Oxygen Depletion
Any atmosphere containing measurably less than 20.9 % of oxygen can be considered depleted. Humans should not be exposed during prolonged periods to concentrations below 19.5 %. The first symptoms and effects of oxygen depletion below normal concentrations and down to 14 % are an increased heart rate and tiredness. Between 14 % and 11 % the agility of mind and body diminish significantly, where as between 11 % and 8 % headaches, dizziness and unconsciousness occur quickly. Between 8 % and 6 % unconsciousness occurs in a few minutes and reanimation is only possible if it occurs rapidly. At concentrations below 6 % unconsciousness takes place almost immediately and severe brain damage and death are almost certain.

Applications include indoor air quality and personal safety. Particularly exposed to oxygen deficiency are sites like deep wells, exca-vations, sewers, tanks, confined spaces, mines, certain industrial plants, cold storage facilities for fruit, as well as sites where compressed gas or cryogenic liquids are present. O₂ depletion often happens in conjunction with the rise in concentration of other gases or gas mixtures, which may also be toxic, e.g. N₂, CO₂, CH₄, CO, H₂S, NH₃, etc. Alarm levels for oxygen depletion sensors are typically set to 19 % or 19.5 %.

Industrial Process Control
Many industrial processes exist where oxygen measurements are crucial, either at high or low concentrations:

• Waste incineration
• Yield improvement of chemical and petro-chemical processes
• Fermentation, bio-reactor process monitoring
• Inerting & purging
• Packaging of food & beverages
• Pharmaceutical manufacturing

Medical Technology
In health care oxygen and oxygen measurements are being used extensively. Many applications in oxygen therapy, pulmonary diagnostics, patient ventilation and critical care exist. Some of them are only accessible with an oxygen sensor providing high temporal resolution. In Pulmonary Diagnostics, O₂ and CO₂ are measured to analyze the gas exchange mechanism of the patient either for pathological diagnosis and treatment, or for sports medicine applications (ergo-spirometry). In Critical Care, a measurement of O₂ and CO₂ in the breath of a patient would enable to draw conclusions on the patients metabolism and thus optimize nutrition via infusion or Total Parenteral Nutrition, (TPN). Today, this is not possible and often patients are kept in critical care units and on ventilators for too long - at a cost of USD 2’700 per day. For both applications described, ^®microLGD gas sensors are the only possible solution for a main-stream measurement because competing gas sensors cannot match the performance at the required speed of operation.

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 oxygen depletion monitoring in safety applications the microLGDs’ most important advantages are its functional safety, especially crucial to avoid costly false alarms or un-noticed sensor failure, its long lifetime, as well as its low cost-of-ownership, as regular replacements or frequent re-calibration are not necessary. Current solutions using electrochemical cells do not offer functional safety and need frequent replacements (lifetime of about 1 year).

For industrial applications the microLGDs' high selectivity toward oxygen, its functional safety, as well as its measurement speed is important. Environments as well as measurement speeds are often challenging for E-Chem or paramagnetic sensors. In medical pulmonary diagnostics and critical care applications a high temporal resolution, as well as high accuracy is required in a main-stream measurement, which cannot be matched by gas sensor systems competing with IR Microsystems' microLGD at a similar price point. Additional advantages are functional safety, low cost-of-ownership, as well as the potential for optical fiber based, light and small systems.

Several tests were carried out to calibrate and evaluate the performance of the oxygen sensor. A climate chamber was used to specify the performance at varying temperatures.

Sensor Set-Up

• 760 nm diode laser, temperature stabilized
• Absorption path length of 16 cm
• Cable connector from sensor to electronics
• Standard electronics module
• Concentration output every 0.7 second (moving average over 2 seconds)
• Festo fittings (for polymer tubing 6 mm OD) for flow-through measurements

Operation Range
The sensor was tested for an operation between -40°C and +60°C, and 0 to 25 % O₂. Sensor Stability with Temperature Changes The sensors’ response to ambient temperature changes was tested in a climate chamber. Using an internal temperature correction, the sensor shows a flat response, as shown below. The measurement was carried out with a 5 second integration time and resulted in sigma s = 290ppm, and a peak-to-peak noise of 1800ppm with a mean O₂ value of 20.975 %.

Verification of the Calibration Linearity
The linearity of the oxygen sensor calibration was tested in alternating set oxygen gas concentrations in nitrogen with pure nitrogen gas, each time for two minutes. The set oxygen concentrations, represented by the numerical values in the graph below, are compared to the senor response (red curve).

Mid-Term Stability Test
The measurement in the following graph shows a mid-term measurement of ambient air for about 8 hours.

The table below compares the system performance at various integration times.

* The precision in “relative percent” is sigma in [ppm] divided by the experiments’ mean O2 concentration of 21 %.

Tests on IR Microsystems’ microLGD laser diode gas sensor have shown that stable, measurements of oxygen are possible. The sensor positions itself as an alternative to current detectors that have shortcomings in terms of stability, speed and lifetime. The microLGD O₂ sensor thus opens a new perspective on applications in medical technology, industrial safety and process control.