Sensor for dissolved CO₂ measurement

Carbon Dioxide in the biotechnology

Carbon dioxide is a product of respiratory and fermentative metabolism of micro organisms. During fermentation the carbon dioxide emission correlates with the biomass concentration and the substrate consumption.

Depending on concentration carbon dioxide may influence the growth and metabolism of micro organisms. For example a high concentration of carbon dioxide leads to a delay of budding of Saccaramyces cerevisiae, so the reproduction of yeast is influenced negatively. Further the negative effect of a very low carbon dioxide concentration on the metabolism during aerobic substrate limited yeast fermentation is well known and results in a yield loss. Also the formation of products and by-products in fermentation processes frequent depends on the content of carbon dioxide in the medium.

On-line Monitoring of dissolved Carbon Dioxide

Continuous detection of carbon dioxide by the presented measuring system is realized by combination of a silicone-tubing-probe and a carbon dioxide sensor.

The measurement is carried out:

• directly in the fermentation broth, where carbon dioxide is formed and where the micro organisms are affected by
• also in the gas stream
• both below and above the carbon dioxide saturation achieved under atmospheric pressure
• without any preparing of the exit gas
• with high stability
• with short response time

SILICONE-TUBING-PROBE

The silicone-tubing-probe was developed by BIOTECHNOLOGIE KEMPE GmbH (EU and US patent). It is steam sterilizable.

The probe inserted into the broth consists of a silicone coated channel which is flown through by a carrier gas. Carbon dioxide permeates through the silicone membrane and is carried to the CO₂-sensor.

The measuring principle of the system bases on the linear relation between the partial pressure of carbon dioxide inside the broth and the amount of carbon dioxide permeating through the membrane. The concentration of CO₂ in the carrier gas is far from the equilibrium concentration so that permeation back from the carrier gas into the broth is negligible.
The carrier gas control and the signal converter are located inside a separate box.

SIGNAL PROCESSING

The gas is measured by its absorption of infrared radiation inside the CO₂-sensor located in the housing of the ST- probe. Infrared radiation emitted by a lamp passes through a filter working at the selective wavelength which is characteristic for absorption by carbon dioxide. If there is carbon dioxide inside the measuring cell the intensity of light decreases corresponding to the actual CO₂-concentration.

The described ST-probe is equipped with a modular CO₂-sensor. The CARBOCAP sensor is silicone based and its operation is based on the NDIR Single-Beam Dual-Wavelength principle. Basing on this new measuring principle the main advantages of the sensor are long-term stability and long lifetime.

The output of the sensor (Signal Converter Module) is 0(4) - 20 mA and a serial interface RS- 232 is available.

Additional signal processing is realised by connecting the CO₂ probe to a measuring computer FMC-MINI LCD. This is a modification of an existing measuring computer which is successfully used in Biotechnology to detect volatile organic compounds. The system to measure carbon dioxide is now a new development of BIOTECHNOLOGIE KEMPE GmbH.

The carbon dioxide concentration in the carrier gas is shown as % Vol. after processing by the computer.

Based on the detected concentration in the carrier gas the computer calculates the partial pressure of CO₂ in the fermentation broth. The next step is the calculation of the dissolved CO₂ by means of the additionally measured temperature. Influence of other compounds on the solubility of CO₂ can be compensated  by means of the computer programme. The measuring computer realizes the collection of values, processing, calibration and graph.

MEASURING RANGE

Several CO₂-probes are offered for measuring the dissolved carbon dioxide concentration by the described principle, in accordance to the measuring range.
The measuring range is fixed by the carrier gas flow rate which depends on the used nozzle. Depending on the chosen nozzle a measuring range of 0-50% or 0-100% of saturation can be realised. This probes are offered for using during no-pressure fermentation.
Furthermore a probe is offered which covers a range of 0-500% saturation, used during pressurised fermentations.

APPLICATIONS

Carbon dioxide is a product of cellular metabolism of micro organisms used in biotechnology. During fermentation the carbon dioxide content is the result of carbon dioxide formation by micro organisms and its transport by aeration. By means of the CO₂-probe it is possible:

• to detect such carbon dioxide concentrations which inhibit metabolism and
  growth of micro organisms. So the user is able to start compensating steps, e. g. aeration,
   
• to get information’s about the growth and activity of micro organisms during fermentation
   
•  to realise the control of anaerobic fermentations.

Furthermore carbon dioxide plays an important role as a quality parameter in beverage and brewery industry. It can be detected by the measuring system in order to insure the quality of the final product.