Both for reasons of safety and to prevent interruption of refrigeration service, it is important to sense leaks of refrigerant. Since halocarbons are odorless, sensors are important in installations using such refrigerants to ensure that a leakage of a major portion of the refrigerant charge does not go undetected. Also, leakage of large quantities of a halocarbon represents a danger to humans who are not aware of being engulfed in high concentrations. When an ammonia leak occurs where people are present, the odor of ammonia provides its own warning. But for unoccupied spaces, such as refrigerated storage areas and unattended machine rooms, automatic sensors are crucial to providing an early warning of a potential problem.
The principles of operation19 of the most popular ammonia sensors are: (1) detector tubes, (2) solid state, (3) electrochemical, and (4) infrared (or ultraviolet). The concentrations of ammonia and the applicable temperature ranges of these types of sensors are enumerated in Table 13.2.
The most widely used concept for sensing ammonia is the solid-state type because of its relatively low cost and ease of installation and maintenance. The solid-state sensor works well for initiating alarms in the range of ammonia concentration of several hundred ppm, but it is normally not very precise in the range of 20 to 50 ppm, which might be of interest for personnel exposure regulations. One experience report suggests that solid-state sensors employed
in low-temperature spaces should be allowed several weeks to age before taking seriously the indications from the sensor.