A special feature has been designed into the OMNI-4000, SMARTLOGGER and QUADRANT instruments which measures the deterioration of sensors as they age. In the OMNI-4000 and SMARTLOGGER it is called a wear rate, in the QUADRANT it is called the usage rate. This feature is only visible when using the COM 4000, SMART-COM or COM 2000 Software packages.
Theoretically for each type of sensor and type of gas, there is a point where the sensor sensitivity to its target gas drops and the sensor becomes unstable. The wear rate is a measurement of how close the sensor is to that point.
For example when a carbon monoxide (CO) sensor's signal with gas applied, has dropped to 50% of its original value, the sensor is close to failure. Given some variation from sensor to sensor, a safety factor at 60% is used. Wear rate relates how close the sensor is to the 60% value.
For example:
Age
Output
Signal Level
Wear-Rate
Brand New
100 ppm CO = 100 mV
100%
0%
1 yr sensor
100 ppm CO = 84 mV
80%
50%
2 yr sensor
100 ppm CO = 68 mV
68%
80%
3 yr sensor
100 ppm CO = 60 mV
60%
100%
Wear rate should really only be used as general guide to sensor lifetime. Typically a sensor shows a wear rate of 0-20% for a majority of its life. A sensor with a 50% wear rate does not necessarily imply that the sensor is 'half' consumed. Caution should be taken when the wear rate approaches 80% as that is an indication of impending sensor failure. A sensor with an 80% wear rate may stop working in another 3 days, 3 weeks, or 3 months. It's impossible to say for sure. A sensor in that condition should be watched and calibrated or field tested more frequently than usual.
