CO2 in Ecological Soil, Snow and Water Measurements

Vaisala's GMP343 probe is specifically designed for outdoor ecological measurements: soil respiration, ambient CO2, and plant growth chambers. This note focuses on the equilibrium sampling method used in soil, snow, and aquatic ecosystem research. For the general principles of infrared measurement, see the technical note on how to measure CO2.

Unlike a controlled industrial environment, the challenge here is measuring without disturbing the very system being studied — neither through the sampling method nor through the instrument itself.

This note covers

• What equilibrium sampling is and how it's used in soil, snow and water
• Why sensor heating can skew field measurements
• What measurement range is needed for deep soil profiles

What Equilibrium Sampling Is

Equilibrium sampling circulates air through a closed loop that includes the CO2 probe, a pump, and a length of semipermeable tubing. The membrane allows gas exchange with the surrounding medium (soil, snow, or water) until the concentration inside the loop reaches equilibrium with the outside concentration — without needing to draw a liquid sample or physically disturb the measurement point.

Response time depends on the tubing material: a 1-meter length of silicone rubber submerged in water takes around 1.18 hours to reach half of the total change (half-life), while a 2-meter length takes about 0.51 hours. Sintered PTFE tubing responds faster (around 0.32 hours), though it adds installation complexity.

Why Sensor Heating Can Skew Field Measurements

The GMP343 heats the sensor head to prevent condensation on the optical components — the same solution that solves the humidity problem in closed industrial environments. But in outdoor ecological measurements, that same heat can become a source of error: in snow measurements, snow around the probe was observed to melt earlier than expected, likely due to the heating effect, altering the very condition being measured.

The practical recommendation is to use heating cautiously in field studies, and to monitor soil or snow temperature 1 cm away from the probe to detect whether the instrument's own heat is affecting the immediate measurement environment.