Optical sensors - Electrochemistry - Chromservis.cz

Optical sensors for DO measurement

Classical oxygen sensors are based, as is common in the industry, on Clark Cell technology, in which oxygen diffuses through a membrane and is reduced electrochemically on a precious metal. The electrons involved in this process generate a very small current (nanoamperes) which is converted to an oxygen measurement signal by a measurement amplifier. Sensors such as these have served well for decades, but have their limitations.

Measurement principle

While other suppliers of optical oxygen sensors utilize fragile optical fibers, a single light channel, or two different LEDs, Hamilton prefers a mechanically and thermally stable symmetrical design. The unique design of the VISIFERM™ DO enables to monitor the status of the sensor's blue LED using one of the photodiodes. The photodiode with the red filter measures the oxygen-dependent red light generated on the luminophore through luminescence (fluorescence) caused after exitation by the blue light. Electrons are excited to a higher energy level, and return to their original level after emission of red light. When the luminophore comes into contact with elemental oxygen, the O2 molecules absorb the energy, resulting in reduced intensity of red light emission. This difference in intensity is analyzed by the instrument's self-monitoring system to pinpoint photobleaching (bleaching of the luminophore).

High precision measuremet of the optical phase shift between the blue and red light pulses provides accurate indication of oxygen concentration. Normally, the luminophore's excited electrons remain in this state for some time. However, in the presence of oxygen they return to their ground state more quickly. Between the pulsed excitation of the luminophore with blue light and the emission of red light, there is an oxygen-dependent time shift which can be measured as an angle of phase. Measurement, calculation, and output of the measured value occur entirely inside the sensor.

Notice that VISIFERM™ DO sensors measure the partial pressure of oxygen (pO2) just as classical sensors do. This can be displayed as % air saturation, concentration in mg/l, ppm, or even as ppb. The measurement range is currently limited to 0.05% to 300% air saturation (4 ppb to 25 ppm). For most applications this measurement range is more than adequate. When calibrating the sensor well, the limit is even below 1 ppb.