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CO₂ probe

Dissolved CO₂ sensor for in situ measurement

Sonde CO₂ dissous

The measurement of dissolved carbon dioxide is a very important parameter for the assessment of pond water quality in fish farming, the optimization of industrial processes and the monitoring of ma- rine and freshwater waters.

The determination of dissolved CO₂ concentrations by techniques using samples, proves tedious and imprecise due to the influence of other compounds on the pH, such as phosphates, silicates, hy- drogenocarbonates and carbonates. In addition, a direct-reading immersed sensor is necessary to allow measurement without alte- ring the temperature and pressure parameters which have a direct influence on this dissolved gas.

The CO₂ sensor allows the measurement of the partial pressure of carbon dioxide directly in the water, thanks to a specific membrane permeable only to gases covering an infrared sensor. This partial pressure must be coupled by a measurement of the temperature of the field for the calculations of concentrations, calculation formulas are provided.

The CO₂ probe covers a wide range of applications thanks to its titanium body and its possible use for water up to 1000 meters deep (1000 dbar pressure) and temperatures up to 60 ° C.

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Measurement principle

separation membrane and absorption NDIR sensor :

The probe is equipped with a silicone membrane making it possible to isolate the volume of the measuring chamber from the liquid medium in which the probe is immersed. Only the gases pass through the side walls, so once the sensor is immersed, the partial pressures of CO₂ equalize between the middle and the internal volume of the sensor.
An optical sensor is placed inside, it uses the principle of non-dispersive infrared absorption (NDIR). The CO₂ molecules present in the measurement chamber absorb some of the infrared radiation emitted by the sensor. The radiation attenuation is therefore directly proportional to the amount of CO₂. This partial pressure measurement must be coupled by the values of temperature and atmospheric pressure to calculate the concentration of dissolved CO₂ in mg/l (formulas provided).

Applications

  • Monitoring of fish ponds
  • Determination of CO2 in food industry
  • Carbon balance studies in lakes and rivers
  • Management and optimization of industrial processes
  • In situ measurements in oceanography

Advantages

  • No disturbances due to silicates, phosphates, HCO3- et CO32-
  • RS485 digital output, or analog 4-20 mA / 0-5VDC possible wit- hout additional external transmitter
  • Stability of measurements < 1% drift of measurement per year
  • Measuring chamber heated to prevent condensation
Principle of measurementsilicone membrane
NDIR dual-beam infrared single beam optical sensor
absorption by attenuation measurement
Measurement ranges- type I : 0 ... 15 mg/l CO₂
- type II : 0 ... 30 mg/l CO₂
- type III : 0 ... 340 mg/l CO₂ (other on request)
Accuracy0 à 5 mg/l CO2 ..... +/- 0,06 mg/l
5 à 15 mg/l CO2 ..... +/- 2 % of the measuring value
15 à 50 mg/l CO2 ..... +/- 3,5 % of the measuring value
50 à 80 mg/l CO2 ..... +/- 1,5 mg/l
80 à 340 mg/l CO2 ..... +/- 6 mg/l
Dependence on temperaturecompensation calculations delivered with the sensor (adapted to the range and the output signal)
Dependence on pressure0.15% of the measured value per hPa
No linearityLess than +/- 1 FS
Probe Body Materialstitanium
Dimensions (L x d)225 mm x 33 mm
ConnectorSubConn MCBH4M
Power supply12...30 VCC (with digital output or 0-5 VCC) - 20...30 VCC (with 4-20 mA output)
Consumption< 0,5 W
Output signal- analog 4-20 mA
- analog 0-5 VCC
- digital RS-485
Compensationautomatic in case of moisture in the membrane
Maintenance<0.1h / week (standard use - membrane cleaning)
Calibration interval24 months - drift <1% of the measure per year
Maximum pressure of use1,000 meters / 1,000 dbar
Temperature of the medium / sample+ 0 ... + 60 °C