Howard P. Isermann Department of Chemical and Biological Engineering
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Biosensor is the term used for a whole class of sensors based on biochemical reactions to determine specific compounds.
Continual measurements of raw materials and products are important for the control of Biochemical processes.
Monitoring of important organic pollutants is also desirable for environmental control. Recently, many biosensors have been developed and provide methods of rapid and continuous measurements of various compounds.
A biosensor is generally an immobilized enzyme or cell that is combined with a transducer to monitor a specific change in the microenvironment.
The probe tip is immersed in the liquid phase and is in contact with the process either directly or through a membrane.
To date, these devices have not seen widespread use because of serious disadvantages. These include:
Microbial sensors are suitable for the industrial process because they may be stable for a long time. Two different types of microbial sensors measure organic compounds.
An inability to be steam sterilized
- They react with the product
- And are oversensitive
1.) Sensors consisting of immobilized whole microbial cells and an oxygen probe.
The concentration of compounds is determined from microbial respiration activity directly measured by the oxygen probe.
2.) Sensor consisting of immobilized microorganisms and an electrode for something other than oxygen.
The concentraion of compounds was indirectly determined from electroactive metabolites such as protons, carbon dioxide, hydrogen, formic acid, and reduced co-factors which can be measure by the electrode.
(This portion of the text was written by Craig Pohan and Matt Armstrong at Rensselaer Polytechnic Institute (c) 1995, The following portion was authored by Urban Jenelten)
Sensors are crucial when it comes to controlability, reliability and last
but not least to profitability of a process.
Why are sensors so important?
Sensors provide the connection between the "real" world and the world of process
control and computers. The sensors' accuracy determines the overall accuracy and reliability of the control system. The most
sophisticated analyzer or computer system with cannot correct for faulty measurements.
Both fed-batch and continuous cultures depend heavily on good sensors and
accurate measurement for good performance.
What are the general properties of a good sensor:
- optimum measurement accuracy (not as good as possible, but as good and as
accurate as necessary)
- ease of calibration and reconditioning
- sensitivity and good resolution
- provision of reproducable measurements
- long term stability
- fast response (important for control)
- continuous operation
- insensitivity to electrical and other environmental interference
- low operation and maintenance cost
- acceptance by users
- meet safety requirements
Fig 1: Placement of a sensor in a control system
If the sensor and the control unit are a considerable distance apart, the
sensor should be equipped with an amplifier
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