near 60 pounds per square inch (4.14 bar).
  • Power Button: A simple analog switch is in while on and out while off. Be sure it is in the off position before connecting the power cable.
  • Equilibrators: The two-stage equilibrator brings pCO2 in the water sample into equilibrium with the head-space air. Each equilibrator has a tube that is open to ambient air to equilize pressure.
  • Flow Meter for SBE 45: The flow meter shows the water flow rate through the SBE 45 Thermosalinograph.
  • SBE 45 Primary Valve: Adjusts the water flow rate through the SBE 45 Thermosalinograph. The ideal flow rate is 20 gallons per hour (76 liters per hour). The SBE 45 requires flow between 10 to 28 gallons per hour (38 to 106 liters per hour).
  • SBE 45 Secondary Valve: Adjusts the water flow rate in combination with the primary flow valve. After adjusting the primary flow valve to achieve the ideal flow rate, adjust the secondary valve to achieve the ideal flow pattern in the equilibrators.
  • Oxygen Optode: Aanderaa 4835 Dissolved Oxygen Sensor measures dissolved oxygen in the sample (optional).
  • Water Discharge: The water discharge is for water after it has been measured. It can be connected to a 3/4" NPT connector. Discharged water should drain into a sink or floor drain. The discharge pipe must be kept downward since the flow is driven by gravity.

On the right side of the pCO2 module, there are banks of ports and connectors. From this perspective, you can also see the two-stage equilibrator and other components labeled in Figure 1‑2.

Figure 1‑2. Underway pCO2 system side panel with connectors and visual access to the two-stage equilibrator, air filters, and other components.
  • CO2 Standard Connections: Five ports (numbers 1 to 5) are for reference CO2 gas standards. Gases connect with stainless steel compression fittings that are compatible with 1/8" OD copper or stainless steel tubing.
  • To Gas Analyzer: Connects to the gas analyzer air inlet with the included 1/4" OD Bev-a-line tubing (part number 222-01824).
  • From Gas Analyzer: Connects to the gas analyzer air outlet with the included 1/4" OD Bev-a-line tubing (part number 222-01824).
  • Ambient Air In: Connects to sample tubing that brings ambient air in for analysis. Accepts 1/4" OD steel, copper, or Bev-a-line tubing.
  • Airmar Port: Accepts the data cable from an Airmar Weather Station; it is an RS-232 serial port with a DB9 connector. The system records time in UTC, GPS location, and weather data from the Airmar Weather Station.
  • Main Fuse: The system uses a 3-amp slow-blow fuse for protection.
  • Power In: The pCO2 system is powered from an AC power supply (110 or 240 VAC; 60 or 50 Hz). Be sure the power button is in the OFF position (not pushed in) before connecting the power cord to a power supply.

Caution: Be sure that the instrument is compatible with the power supply. Connecting to an incompatible power supply may damage equipment or cause injury.

  • To Computer: To connect the computer; this USB port is connected to a USB-to-serial adapter inside the system. Accepts a USB-A cable.
  • Sea-Bird SBE 45: The thermosalinograph reports temperature and salinity of the sample water.
  • Flow Meter: Measures air flow through the system.
  • Condenser: Cools air with peltier coolers to reduce humidity. Condensation in the condenser is purged with the peristaltic pumps.
  • Air Filters: Three filters separate particulates from the air stream to protect equipment.
  • Peristaltic Pumps: Two pumps move condensate out of the condenser on a scheduled interval. The pumps discharge water to the main water discharge tube.

Spares and accessories kit

Part number 9953-043

Each pCO2 system includes an assortment of accessories and replacement parts. These parts include tubing, connectors, and service items.

Description Part Number
Bev-a-line tubing (1/4" OD; 3 meters) 222-01824
Coupling; 3/4" socket (1) 300-21741
Coupling ; 3/4" plug (1) 300-21742
PVC pipe; 3/4"; 1 1/2" (3.8 cm) long ( 1) 300-21743
2-socket bushing (2) 300-21744
USB cable type A-to-A (1) 392-21740
Air filters (2) 9967-008

Airmar Weather Station

Part number 294-24156

The Airmar Weather Station provides ancillary data to the pCO2 system, including location and weather data. The weather station is an external accessory that should be installed in open air.

Sea-Bird SBE 45 Micro TSG Thermosalinograph

Part number 294-21451

The Sea-Bird SBE 45 thermosalinograph measures water temperature and conductivity of the sample. It is installed prior to delivery.

Aanderaa 4835 Dissolved Oxygen sensor

Part number 294-21452

The Aanderaa Oxygen Optode 4835 measures the dissolved oxygen. It is installed prior to delivery.

Turner Designs Cyclops-7F chlorophyll sensor

Part number 294-21453

The Cyclops-7F Fluorescence sensor that measures chlorophyll concentration (µg/L). It is installed prior to delivery.

Power cables

The instrument will include a power cord. The power adapter and cord are for indoor use; they are not rated for outdoor deployment.

Description Part Number
For USA, Canada, Mexico, and Japan; NEMA 5-15; Type B 390-00787
For Europe, South America, and Asia; CEE 7/7; Type C 390-01486
For United Kingdom; Type G 390-03089
For Israel; SI32; Type H 390-03059
For Australia; Type I 390-03095

Warning: Use the safety-approved power cord supplied with the instrument. The instrument power supply is equipped with a 3-wire grounding plug. This plug will only fit into a grounded outlet. Do not defeat the purpose of the grounding-type plug. Do not place the instrument where the power cord will be walked on or exposed to water or chemical spills.

LI-7815 CO2/H2O Trace Gas Analyzer

Each LI-5405A system includes one LI-7815 analyzer. The gas analyzer includes its own instructions, power supply, accessories, and batteries. Refer to the gas analyzer instructions for information beyond what is described here. The gas analyzer is delivered with a reduced flow rate kit installed to provide a nominal flow rate of 70 sccm, as required by the system.

Computer with software

Part number 616-21277

A Dell™ Latitude™ 3450 laptop computer running Windows® 11. Includes a universal power supply and battery. The computer is equipped with a licensed version of the software required by the instruments, and should be dedicated to the control and operation of the device.

User-supplied materials

To use the pCO2 system, you will need to acquire between three and five gas standards and additional hardware to connect the gases.

Tool kit

You will use some basic tools to complete the installation. Two adjustable wrenches or a basic SAE wrench set will be used to connect gas standards and other tubing. Absorbent rags will be useful to clean any spilled water. Keeps some spare hose washers nearby in case one gets lost or damaged.

Gas standards (reference gases)

Three or more tanks of air-balanced CO2 gas (gas standards) are needed. Up to five gases with different CO2 concentrations can be used for better accuracy. The LI-7815 requires air-balanced gas standards. The standards should cover the pCO2 range you expect to measure (you might choose standards of 200, 425, 550, 800, and 1500 ppm CO2) - be sure a standard with a low concentration is lower than the expected concentrations and a standard with a high concentration is higher than the expected concentrations. One standard should be close to ambient atmospheric CO2 (~422 ppm as of December 2024). Do not use CO2-free air as one of the reference gases because it takes too long for the analyzers to reach the true zero value. Thus, a low CO2 air (such as 120 ppm) should serve as the lowest CO2 reference.

Primary CO2 standards can be purchased from NOAA. Less precise and lower-cost standards may be used for estuarine and coastal research, or they can be calibrated against the primary standards.

Two-stage low-pressure regulators

Each tank should be equipped with a high-quality low-pressure regulator capable of delivering pressure of 15 psi (1 atm). Regulators with a full delivery pressure range from 0 to 50 psi (0 to 3.4 atm) are recommended for the CO2 gas standards. A maximum range of 30 psi (2 atm) will provide higher resolution adjustment at low pressures. Type CGA-590 gas regulators are needed for air-balanced gas tanks.

Stainless steel or copper tubing

1/8" OD tubing is used to connect the gas standards to the LI-5405A.

Water supply tube and drain tubes

Tubing (3/4" OD pipe or soft hose) will deliver water to and from the pCO2 system. Discharged water should flow to a waste water drain after measurement. The water supply accepts a 3/4" US garden hose fitting. The discharge accepts a 3/4" NPT connector.

Water supply or pump to provide sample water

Some vessels are equipped with a water supply; in other circumstances, you may need to provide a pump to deliver the water to the system. Water should be provided at a flow rate of 3.5 liters per minute (lpm) or in the range of 2.5 to 5 liters per minute (lpm) at 60 pounds per square inch (4.14 bar) pressure. If using an external pump, it should have an automatic shut-off to prevent overheating if the water flow becomes blocked.

Surface water pCO2 measurement principle

In natural water, the species of dissolved inorganic carbon are CO2+H2CO3, HCO3-, and CO32-. The partial pressure of dissolved molecular CO2 (i.e., CO2+H2CO3), or pCO2, is defined thermodynamically as the CO2 gas pressure in an air parcel that is in equilibrium with the water.

In practice, such an equilibrium phase can be established via a gas-water equilibrator. The equilibrator used in LI-COR pCO2 systems follows the design described in Wanninkhof and Thoning, 1993 and Feely et al., 1998 and detailed in Pierrot et al., 2009. Figure 1‑3 shows the main components of the air and water handling system.

Figure 1‑3. The pCO2 system handles water and gases with a system of pumps, valves, and tubing. Water enters through the intake and is discharged after analysis. The peristaltic pumps drain condensate from the condenser on a configurable schedule. The equilibrator drain should be opened to drain water when measurements are done.

The measurement sequence

In a measurement sequence, sampling begins with measurements of gas standards. Each standard is measured a set number of times (set in Exp. Config > EXPERIMENTAL SETTINGS in Configuring the measurement sequence; depicted in Figure 1‑4) until statistical criteria are satisfied.

Figure 1‑4. The measurement sequence consists of the measurement of gas standards, which are repeated to achieve statistical stability. This sequence has 10 standards repeated 3 to 5 times followed by air measurements and air intervals (5), and finally, seawater measurements and seawater intervals (25). New Source Purging is a time period to elapse when switching from one gas to another. The sequence will repeat Air-Seawater measurements the set number of times. Then it repeats the whole cycle a set number of times or continuously.

The default settings require the Standard Passing Criteria to have a standard deviation of <0.1 ppm for 10 consecutive readings of the standard. If the standard deviation is >0.1 ppm for the first 10 readings, the system takes more readings and computes the running average of the most recent 10 readings until the standard deviation is <0.1 ppm. When the passing criteria are met, the average is recorded as the reading of this standard gas. The system takes a maximum of 100 readings by default (the maximum is user-configurable). The system repeats this with all gas standards (a minimum of three, but up to five).

After measuring standards (and passing), the instrument proceeds to the ambient Air Measurement a set number of times, honoring the Air Interval setting between each measurement. Next, it proceeds to the Seawater Measurements with a configurable Seawater Interval between each measurement.

When the valve is switched to a different source (atmospheric air, equilibrator air, or standard gases), the system will wait a configurable time between measurements - the air interval and seawater interval. No data are recorded during the intervals. The New Source Purging time is used to purge the system after switching to a new gas. If the flow rate is reduced, new source interval should be extended to ensure the tubing is flushed between measurements.

The system will repeat the air-seawater measurement a set number of times (Air-Seawater Repeats), and it will repeat the whole cycle a set number of times (Cycle Repeats) or until the operator stops the measurement if set to Continuous.

References

1 Feely, R.A., Wanninkhof, R., Milburn, H.B., Cosca, C.E., Stapp, M. and P. Murphy, P. (1998). A new automated underway system for making high precision pCO2 measurements onboard research ships. Analytica Chimica Acta, 377(2-3), pp.185–191. doi:https://doi.org/10.1016/s0003-2670(98)00388-2.
2 Pierrot, D., Neill, C., Sullivan, K., Castle, R., Wanninkhof, R., Lüger, H., Johannessen, T., Olsen, A., Feely, R.A. and Cosca, C.E. (2009). Recommendations for autonomous underway pCO2 measuring systems and data-reduction routines. Deep Sea Research Part II: Topical Studies in Oceanography, 56(8-10), pp.512–522. doi:https://doi.org/10.1016/j.dsr2.2008.12.005.
3 Takahashi, T., Sutherland, S.C., Sweeney, C., Poisson, A., Metzl, N., Tilbrook, B., Bates, N., Wanninkhof, R., Feely, R.A., Sabine, C., Olafsson, J. and Nojiri, Y. (2002). Global sea–air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects. Deep Sea Research Part II: Topical Studies in Oceanography, 49(9-10), pp.1601–1622. doi:https://doi.org/10.1016/s0967-0645(02)00003-6.
4 Wanninkhof, R. and Thoning, K. (1993). Measurement of fugacity of CO2 in surface water using continuous and discrete sampling methods. Marine Chemistry, 44(2-4), pp.189–204. doi:https://doi.org/10.1016/0304-4203(93)90202-y.