Anyone who has used ammonia sensors in refrigerated areas is aware of their shortcomings. Temperatures in refrigeration plants can range from –22 to +122°F during defrost periods, significantly shortening a sensor’s life. Various factors in the work place, such as cleaning detergents or processing equipment, like inkjet printers, may cause the sensor to respond and can lead to unnecessary and costly false alarms. To avoid false alarms and protect personnel and property, a sensor that can withstand the harsh demands of a refrigeration plant is essential. These requirements have led to the development of the new CI 21 Transmitter.

Cooling Medium R717, Ammonia
The use of ammonia as a refrigerant (R717) has increased substantially in recent years. As an alternative to using fluorinated and chlorinated hydrocarbons for refrigeration, ammonia is not ozone-reducing and it is less costly than other cooling mediums. Along with these advantages, however, there are also disadvantages. For instance, ammonia is a colorless, toxic gas that is lighter than air and is explosive at concentrations above 15% volume. Due to its high solubility in water, ammonia will cauterize the respiratory tracts and cause death at concentrations of 5,000 ppm. An undetected ammonia leak may endanger lives as well as cause expensive food spoilage or production failure.

Temperature Influence
The new Charge Carrier Injection (CI) sensor is the solution to temperature fluctuations. Utilizing a controlled sensor voltage, a constant temperature in the sensor is maintained throughout the whole temperature range in the refrigeration plant. This, combined with a sensor that is less sensitive to temperature variations in general, gives the CI 21 extremely good stability.

Chart 1 shows the temperature behavior of an electrochemical sensor, a conventional semiconductor, and the newly designed CI sensor from GfG. The alarm threshold is set at 200 ppm of ammonia. The sensors are calibrated with 200 ppm of ammonia at 77°F. At a temperature of –22°F, the CI 21 displays 180 ppm when supplied with 200 ppm of ammonia. A conventional semiconductor in the same conditions will display only 110 ppm of the 200 ppm applied. Although the original measurement value changes during cooling, the alarm threshold value remains the same, meaning that the alarm responds much too late. Similar behavior is observed when ammonia is applied to an electrochemical sensor.

If the calibration is performed at a lower temperature, such as –22°F, each sensor shifts to a higher ppm indication. If temperatures increase during defrost periods, the CI 21 operates with the same reliability. The semiconductor and electrochemical sensors will indicate an alarm at concentrations below the alarm threshold due to the higher slope of the indication line.

The CI 21 is very reliable over a wide temperature range.

Picture 1: Behavior of sensors with respect to temperature after calibration with 200 ppm at 50% r.M. (all sensors are without temperature compensation)

Humidity Influence
Sensor stability has not only been improved for temperature fluctuations, but also for changing humidity levels. Conventional semiconductors require a minimum humidity level in order to respond to ammonia. Humidity and oxygen are required for the sensor surface to absorb ammonia and initiate a signal.

The CI 21 is no longer dependent on humidity. In contrast to a conventional sensor, which requires hydrogen as a relief gas, the CI 21 can perform a direct calibration with ammonia as a target gas in low humidity. Low humidity is, of course, present at low temperatures, such as in refrigeration plants.

As shown in diagram 2, the CI 21 is considerably less influenced by fluctuations in humidity than other sensors. As with the electrochemical sensor, stability and accuracy are inherent.

Picture 2: Influence of moisture

Sensor Selectivity
Semiconductors typically work with broad cross-sensitivities to other gases and can rarely be more narrowly defined. They not only indicate the presence of ammonia but also components such as water, alcohol, organic detergents, and others. This cross-sensitivity can often lead to costly false alarms. In one instance, a simple painting and varnishing job caused an alarm and an expensive visit from the fire department. Battery charging, such as for fork lifts, releases hydrogen, which can exceed alarm thresholds on an ammonia sensor at levels under 200 ppm of hydrogen.

In Diagram 3, cross-sensitivities of conventional sensors and the CI 21 are plotted on a logarithmic axis. As a point of reference, the first alarm threshold, 200 ppm, is shown. The CI 21 shows a dramatic improvement in cross-sensitivity to hydrogen. With 200 ppm of hydrogen applied, the CI 21 only indicates 2 ppm of ammonia. The conventional semiconductor indicates 225 ppm of ammonia and sets off the alarm. Cross-sensitivities to alcohol and detergent are also significantly reduced.

Picture 3: Cross-Sensitivities

Results
The new CI 21 is available with a measuring range of up to 10,000 ppm. Life and property can be protected with a level of accuracy and reliability unavailable until now. The many advantages of the CI 21 include reasonable acquisition costs and a low cost of ownership. Costly false alarms are nearly eliminated due to low cross-sensitivity and temperature and humidity influence. Furthermore, the CI 21 calibrates better than other sensors because even at temperatures below 32°F, the sensor can be calibrated with ammonia. No hydrogen is needed as a relief gas. Since the variation in response is negligible, ammonia can be monitored even if the surrounding temperature varies frequently.

The new CI 21 transmitter is a progressive development to which all other ammonia transmitters will be compared. The CI 21 is the new technical standard.

By Lambert Gonschorek