How fast do optical O2 sensors respond?
Optical O2 sensors work in gases and liquids, slurries, tissue and other fluids. In gases the response can be very fast — with T90 response rates well under 1 second. That is essential for applications like capnography which looks at the rate of change of breathing gases as an indicator of pulmonary function.
But how fast do they work in water and other fluids? What governs the rate? Optical O2 sensors do not consume oxygen, like polarographic electrodes, but, O2 must diffuse from the media into or out of the sensor matrix. So ultimately diffusion will limit the response rate to a change in pO2 in the sample.
Measuring that rate requires the right kind of experiment. Creating an instantaneous change in pO2 is very difficult in a fluid. For example bubbling gases in water may take 10s of minutes to create a new equilibrium pO2 level. So we devised a simple method that bypasses this limitation.
The experiment involves taking a sensor measuring pO2 in air, and thrusting it into de-oxygenated water. Except for the time required to get the probe into the water, the only limitation is diffusion of O2 out of the sensor matrix and into the water.
Here is what we found!
The spin coated AP chemistry (polystyrene matrix) responded quickly, with a T90 of ~2.0 seconds. Spin coating is our most effective method for making thin layers of the sensor matrix. The thickness and permeability to O2 will govern the diffusion rate,
In contrast, a dip coated fiber sensor with AP chemistry was considerably slower. T90 was ~ 6.5 seconds. Since this is an identical material, the difference is due to the thickness of the dip coating. This is further confirmed by the variation aming dip coated sensor response rates. Dip coating is not as precise as spin coating.
The sensor matrix also influences response rate. Our HCR (hydrocarbon resistant sensor material) is a sol-gel based system. response rates for both patches and probes were fast. In fact the HCR probe response rate in gases was on the order of 200msec. The faster response may be due to permeability of O2, and/or the viscosity and ultimate layer thickness that can be realized with sol-gels.