Ammonia is a toxic gas and a severe irritant to the respiratory tract. Several international standards exist that define the level of toxicity for humans. Generally, short-term exposure over 15 minutes needs to be limited to 25-35 ppm and the Time-Weighed-Average (TWA) over a 8-hour period should not exceed 25 ppm. Values for Immediate Danger to Life & Health (IDLH) are at 300 ppm. Concentrations of 1’500 ppm and more can cause fatal pulmonary oedema and ammonia becomes irritating to moist skin at about 10’000 ppm. The Lower Explosive Limit (LEL) for ammonia is 15 %. Depending on conditions and sensibility, humans can first notice the smell of ammonia anywhere between a few and 50 ppm. Accidental ammonia releases are far from being rare. A survey has shown that over a 5-year period in the 90ies a total of 40’000 people were evacuated as a result of roughly 600 incidents, whereas 250 led to a total of 1’400 injuries. This shows the critical need of reliable and effective ammonia monitoring in the industry sectors mentioned below.

Industrial Applications
Refrigeration: Due to its interesting thermo-dynamic properties ammonia has been used for decades in industrial style refrigeration. Apart from its toxic properties in case of an accidental release, it is considered to be efficient, economical and environmentally friendly because it does not deplete the ozone layer or contribute to global warming, which is not the case for most other refrigerants. Applications include:

• Cold storage
• Food manufacturing, processing and preservation
• Beverage production
• Ice rinks

Chemical Industry: Today’s ammonia production of more than 100 million tons per year principally uses hydrocarbons as an educt in a catalyzed process.

Applications include:

• Fertilizer production (more than 80% of the produced ammonia)
• Explosives production
• Plastic manufacturing
• Pharmaceutical products

Agriculture and Livestock Farming
Most ammonia emissions result from metabolic activities related to bacteria in livestock farming of poultry, cattle and pigs. Ventilation in the stables must be managed in order to avoid long-term over-critical exposure of the animals to ammonia, causing stress, pour health and reduced productivity. In poorly ventilated stables the health of the personnel also can be endangered. At the same time, ventilation and heating must be minimized/optimized to safe energy while keeping temperatures at an adequate level, especially when young livestock is present. Fertilization in intensive agriculture is another source of ammonia.

Emission Control
While NO_x reduction by 3-way catalysts has found a lot of attention in environmental protection, there is a growing awareness to reduce significant NH3 emission from fuel-rich operated thermal engines. Ammonia contributes to poor air quality by fostering the formation of noxious dust-size airborne particles.

Heat & Power Generation: Gas and oil power & heat plants, as well as stationary diesel engines for power generation use selective catalytic reduction (SCR) to reduce NO_x emissions. In this process an aqueous urea solution is injected to react with the exhaust gas. Ammonia slip occurs when over-stoichiometric amounts of urea are present. NH₃ detection in mobile diagnostics and engine management can be used to optimize the catalytic process, reduce NO_x and NH₃ emission, increase catalyser life span and reduce urea injection.

Truck Emission Control: Compliance with 2005 (Euro IV) and 2008 (Euro V) emission standards for heavy-duty trucks will require exhaust after-treatment. Selective Catalytic Reduction (SCR) to reduce NO_x emission has been adopted for mid- and heavy-duty trucks by several major manufacturers. In order to optimize performance and to reduce cost, the ammonia slip related to the necessary urea solution injection may need to be monitored, as NH₃ levels in transient regimes reach up to 50 ppm.

Medical Applications
Ammonia is produced in small quantities by the human body. The normal level can be altered as a result of kidney disorder or ulcer, where urea conversion leads to presence of ammonia in exhaled air. A measurement of ammonia on the 50-100 ppb level would allow fast and non-invasive diagnostics.

IR Microsystems’ ^®microLGD gas sensors are stand-alone or ready-to-use OEM subsystems for selective detection and monitoring of gases. The sensor is based on a technology called “TDLS” – or Tunable Diode Laser Spectrometry, which has proven its validity in high-end laboratory and process control applications. It uses a laser to scan the specific absorption lines of a target gas with an extremely high resolution, which enables a precise measurement of the gas concentration without any cross-sensitivity. IR Microsystems proprietary approach to TDLS leverages this technology to low-cost, high-volume gas detection & monitoring applications: The use of low-cost telecom-type laser diodes as light source, combined with IR Microsystems intellectual property of reference channel-free devices, reduces the gas sensor to a set of generic components and enables significant cost synergies across applications.

The microLGD gas sensor technology brings a competitive solution to most of the drawbacks of current sensors:

• Extremely high selectivity to the target gas
• Functional safety, continuous status reporting
• Long lifetime (10+ years)
• Fast response times
• Low power consumption possible
• Very low cost-of-ownership (no regular replacement and/or calibration)
• Low cost of the gas sensor through excellent scaling costs of the components
  

For industrial applications the microLGDs’ high selectivity toward ammonia and functional safety are especially crucial to avoid costly false alarms and un-noticed sensor failure. Traditional sensing solutions typically offer neither. In the case of electrochemical cells the presence of under-critical ammonia concentrations can lead to premature ageing and failure of the cell, leaving a site without protection. The microLGD offers the additional advantage of low cost-of-ownership as regular replacement or frequent re-calibration is not necessary.

The following shows typical test data of the ^®microLGD Ammonia Sensor in terms of detection limit, repeatability, drift and stability against temperature variations for an operation between -40°C and +40°C, and 0 to 1,000 ppm NH₃.

For a better control of the set concentration values, the ^®microLGD Ammonia Sensor was operated in a “flow-through” configuration.

The span data shows an extreme linearity over the entire range of concentration, and the calibration is highly stable, even after 11 weeks of operation.

Detection Limit
The extremely low RMS noise of 150 ppb gives an accuracy of 300 ppb, and a detection limit of 500 ppb at 3s, or a detection limit of 1 ppm at 6s.

Repeatability
The figure below shows a perfect repeatability

Stability & Temperature Drift
Short-term stability (80 minutes) at 20 ppm yields a reading of (20.1  +-0.45) ppm, corresponding to an extremely low RMS noise of 0.15 ppm.

The zero-concentration temperature drift (13.5 hours, see insert in above figure) was recorded under a variation of ambient temperature from +40°C to -40°C, yielding a reading of (-0.3 +-2.5) ppm, with a RMS noise of only 1 ppm.

High Concentration Temperature Drift
A similar test at 100 ppm ammonia concentration (temperature variation from +40°C to -15°C over a period of 6 hours) gives a reading of (101.4 +-2.5) ppm an a RMS noise of 0.8 ppm.

Tests on IR Microsystems’ microLGD laser diode gas sensor have shown that stable, low ppm range measurements of ammonia are possible. This sensor positions itself as a real alternative to current electrochemical detectors that have serious shortcomings in terms of stability, lifetime, cross-sensitivity and speed.

The microLGD NH₃ sensor thus opens a new perspective on applications in agriculture, industrial safety, automotive and medical technology