Selecting the right ammonia sensor requires an understanding of the monitoring environment as well as the specific benefits and limitations of the various sensor types.
Almost anyone who has used ammonia sensors in refrigerated areas is aware of the limitations of using sensors in such a challenging environment. Temperatures can range from -40°F (-40°C) in some flash-freezing operations to 104°F (40°C) during defrost periods. Strong jets of water, steam and detergents used during washdown procedures can damage sensors and other system components. Interfering contaminants f rom propane-powered forklifts, alcohol-based disinfectants, or even the products being stored or manufactured can have an adverse effect on sensor performance. Additionally, different types of ammonia sensors have their own inherent limitations. The secret to choosing and installing a trouble-free system is to understand the limitations of the sensors and choose the correct sensors for the locations and conditions in which they will be operated.
The four most common types of fixed sensors used to detect ammonia vapor are infrared, chemosorption (frequently referred to as “MOS” sensors), electrochemical and solid-state charge-carrier-injection sensors.
Non-dispersive infrared (NDIR) sensors measure gas as a function of the absorbance of infrared light. Molecules consist of atoms that are held together by chemical bonds. The bonds in a particular type of molecule (like ammonia) absorb energy at specific wavelengths. When a chemical bond absorbs infrared light, it continues to vibrate at the same frequency but with greater amplitude after the transfer of energy. In other words, molecules that are able to absorb energy at that wavelength are heated to a higher temperature than molecules that are not able to absorb light at that wavelength.
When infrared radiation passes through a sensing chamber containing a specific contaminant, the only wavelengths that are absorbed are the wavelengths that correspond with the chemical bonds in that contaminant. The rest of the light is transmitted through the chamber without hindrance. Because most chemical compounds absorb at a number of different frequencies (wavelengths), infrared absorbance can provide a “fingerprint” for identifying unknown contaminants. Alternatively, for some molecules, it might be possible to find an absorbance peak at a specific wavelength that is not shared by other molecules likely to be present. In such instances, absorbance at a particular wavelength can be used to provide a substance-specific measurement for a specific molecule.
For example, ammonia has a usable absorbance peak at a wavelength of about 1.53 micron Absorbance at this wavelength is proportional to the concentration of NH3 present in the sensor’s sensing chamber. The absorbance is not linear per concentration unit, but it is mathematically predictable and easily calculated by microprocessor-equipped portable gas detectors.
Optical filters or “choppers” are used to limit the wavelengths of the infrared light passing through the sensing chamber to the specific frequencies at which absorbance by the target molecules occur. Some NDIR detectors use “thermopile” detectors to measure the amount of infrared light absorbed at specific wavelengths. Other “photo-acoustic” infrared detectors use a microphone to measure pressure changes in the sensing chamber due to the absorbance of infrared light. In this case, the microphone is used to measure pressure changes in the sensing chamber due to the heating effects of the absorbance of infrared energy by the ammonia molecules.
The chief benefits of infrared sensors are their high specificity to ammonia, their long-term stability and their reduced need for calibration adjustment compared to other sensor types. Infrared detectors have a wide dynamic range and are not degraded or consumed by exposure to high concentrations of ammonia. Their chief limitations are the large physical size of the detector assembly, the need to protect the detector against the potential effects of fluctuating temperature and humidity, and their higher cost compared to other detector types.
Chemosorption sensors consist of a metal-oxide semiconductor (MOS) such as tin dioxide (SnO2) on a sintered alumina ceramic bead contained within a flame arrestor. In clean air, the electrical conductivity of these sensors is low. Oxidation of the measured gas on the sensing element increases conductivity. An electrical circuit is used to convert the change in conductivity to an output signal that corresponds to the gas concentration. Sensitivity to a particular gas is alterable by changing the temperature of the sensing element.
Chemosorption sensors generally are designed to respond to the widest possible range of toxic and flammable gases and vapors. The idea is to provide a “broad range” response to the presence of contaminants. These sensors are able to detect chlorofluorocarbon refrigerants and other contaminants that are difficult to detect by other means as well as ammonia, carbon monoxide, hydrogen, alcohols and many other gases and vapors. This nonspecificity can be advantageous in situations where unknown toxic gases might be present and a simple go/no-go determination is sufficient. However, because the sensors are not ammonia-specific, they can generate false alarms if they are installed in environments that are subject to the presence of interfering contaminants.
The chief benefits of chemosorption sensors are their long operational life and low cost. These sensors are the least expensive type of fixed ammonia sensor, and they are not damaged or consumed by chronic exposure to ammonia. Chemosorption sensors can be used to detect ammonia from concentrations as low as 30 ppm and as high as the flammable range. However, because their output signal is highly non-linear, they need to be calibrated and adjusted for use in the desired range. Adjustment for use in high-concentration environments reduces accuracy and resolution.
The chief limitations of chemosorption sensors are the difficulty in interpreting readings, the potential for false alarms and the effects of humidity on the sensor. As humidity increases, the sensor output increases. As humidity decreases to low levels, the sensor output might fall to zero even in the presence of the contaminant being measured. The extremely low humidity seen in many refrigerated areas (especially flash freezers) can effectively make it impossible to use this type of sensor. In addition, the user must exercise caution in making assumptions about other contaminants that might be present in the area being monitored.
The best use for chemosorption sensors is as a go/no go alarm device that is adjusted for maximum accuracy near the hazardous condition or “take action” threshold. These sensors also can be used to measure refrigerant gases such as Freons that are difficult or more expensive to detect by other means.
Substance-specific electrochemical (EC) sensors are offered for many of the most common toxic gases. EC sensors are compact, require little power, exhibit good linearity and repeatability, and are comparatively less expensive. The detection technique is straightforward: Gas that enters the sensor undergoes an electrochemical reaction that causes a change in the electrical output of the sensor. The difference in the electrical output is proportional to the amount of gas present. EC sensors are designed to minimize the effects of interfering contaminants, making the readings as specific as possible for the gas being measured.
In the case of ammonia sensors, the electrolyte includes an active ingredient that is consumed in the electrochemical reaction used to detect the ammonia. Thus, sensor lifespan is directly related to the sensor’s exposure to NH3. Generally, lifespan is listed in parts-per-million (ppm) exposure hours. For instance, a sensor rated at 17,520 ppm hours would have a lifespan of one year when continuously exposed to a constant concentration of 2 ppm of ammonia (2 ppm x 365 days x 24 hours = 17,520 ppm hours). The same sensor would last only six months if continuously exposed to 4 ppm, three months when exposed to 8 ppm, etc.
There are several variations on the specific electrochemical reaction used to detect ammonia. Some EC sensors depend on a straightforward oxidation reaction, where ammonia (NH3) is converted into nitrogen (N2) and hydrogen protons (H+) at the sensing electrode. For every two molecules of ammonia that are oxidized, six electrons (e-) of electricity also are produced. This electrical current output is what the instrument uses to determine the concentration of ammonia present. The second half of the electrochemical reaction occurs at the “counter” electrode, where the hydrogen protons produced in the first half of the reaction react with oxygen to produce water.
The sensor is filled with an organic gel electrolyte mixture in which the reaction occurs. Active ingredients in the electrolyte are incrementally used up as the sensor is exposed to ammonia. Once the “ppm hour” exposure life of the sensor is exceeded, the sensor no longer is capable of detecting gas and needs to be replaced. The benefits of this type of sensor include good low-range resolution and good cold temperature performance down to -40°F (-40°C).
In other EC sensors, a patented set of “pH shift sensitive” electrochemical reactions is used to detect ammonia. Once again, the electrical output of the sensor is proportional to the amount of ammonia oxidized at the sensing electrode. These sensors are based on a hydrous electrolyte system that, while slightly less able to withstand cold temperatures, can provide extended measurement ranges of up to 5,000 ppm, with exposure life of up to 50,000 ppm hours.
Electrochemical ammonia sensors should be used only when the normal ambient background concentration of ammonia is sufficiently low to allow a reasonable operational life. For example, this type of sensor would not be recommended for use at a poultry farm or nitrate fertilizer plant where the routine ambient concentration of ammonia might be as high as 20 to 30 ppm. In this environment, the lifespan of the sensor could be a matter of weeks. However, EC sensors are preferred in many environments due to their high specificity to ammonia and low cross-sensitivity to other interfering contaminants such as carbon monoxide that might be present.
Charge Carrier Injection Sensors
Charge carrier injection (CI) ammonia sensors are based on a different detection principle. CI sensors depend on the adsorption of ammonia by “charge carrier” molecules in a solid-state substrate. By absorbing ammonia, the charge carriers are “injected” into the sensor element, causing a change in resistance that is proportional to the concentration of ammonia present.
The materials, morphology and substrate layering are selected carefully to maximize the affinity of the sensor to ammonia while minimizing the effects of interfering contaminants. CI sensors are able to be used over a wide detection range, from low parts per million concentrations to 30,000 ppm or higher. Because the sensing element is operated at an elevated temperature, the sensor functions as its own “heater” and is well-suited for operation in flash freezers and other environments subject to extreme cold (-40°F or colder). Because the detection principle is not based on the oxidation of ammonia, the sensor also can be used to measure ammonia in oxygen-free environments. Most importantly, the sensor is not consumed or permanently altered by exposure to ammonia. In the presence of ammonia, the charge carriers are injected into the sensor element; in fresh air, the ammonia is desorbed.
CI sensors are stable and typically have an operational lifespan of five years or more. They are not affected by shifts in humidity, and they can offer good performance in the extreme low humidity associated with flash-freezing operations.
In conclusion, no single type of ammonia sensor is perfect for all applications. The key to choosing the right sensor lies in understanding the monitoring environment as well as the specific benefits and limitations of the various sensor types.
The Importance of Ammonia Monitoring
Anhydrous ammonia (NH3) is widely used as a coolant in large industrial refrigeration systems. In fact, the use of ammonia as a refrigerant (R717) has increased substantially over the past several years as a replacement for environmentally unfriendly chlorofluorocarbon refrigerants. While ammonia refrigeration has long been a standard in the food and beverage industry, it also is now found in pharmaceuticals production, in air-conditioning equipment for many public buildings, and in electric power generation plants.
Despite its increasing popularity, ammonia is regarded as a highly hazardous chemical - and rightly so. According to the European Environmental Agency (EEA), the worst industrial accident of the last 20 years occurred on September 21, 2001, when an ammonia/ammonium nitrate explosion at a fertilizer plant in Toulouse, France, killed 30 and injured more than 2,000 workers and nearby residents. According to the government investigation, as horrific as the accident was, it could have been much worse if intervening buildings had not broken the force of the explosion, preventing the potential detonation of 20 more railroad tank cars full of anhydrous ammonia.
Smaller incidents are extremely common. A study published by the State of New York Department of Health illustrates just how commonly these accidents occur.1 The report documents 107 serious ammonia spills that occurred in New York State during 1993 through 1998. Sixty-one people were seriously injured and one person killed in the reported accidents. Equipment failure caused 58 percent of the releases, and most of the releases involved piping (44 percent).
Ammonia is a highly toxic gas and also is corrosive to the skin, eyes and lungs. Proper safety-monitoring procedures and equipment must be in place at all times to avoid serious accidental injury or death. The most widely recognized exposure limits for ammonia are an 8-hr time weighted average (TWA) of 25 ppm, with a 15-min short-term exposure limit (STEL) of 35 ppm. Exposure to 300 ppm is immediately dangerous to life and health. Fortunately, ammonia has a low odor threshold (20 ppm) with good warning properties, so most people seek relief at much lower concentrations.
Besides its toxic properties, ammonia also is an explosively flammable gas, with a lower explosive limit (LEL) concentration of approximately 15 percent volume. Although ammonia vapor is not flammable at concentrations of less than 15 percent, it can explode or catch fire throughout its flammability range of 15 to 28 percent by volume. Ammonia contaminated with lubricating oil, however, can catch fire or explode at concentrations as low as 8 percent.
In the United States, the use of monitoring instrumentation to provide early warning of releases of highly hazardous chemicals such as ammonia is required under 29 CFR 1910.119, “Process Safety Management of Highly Hazardous Chemicals.