2.1.5. Effect of Cathode Diameter on Sensor Performance
Effect of Sensor Size on Flow Dependency. When a DO sensor is placed in a stagnant liquid a diffusion field is generated due to the consumption of oxygen at the cathode surface.The size of the steady-state diffusion field is proportional to the size of the cathode. When the liquid is stirred, the diffusion gradient can no longer be extended beyond the liquid film around the membrane. Since
Fig. 2.10. Temperature dependency of DO sensor.
Fig. 2.11. Effect of cathode diameter on flow dependency of DO sensor.
the diffusion gradient becomes steeper with decreasing liquid film thickness, the sensor output current increases with increase in liquid velocity as shown in Fig. 2.11. This flow dependency is larger for larger cathode area because the size of the stagnant diffusion field is proportionally larger. For proper operation of the sensor, the liquid has to be stirred beyond a certain level to maintain membrane control of oxygen diffusion. The critical velocity, Vc, of the liquid is the velocity where the probe output reaches 95% (95-99% depending on the definition) of the steady state value. For a given liquid, Vc is smaller for smaller cathodes. For example, with a 25 µm Teflon membrane, a cathode of 5 mm diameter requires Vc of 70 cm/s in water, whereas only 5 cm/s is required for 25 µm cathode. When the cathode diameter is less than 1 µm, the sensor becomes insensitive to liquid flow even without the membrane. In this case, the diffusion field of the cathode is so small that it is always contained inside the liquid boundary layer around the cathode.
Effect on Current Output. One obvious effect of the cathode size is the area effect. Eq. (17)shows that the current output is directly proportional to the sensor area A. When the current produced by the sensor is too small (when the cathode area is small), the sensor signal tend to be more susceptible to noise. The current output should be greater than 10-6 to 10-7 range for ease of signal amplification.