2.1.4. Effect of Temperature on Sensor Performance
Temperature Dependancy. It has been observed that there is 1 to 5% increase in sensor output current per oC increase in temperature (Fig. 2.10a). The temperature effect comes from Pm and PL in Eq. (17) because they are functions of temperature. When the sensor is operated under membrane-diffusion control, the temperature dependency of Is should come entirely from Pm whereas when the liquid film resistance is not negligible, both Pm and PL contribute to the temperature dependency. Generally, Pm is expressed as:
where E is the activation energy for the permeation and amounts to be 8.8 kcal/g-mol for polyethylene and 7.8-9.6 kcal/g-mol for polypropylene membranes. The response time of the sensor also depends on temperature but the temperature dependency originates from diffusivity (see Eqs. (18) and (19)). As the temperature increases, the diffusivity increases and this makes the sensor respond faster (Fig. 2.10b). In commercial instruments, thermistors are used for compensating temperature variation in sensor output (see Fig. 2.10a).