Graph 1 compares the temperature behaviors of metal oxide (MOS) and electrochemical sensors with the CI 21. The alarm threshold is set at 200 ppm, and each of the sensors is calibrated to 200 ppm NH3 at 25ºC (77ºF). At lower temperatures, the response of the CI 21 is extremely accurate, whereas the MOS and electrochemical sensors drift considerably.
If calibration is performed at lower temperatures, the identification lines shift to a higher ppm indication. As temperatures increase, the CI 21 operates with the same reliability, whereas the MOS and electrochemical sensors indicate alarm conditions due to the higher slope of their indication lines.
Fluctuating humidity levels are no longer an issue with the CI 21. MOS sensors require a minimum humidity level in order to respond to leaks of ammonia, while the CI 21 does not!
Low humidity is a typical condition of refrigerated areas due to lower temperatures. With the CI 21, a direct calibration with ammonia test gas can be accomplished with low humidity. As shown in graph 2, the influence of humidity on the CI 21 is considerably less than MOS sensors.
MOS sensors typically interfere with other gases and are rarely specific. Cross interferences occur with alcohol, cleaning detergents, water, carbon monoxide, and many other substances. Interfering alarms become a nuisance that can lead to work stoppage and expensive shut downs.