44
Field tests coordinated by the Alliance for Coastal Technologies of five glass
electrode instruments, one dye-based sensor, and one ISFET instrument have largely
confirmed these advances. The combination ISFET/chloride electrode demonstrated
robust performance in the marine environment. The colorimetric instruments and a
single glass electrode also showed good precision but greater uncertainty. Glass
electrode performance varied widely with only one of the five demonstrating comparable performance to the newer approaches.
pCO 2 Sensors
Automated measurements of CO 2 partial pressure (pCO 2 ) rely either on spectroscopic determination of infrared radiation (IR) absorption or spectrophotometric
dye-based applications. The former use nondispersive IR (NDIR) photometers measuring gas absorption in the 4.3 μm band relative to a reference band at 4 μm.
Ceramic-coated resistances generate the radiation that is propagated across a gas
cell to an IR-sensitive photodiode. Additional sensor channels to measure water
vapor which also absorbs IR may be added to eliminate the necessity for prior drying of the gas stream. Modular gas analyzers are widely available.
For atmospheric measurements of pCO 2(g) , the gas analyzer cell is flushed by a
pump and valve arrangement alternating samples with calibration gases. Chemical
dryers may be used to remove water vapor from the sample stream. For continuous
determination of surface seawater pCO 2(a) , an air well may be used for internal
equilibration of seawater pCO 2 with a near-surface air stream across a gas permeable membrane. In surface instruments the membrane may be exposed directly to
ambient water. Buoy-based field instrument performance evaluations by ACT over
an effective range of 300–800 μatm pCO 2 show enhanced performance for those
NDIR instruments using onboard calibration with a zero gas (achieved by chemically removing all CO 2 from an air sample) and a further span reference mixed gas
preparation such that the sample values are bracketed within these two standards.
Biological fouling was found to be well controlled by the use of copper foil, wire
mesh, or perforated sheets protecting the active membrane or intake. Air wells on
buoy-mounted instruments are purposely constructed using a nickel copper alloy
with anti-fouling properties.
Shipboard systems may use a showerhead dispensing seawater spray into a cylindrical housing flushed with boundary layer air pumped from the ship’s bow for
equilibration rather than the membrane approach. This air equilibrates with the seawater and is then piped across a drying tube to a NDIR detector. Dry near-surface
air piped directly to the detector without prior equilibration serves to determine
atmospheric pCO 2(g) . Shipboard shower head systems and the buoy-mounted membrane system achieve greater accuracy by periodically measuring a blank and a socalled span reference sample from an onboard tank dispensing a very precisely
measured calibration gas. Since gas concentration in the gaseous state varies with
water content and atmospheric pressure, systems measuring the gaseous phase
report concentration as the mole fraction (xCO 2 ). The sample is assumed to be satu2 Electronic Sensors and Instruments for Coastal Ocean Observing
Field tests coordinated by the Alliance for Coastal Technologies of five glass
electrode instruments, one dye-based sensor, and one ISFET instrument have largely
confirmed these advances. The combination ISFET/chloride electrode demonstrated
robust performance in the marine environment. The colorimetric instruments and a
single glass electrode also showed good precision but greater uncertainty. Glass
electrode performance varied widely with only one of the five demonstrating comparable performance to the newer approaches.
pCO 2 Sensors
Automated measurements of CO 2 partial pressure (pCO 2 ) rely either on spectroscopic determination of infrared radiation (IR) absorption or spectrophotometric
dye-based applications. The former use nondispersive IR (NDIR) photometers measuring gas absorption in the 4.3 μm band relative to a reference band at 4 μm.
Ceramic-coated resistances generate the radiation that is propagated across a gas
cell to an IR-sensitive photodiode. Additional sensor channels to measure water
vapor which also absorbs IR may be added to eliminate the necessity for prior drying of the gas stream. Modular gas analyzers are widely available.
For atmospheric measurements of pCO 2(g) , the gas analyzer cell is flushed by a
pump and valve arrangement alternating samples with calibration gases. Chemical
dryers may be used to remove water vapor from the sample stream. For continuous
determination of surface seawater pCO 2(a) , an air well may be used for internal
equilibration of seawater pCO 2 with a near-surface air stream across a gas permeable membrane. In surface instruments the membrane may be exposed directly to
ambient water. Buoy-based field instrument performance evaluations by ACT over
an effective range of 300–800 μatm pCO 2 show enhanced performance for those
NDIR instruments using onboard calibration with a zero gas (achieved by chemically removing all CO 2 from an air sample) and a further span reference mixed gas
preparation such that the sample values are bracketed within these two standards.
Biological fouling was found to be well controlled by the use of copper foil, wire
mesh, or perforated sheets protecting the active membrane or intake. Air wells on
buoy-mounted instruments are purposely constructed using a nickel copper alloy
with anti-fouling properties.
Shipboard systems may use a showerhead dispensing seawater spray into a cylindrical housing flushed with boundary layer air pumped from the ship’s bow for
equilibration rather than the membrane approach. This air equilibrates with the seawater and is then piped across a drying tube to a NDIR detector. Dry near-surface
air piped directly to the detector without prior equilibration serves to determine
atmospheric pCO 2(g) . Shipboard shower head systems and the buoy-mounted membrane system achieve greater accuracy by periodically measuring a blank and a socalled span reference sample from an onboard tank dispensing a very precisely
measured calibration gas. Since gas concentration in the gaseous state varies with
water content and atmospheric pressure, systems measuring the gaseous phase
report concentration as the mole fraction (xCO 2 ). The sample is assumed to be satu2 Electronic Sensors and Instruments for Coastal Ocean Observing