Frequently Asked Questions:
Advantages offered by AGS™ Solid State technology
A constant status check confirms the sensor is functioning. That feature eliminates the hassle, expense and liability of routinely bump testing to look at the sensor as is required with every other portable monitor.
No calibration is necessary. We guarantee our sensor will hold its calibration for five years. If it should wander in that time, we will replace it without charge.
We offer the industries only five year warranty. Every part of the monitor is warranted for five years, except the AA batteries that power it.
The monitor self manages. Training is reduced to sliding on the switch and waiting a few seconds for a green status light. The green status light is confirming the sensor is functioning, the electronics are good and the sensor has a good "zero" – all automatically.
We self "zero". Contamination, for example, is probably the most common problem in sensors – resulting in frequent need to recalibrate in older technologies. Our monitor will burn off most contamination and return itself to zero – automatically. During that time the monitor will make you aware there is a problem. Simply make sure it has good batteries and place the monitor in clean air for a while – no re-calibration necessary.
All sensors, including ours, can be overwhelmed by some contaminants – holding a monitor in the exhaust of a vehicle, for example – or spraying with alcohol or silicone based cleaners. Again, our monitor will tell you it has been damaged when you turn it on – you don't need to bump test to find that out. In most cases it will clean itself, but, like any sensor, it can be overwhelmed by mishandling.
Our technology stores without degrading. A specially designed case allows atmospherically neutral storage. That ability will eliminate a large percentage of the problems other monitors experience from degradation and contamination while not being used.
The AI-1201 dual gas model is an exceptional value at $515.00. It is constantly looking for carbon monoxide and flammable gases. We calibrate the LEL side of the monitor to methane since that can also be a silent gas, but it will "see" most of the hydrocarbon gases like; gasoline, natural gas, propane, hydrogen, acetylene, etc. The ability to see if the ambient air is saturating with an explosive gas and monitor the trend can be lifesaving for first responders and others working around those gases.
Airspace monitors are not disposables. A new sensor can be installed every five years. The cost is presently $150.00. That feature brings costs down even more dramatically.
Carbon Monoxide (CO) is a tasteless, colorless, odorless gas that causes headaches, disorientation, nausea, and death, even in very low concentrations. Often misdiagnosed as symptoms that mimic the flu, it is the leading cause of accidental poisoning deaths in the United States and throughout the World.
Carbon Monoxide poisons by inhibiting the blood’s ability to carry oxygen to body tissues including vital organs such as the heart and brain. When a person breathes, oxygen in the lungs combines with hemoglobin in the blood and travels to the body’s cells. When CO is inhaled, it tightly binds with the oxygen carrying hemoglobin of the blood, forming carboxyhemoglobin. Once combined with the hemoglobin, oxygen is replaced and the oxygen-carrying capacity of the blood is reduced. How quickly the carboxyhemoglobin builds up is influenced by three main factors: (1) the concentration of the gas being inhaled (measured in parts per million or ppm), (2) how long the exposure lasts, and (3) rate of respiration and circulation (affected by workload, temperature, altitude, and one’s health).
Adding to the effects of the exposure is the long half-life of carboxyhemoglobin in the blood. Half-life is the measure by how quickly levels return to normal. The half-life of carboxyhemoglobin is approximately 5 hours. For a given exposure level, it will take about 5 hours for the level of carboxyhemoglobin in the blood to drop to half its current level after the exposure is terminated. The following chart shows the maximum allowable exposure limits and symptoms developed for CO inhalation.
* Exposure to CO will have varying effects depending upon the person (size, age, sex, and health) and the environment (temperature and altitude).
Like Carbon Monoxide, Methane (CH4) is a colorless, odorless gas with a wide distribution in nature. It is the principal component of natural gas, a fossil fuel. It is released into the atmosphere when organic matter decomposes in environments lacking sufficient oxygen. Natural sources include wetlands, swamps and marshes, termites, and oceans. Man-made sources include the mining and burning of fossil fuels, digestive processes in ruminant animals such as cattle, rice paddies, and the burying of waste in landfills.
At room temperature, methane is a gas less dense than air. It melts at -183°C and boils at -164°C. It is not very soluble in water. Methane in general is very stable, but mixtures of methane and air, with the methane content between 5 and 15% by volume, are explosive. Unlike Carbon Monoxide, Methane is not toxic when inhaled, but it can produce suffocation by reducing the concentration of oxygen inhaled.
Home CO Detectors
Early home CO detectors were found to be "too" sensitive to CO and triggered nuisance alarms. To address this situation, Underwriters Laboratory (UL) required home CO detectors meet a characteristic standard UL 2034. The most recent being UL2034-98. CO detectors for home use must meet UL2034-98. This requirement effectively "desensitizes" the response to CO by utilizing a formula called Time Weighted Averaging or TWA over an 8-hour period.
For example, if the alarm is exposed to 100 ppm for 1 hour:
(1 hour x 100ppm + 7 hours x 0 ppm) / 8 hours = 12.5 ppm
or, if the alarm is exposed to 30 ppm for 4 hours and 200 ppm for 4 hours:
(4 hours x 30ppm + 4 hours x 200 ppm) / 8 hours = 115 ppm
The formula is basically taking the accumulated reading divided by 8 hours.
This is an important consideration when comparing the reading of a personal CO detector to that of a home unit.
The Airspace Monitor gives an instantaneous CO reading with no averaging. The immediate reading is the priority when the user needs to be informed of the presence of lethal CO!
Airspace AGS TechnologyTM Theory of Operation
There are a number of basic gas-sensing technologies used in the detection lethal gases. These include: selective chemical detection (or "bio-mimetic"), electrochemical, semiconductor, non-dispersive infrared (NDIR) optical sensor, and Hot-wire (Catalytic Element). Due to the cost and power requirements, NDIR and Catalytic Element technologies are typically not used in portable applications. Another technology, chemical stick-type “spot” detectors, is not considered to be fast or accurate enough for personal protection devices. Airspace AGS TechnologyTM (AGS) addresses the limitations of the existing technologies. To understand the advantages of AGS, the following is a review of these existing technologies and an explanation of AGS.
Principle of Operation:
Biomimetic means, "to mimic Life”. This sensor technology attempts to monitor the amount of Carbon Monoxide (CO) in your body. A chemical tablet absorbs and releases CO just as the human body does. As the tablet absorbs CO, it darkens in color. An LED and detector monitor the color change and trigger an alarm at a preset color. Note that the amount of CO in the immediate environment is not the important factor. To be accurate, the person being monitored must carry the unit at all times. Once triggered, the unit must be “zeroed” by exposing it to fresh air for several hours.
Low cost, simple, simulates human response.
Slow reacting and inaccurate. Dangerous ambient conditions are not detected quickly due to inherent time-weighted averaging. The unit must be on the person at all times to accurately mimic hemoglobin response. The sensors have a limited service life which is reduced by extended exposure to CO.
Principle of Operation:
Electrochemical sensors generate an electrical current proportional to the CO molecules interacting, via catalytic reaction, with an acid-based electrolyte. The electrodes and electrolyte are located behind a gas-permeable membrane. The basic construction consists of three platinum electrodes: sense or working electrode (WE), counter electrode (CE), and the reference electrode (RE) soaking in a chemical solution. The platinum electrode is a catalytic metal to CO as it catalyzes the oxidation of CO to CO2. The chemical solution, or electrolyte is a non-metallic liquid that conducts electricity, usually through acids or dissolved salts.
The reference electrode is isolated from any reaction. Its thermodynamic potential is always the same and remains constant. When CO reacts with the solution, it creates an electrical charge in the WE electrode. The counter electrode functions solely as the second half-cell and allows electrons to enter or leave the electrolyte. The current generated is proportional to the amount of reactant gas present. The potential difference between the sense and reference electrode is translated into a CO reading. The chemical reaction of CO oxidizing on a platinum sense electrode is:
CO + H2O => CO2 + 2H+ + 2e-
The counter electrode acts to balance out the reaction at the sensing electrode by reducing oxygen present in the air to water:
1/2 O2 + 2H+ + 2e- => H2O
Varying the electrolyte solution, catalyst material, or applying a bias to the CE can make the sensor more selective. Similar reactions allow for the electrochemical detection of a variety of reactant gases including hydrogen sulfide, sulfur dioxide, chlorine, hydrogen cyanide, nitrogen dioxide, and hydrogen. The CO sensor is typically equipped with a selective external filter. This filter removes potentially interfering gases before they reach the working electrode
Electrochemical sensors have the advantage of linear output, low power requirements, and good resolution. The output signal can be sampled continuously, though the overall response time is similar to semiconductor technology.
Electrochemical sensors have a limited service life. They can be viewed as small “fuel cells” that are constantly reacting with the environment. A two-year service life is typical. Gas exposure, elevated temperature, and humidity will shorten the life.
As the sensor deteriorates, sensitivity decreases and the sensor becomes unstable. When the electrolyte is used up, they must be replaced. Replacement sensors are expensive, more than $200. The sensor must be continuously calibrated to compensate for the electrolyte loss. A monthly calibration is typically recommended and more often if the sensor was exposed to temperature or gas-level extremes. It is possible to have an expended sensor read “zero” in a potentially dangerous environment.
The sensors have a limited shelf life and are typically supplied in airtight bags to prevent premature wear. The electrode leads are sometimes tied together with shorting bars to prevent a potential from developing. These must be removed prior to installation.
Temperature, humidity, and pressure cause the chemical reaction to change and must be compensated for. The sensor can become ineffective at low temperatures as the electrolyte may freeze. As noted in the chemical reaction equation above, oxygen is required for the sensor to operate.
Another limitation of electrochemical sensors is the effects of interfering contaminants on toxic gas readings. Since electrochemical sensors are based on chemical reactions, it is always possible to have certain compounds react very similarly. That's why some electrochemical sensors can be very specific like oxygen, (not many gases react like it) and others are less specific (more cross sensitive to a family of acid gases or oxidizers). Even though care has been taken to reduce cross-sensitivity, some interfering gases may still have an effect on toxic sensor readings. In some cases the interfering effect may be "positive" and result in readings which are higher than actual. Other times, the interference may be “negative” and produce readings which are lower than actual.
Semiconductor (MOS) Sensors
Principle of Operation:
Metal Oxide Semiconductor (MOS) sensors employ a heated mixed metal (iron, zinc, tin) oxide bead, contained within a flame arrestor, that is chemically-doped to selectively burn CO on its conductive surface. The combustion of CO on the sensor's surface will substantially decrease the resistance of the sensor and this change is proportional to the concentration of CO near the sensor bead. This behavior can be fitted either to a logarithmic curve, or one, which varies, as the square root of the concentration of the target gas. The resistance change is translated into a gas concentration value. After a reading is taken, the sensor cleans itself by heating the surface and burning off the CO that is there. This prepares the surface for the next reading cycle.
The electrical resistance of the sensor material depends upon the temperature, and also on the chemical composition of the surrounding atmosphere. When heated, metal oxides change their resistance as the oxygen concentration in the atmosphere changes. This is because the basic conductivity of tin dioxide, absorption of oxygen, reaction between gases and surface oxygen are based on the thermodynamic principle. The equation for this reaction is:
2e- + O2 => 2O-
O- + CO => CO2 + 2e-
MOS sensors may be used for toxic as well as combustible gas monitoring. In clean air the electrical conductivity is low, while contact with reducing gases such as carbon monoxide or combustible gases increases conductivity. Changing the temperature of the sensing element will alter the sensitivity of the element to a particular gas Adding noble metal doping to the tin-dioxide greatly enhances the selectivity while decreasing the operating temperature. There are a variety of different types of MOS sensors that can be used for Lower Explosion Limit (LEL) monitoring of flammable hydrocarbons, ppm level of toxic hydrocarbons, and a variety of other toxic gases such as carbon monoxide, hydrogen sulfide, refrigerants, and ammonia. The CO sensor is typically equipped with a selective external filter. This filter removes potentially interfering gases before they reach the sensor surface.
In addition to the bead type element, thick-film technology and thin-film technology are used to reduce power or increase sensitivity.
MOS sensors are physically small, rugged, and lightweight. They provide long operational life (well in excess of 5 years), without the need for routine replacement. MOS sensors offer the ability to detect low (0 – 100 ppm) concentrations of toxic gases over a wide temperature range. MOS sensors are fast responding and low cost. MOS sensors can be made specific by characterization, temperature point, chemistry and filtering techniques. The sensing mechanism lends itself to a very simple circuit interface.
The output of MOS sensors need to be linearized and require signal characterization. MOS sensors are sensitive to temperature and humidity. As humidity increases, sensor output increases as well. As humidity drops to very low levels, sensor output may fall as well. The humidity effect is mostly compensated for by the temperature correction. The temperature-controlled heater requires power. The chief limitations concerning use of this kind of sensor are the difficulty in the interpretation of positive readings, the potential for false positive alarms, and the effects of humidity on sensor output. Some cleaning solvents and other chemicals are known to give false signals. As with electrochemical sensors, sufficient oxygen is required for the sensor to operate.
Airspace AGS TechnologyTM
Airspace Monitoring Systems is the developer and manufacturer of innovative gas monitors utilizing Advanced Gas Sensor (AGS) TechnologyTM. AGS is based on the latest state-of-art semiconductor gas sensor technology. Patent-pending design innovations incorporating features such as precise heater drive, gas characterization, temperature/humidity compensation, and an accurate multi-point calibration provide an affordable and effective solution to gas sensing requirements. AGS provides a wide range of gas sensors offering high quality, stability, and reliability. Typical sensor lifetimes are guaranteed for 5 years.
Airspace engineers investigated the advantages and disadvantages of each of the prevalent sensor technologies. It was decided to incorporate the best features of each technology to develop the optimal overall sensor design. The solid-state technology was chosen as the core technology due to its long life, fast-response, good sensitivity, low cost, and small size. Overcoming some of the disadvantages of the solid-state sensor included a unique heater power control design, extensive sensor characterization, and addressing intrinsic safety concerns. The result is the Airspace patent-pending AGS TechnologyTM. AGS TechnologyTM addresses the power consumption, temperature/humidity, linearization, selectivity, and intrinsic safety concerns with solid-state sensors.
Airspace sensors incorporate a very small sensing element to provide fast response. With the small sensor size and a unique gas-sensing mode, power consumption is dramatically reduced. A novel heater temperature control method was implemented for obtaining high sensitivity and good selectivity to CO, while improving overall efficiency. The sensitivity to CO increases and the sensitivity to other gases decreases at low temperature ranges (below 150 ºC). Using the sensitivity characteristics at the most suitable temperature for CO detection (approximately 80 ºC), high sensitivity and selectivity to CO is obtained. Additional temperature characterization at higher sensor temperatures resulted in enhanced sensitivity to methane (CH4) utilizing the same sensor. A multi-tiered temperature correction scheme compensates for temperature/humidity effects. Airspace gas monitors are therefore able to provide reliable CO and methane sensing with up to one month of continuous use on two AA alkaline batteries.
The AGS sensor and monitor have been rigorously tested and have the approval by the Underwriters Laboratory, Inc., for use in Class 1, Division 1, Group A, B, C, and D, T3C environments as to intrinsic safety.
The AGS TechnologyTM provides a highly sensitive sensor that responds quickly in detecting lethal gases. Even low concentrations of CO can be detected within seconds. AGS incorporates an intelligent adaptive sampling method. To save power, the unit stays in a “low-power” standby mode until CO is detected. The sensor then rapidly enters a “high alert” active mode, which precisely determines the concentration of gas. This method provides an extended battery life without sacrificing accuracy and response.
The cost of ownership is significantly lower with Airspace AGS Technology™. By eliminating sensor replacement and routine calibration, the Airspace Gas Monitor provides the most cost effective solution.
* Assumes 1 LPM flowing for 3.5 minutes, calibrating once a month.
** Some competitive equipment requires expensive NiCad or NiMh battery replacement and disposal.
Airspace monitors with AGS TechnologyTM provide a very cost effective, reliable, intrinsically safe, portable gas monitor.
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