Clark-type Gas Sensor Explained
A Clark-type sensor is an electrochemical sensor where the chemical element of interest passes an ion-impermeable membrane before oxidation or reduction and measurement takes place.
A Clark-type sensor is also known as an amperometric sensor which is a sensor that produces an electrical current as a function of the analyte concentration. Amperometric sensors need a specialised laboratory instrument, known as a picoammeter, in order to detect the electrical current.
more details on Clark-type sensors
O2 passes through the silicon rubber membrane and into a mildly alkaline electrolyte. O2 then reacts with the gold cathode (reduction) to produce an electrical current. The strength of the current is directly and exactly linearly proportional to the partial pressure of O2 (that is, the more O2 in the sample, the higher the electrical current).
A Clark-type sensor also consists of a reference electrode (anode) and a guard cathode. The reference electrode is the oxidation element in the sensor and completes the electrochemical circuit with the gold cathode. The guard cathode prevents the diffusion of O2 from the electrolyte toward the sensor tip. A guard cathode in a microsensor was introduced by Revsbech (1989) to provide a greater signal stability in the gold sensing cathode.
It is important to note that Clark-type sensors measure the partial pressure of a gas and not its concentration. To know the concentration value, it is necessary to know the temperature and salinity when the measurement takes place. O2 microsensors are calibrated at known temperature and salinity levels and subsequent O2 measurements are automatically corrected. A table of values can also be consulted for any post-measurement corrections.
Other Clark-type microsensors are available to measure hydrogen sulphide (H2S), hydrogen (H), hydrogen peroxide (H2O2) and ozone (O3). The basic principles and sensor design, as outlined above for the O2 microsensor, apply however the cathode and anode surfaces differ. For example, the H2S microsensor has a platinum (Pt) measuring anode, a guard anode, and a reference cathode.
who was Clark?
Leland C. Clark (1918 – 2005) was an American biochemist and Professor who attended, among other institutions, University of Alabama Medical College. Professor Clark published a paper in 1953, with his colleagues, outlining a gas sensor that would later become known as a Clark electrode. More information on Professor Clark can be found here.
Clark, L. et al. 1953. Continuous recording of blood oxygen tensions by polarography. Journal of Applied Physiology, 6: 189-193.
Revsbech, NP. 1989. An oxygen microsensor with a guard cathode. Limnology and Oceanography, 34: 474-478.
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