Figure 1 shows the two biotechnology sensing elements; the sensing disk closest to the LED is red-orange and the other is yellow. The yellow sensing disk is closest to the photodiode as shown in Figure 1. The housing protects the sensing elements from physical and chemical damage. A getter system is used to remove chemicals that can harm the sensing disks. The getter has a finite life and depends on the atmospheric conditions and level of pollution in the environment. The sensor life is limited to 6 years in order to provide the customer with reliable response to CO during this time. After that time chemicals can overwhelm the getter. Therefore, after 6 years there will be an end-of-life signal telling you to replace the alarm. The signal is 2 beeps and 2 flashes every 30 seconds. There is a shock absorber between the getter and sensors and this system is held into place by a retainer clip. The SIR sensing elements are held very tightly in place with the optical alignment system by the sensor housing and other systems.