The measuring principle of an optochemical oxygen sensor is based on the effect of dynamic luminescence quenching by molecular oxygen. This kind of oxygen sensor is frequently used in the pharmaceutical industry, medical research industry, and in the beverage industry. State-of-the-art optochemical oxygen sensors are basically maintenance-free compared to an electrochemical optode (also known as a Clark electrode). Furthermore, the response time is faster and the measurement doesn’t consume oxygen (it is a non-consumptive measurement).
Optochemical oxygen sensor
How does the optochemical oxygen measurement work?
Figure 1: Technical setup of an optochemical oxygen sensor
An LED of suitable wavelength sheds light on the luminescent layer, the dye (luminescent layer) absorbs the emitted light and as a consequence gets transferred to an excited state.
When returning to the ground state, the dye emits light of a different, less energetic wavelength. (The emitted light is called luminescence.) The emitted light is detected by a photodetector and amplified. If an oxygen-free atmosphere, e.g. pure nitrogen, is present, the emitted energy is high. (Figure 2)
The more oxygen is present, the more energy gets transferred from the excited dye to the oxygen molecules. The oxygen, even though it absorbs energy, does not emit light and, thus, once oxygen is present the amount of emitted light by the dye is much smaller.
In short, the more oxygen is present, the less the dye emits. The fact that oxygen takes over energy from the excited dye is called “dynamic luminescence quenching”. (Figure 3)
Figure 2: Luminescence process in the absence of oxygen
Figure 3: Luminescence process with oxygen
Units
The most common units for oxygen measurement are [ppb] or [µg/L], [ppm] or [mg/L], [% O₂ saturation], [% air saturation] and [Torr].
For the beverage industry the most frequently used units are ppb and ppm.
What is one ppm?
ppm = mg/L = part(s) per million = 10-6
What is one ppb?
ppb = µg/L = part(s) per billion = 10-9
In order to get a better idea of what one ppb acutally is, it can be seen as 1 second in almost 32 years!
Benefits
Easy to maintain
Optochemical oxygen measurement requires no electrolyte solutions or exchange of the anode/cathode. Only the O₂ sensor cap has to be replaced.Therefore, an optochemical sensor is easy to maintain and the service expenses are low.
Fast response time
The oxygen doesn’t have to diffuse through a membrane – consequently the reaction and response times are much faster.
Independent of sample flow
Due to the independence of the sample flow, reliable oxygen readings can be achieved.