CO2 Sensors: New Possibilities
Why the latest options for detecting carbon dioxide — which can do so while consuming one-thirtieth the power of other approaches — have significance for a range of industrial and medical applications.
A change in CO2 levels, even one as minor as that equivalent to the change in oxygen levels you experience when going up a steep hill, for example, can affect you. Rising CO2 levels in the Apollo 13 (Figure 1) threatened the lives of crew—they had plenty of oxygen. Increasing CO2 levels result in yawning and dizziness, followed by unconsciousness and finally death.
Figure 1: It was not low oxygen, but rather increasing levels of carbon dioxide that endangered those aboard Apollo 13. (Courtesy NASA)
The normal level of CO2 is around 400 parts per million (ppm). In a closed room with several people in it, this can rapidly rise to 2000 ppm. The meeting might be boring, but the probable reason you are yawning is not lack of oxygen. The stuffy feeling making you yawn is your body reacting to high levels of CO2. Humans are very bad at detecting rising CO2 levels. The challenge for Mother Nature is that you exhale at around 50,000 ppm (5% CO2), so it is hard to notice levels of CO2 in the incoming air, which, at much lower concentrations, can affect your health. Humans have lived in closed spaces for a relatively short time, so there has not been enough time to evolve a detection system.
How do you detect a colorless, odorless gas? Miners solved this by taking canaries in a cage down the mines as they would be affected first, falling off their perches and so giving an early warning to the miners. The Davey Lamp then mainly replaced canaries but amazingly canaries were still used in British mines until 1986. Virtually all modern CO2 detectors now use Non Dispersive Infra Red (NDIR) technology that works by shining Infra Red (IR) through the sample gas. CO2 molecules absorb in the mid range of Infra Red—4.2 to 4.4 micron wavelengths—and so the more of these wavelengths that are absorbed, the more CO2 is present.
Traditionally the source of this IR is a filament similar to what would be found in an old-fashioned light bulb or a micro-heater. Essentially, this hot source produces a range of IR radiation, which must then be filtered to only send the 4.2-4.4 micron wavelengths through the sample. This method wastes substantial energy, and heat issues can distort the readings. In addition, it usually takes several minutes to half an hour for the source to heat up and stabilize, using time and energy. And if you’ve bumped up against a projector when the bulb is on, you’ll recall that a hot piece of wire is fragile and burns out fairly quickly. Remember how often you had to change old-fashioned filament light bulbs before LEDs?
LEDs use much less power and are virtually instant on and off, so there is no wasted energy bringing the source of the IR into operation. Rather than wastefully producing a whole range of wavelengths, LEDs can be tuned to produce specific wavelengths.
However, while LEDs are very good at high energy wavelengths (the blues), drop below visible into the Infra Red, and they do not do as well. UK start up Gas Sensing Solutions (GSS) wanted to use LEDs in CO2sensors, but found no commercial manufacturers of mid range IR LEDs. It solved this problem by developing its own processes to manufacture mid range IR LEDs and matching photodiodes that are specifically tuned to 4.2-4.4 micron wavelengths. GSS invested in its own epitaxy machines so that it could control the manufacturing of these key components in house, an approach which provides the company with a barrier to entry for rivals trying to make LEDs. The processes that have been developed and refined are kept secret within the company without having to use patents, which reveal the hard-won R&D that can often be circumnavigated as patents are in the public domain.
Figure 2: GSS epitaxy machines.
The company does have patents for the parts of its products that can be opened up and possibly reverse engineered. These cover the optical path that the IR travels. For low concentrations of CO2, a long optical path is needed to give the occasional CO2 molecule an opportunity to absorb the IR light. For this the company has patented a horseshoe light path (Figure 3). If concentrations are high, then a short light path is needed, and the company has patented a dome-shaped reflector for this (Figure 4).
Figure 3: GSS patented horseshoe design give a long optical path for low CO2 concentrations.
GSS has created CO2 sensors based on LEDs that use massively less power – often between 10x and 100x less than alternatives/competitors. These sensors boast power consumption of only 35mW at 20 measurements per second, or can operate on a very low duty cycle – enabling a battery life of 10 years in lower-speed applications, and measurement ranges from 0 to 100%. In addition, the sensors can take measurements up to 20 times a second or more with custom designs. Quite a change from needing mains power and several minutes per measurement! So, what does this new LED approach to CO2 measuring make possible?
Figure 5: Breath analysis is now possible using low cost portable devices.
Revealing Medical Concerns
Breath analysis or capnography can reveal a number of medical issues such as heart and lung problems by analysing the rapidly changing CO2 content of exhaled air. However, until now, this has required expensive, mains-powered machines which effectively limits the use to laboratories, hospitals etc.
Many readings per second are needed to monitor how the CO2 levels are changing. LEDs are virtually instant on/off, unlike traditional IR sources that can take seconds or minutes to heat up and stabilize, and thus can be pulsed to give up to 20 readings per second. Speed and accuracy are akin to the previous generation, mains-powered machines.
The last part of the puzzle was the gas sample size. If you take too large a gas sample from the breath, it is effectively sucking the breath out of the patient—you don’t want a patient going blue! The sample size has to be very small, and GSS has engineered this into its new SprintIR®-6S that only needs 2ml per sample in its dome-shaped, gas detection chamber and requires so little power that it can be battery powered.
This opens up a whole new field of application solutions that was not possible before as handheld breath analysis devices can be made at a fraction of the cost of the previous generation of mains-powered, breath analysis machines. As a result, patients could now be provided with devices for home use, giving much better monitoring of their condition on an hour-by-hour, day-by-day basis under the changing conditions of real life, rather than just when in the laboratory or hospital for a couple of hours.
Emergency Medical Services could have hand-held, breath analysis devices to help with the early analysis and diagnosis of pre-hospital patients and as an alert of a worsening condition. Such devices could help check that a breathing tube has been inserted correctly, as often they can be incorrectly inserted into the throat, not the airway, and yet appear to be OK. Detecting CO2 in the exhaled air from the tube proves correct insertion.
In addition, gym and sports venues could monitor how people are responding to exercise programs. And, as this new generation of breath analysis machines with LED CO2 sensors is likely to be priced in the few hundreds, rather than many thousands of dollars range, home fitness use is also a possibility.
Air Quality Monitoring
Everyone is familiar with air conditioning, but mainly for heating or cooling and changing the air. Now people are realizing that CO2 levels need to be kept low for good health—just a percent or so above normal levels can cause drowsiness—yet changing the air has an energy cost to heat or cool it that might not be needed if there is no one present such as at night time or weekends. The U.S. Occupational Safety and Health Administration (OSHA) has set a permissible exposure limit (PEL) for CO2 of 5,000 parts per million (ppm) (or 0.5 %) over an 8-hour work day. They report that exposure to levels of CO2 above this can cause problems with concentration, an increased heart rate, breathing issues, headaches and dizziness.
It is easy to build mains powered CO2 sensors into air quality systems in new builds with the appropriate power and networking cables. But for retro fitting buildings, battery powered LED CO2 detectors can solve the problem more cost effectively, avoiding the costs of cabling and redecorating after cable installation. Even for new builds, these installation cost saving can make battery powered CO2sensors a more cost-effective choice, plus they provide a future proof solution that can be easily adapted as room use changes.
As readings are only needed every few minutes —you don’t want to keep turning the air quality systems on and off all the time, an LED CO2 sensor is so low powered that it can be powered for up to 10 years from a small lithium battery with enough power left over for a wireless control connection.
The importance of the need to maintain low levels of CO2 are now starting to be legislated, for example, with regard to new residential construction in Scotland, as well as for schools, offices and other buildings in various countries.
These divide into two distinct parts. First, high concentration monitoring to detect that enough CO2 is present and the second, at low levels, checking that safe levels of CO2 have not been exceeded. Rapid detection of a spike in CO2 levels can save lives.
High levels of CO2 are used to keep food, including sealed packages of meat and bagged salads, fresher for longer. On the production line, a very fast sensor is needed to measure that the bags are being properly sealed with the right level of CO2 that can be up to 100% concentration. Waiting minutes for a reading can result in many bags being processed incorrectly before the fault is noticed and corrective measures instigated.
Another example is greenhouses, where elevated levels of CO2 are needed to ensure that the plants are in optimal growing conditions.
On the other hand, a CO2 leak in any of these facilities can rapidly take a normal level of around 400 parts per million up to levels where dizziness and disorientation can happen. So having CO2 sensors in the 0 to 5 percent range provides a vital Health and Safety warning. An all-too-often example is found in restaurants, microbreweries, and bars where the cellar is used to store the CO2 cylinders used to carbonate drinks. A CO2 leak collects in the cellar and builds up to levels that knock out someone going down into it. This deadly trap can often claim several victims as more people go down to investigate what happened.
Again, a battery powered CO2 detector provides a simple, ‘fit and forget,’ retrofit, reconfigurable safety solution.
Three Critical Decisions
Lastly, when deciding which is the appropriate sensor to use for a given application, one has to decide the CO2 concentration range to be measured over, the power budget and the frequency of the readings, as there are usually trade-offs to be made. For example, frequent readings will rapidly drain a battery but, for some applications, that might not be an issue if the portable device is rechargeable. Whereas taking a reading every few minutes means that the battery life is measured in years.
Ralph Weir has used his BEng in Electronic & Microprocessor Engineering throughout his career in the Electronics Industry. His first senior role was Marketing Manager at Loughborough Sound Images, then VP of Sales and Marketing at Hunt Engineering, Elixent and then Mirics. He then became CEO of Phase Vision, Polar OLED, and Cambridge Nanotherm, and is now CEO at Gas Sensing Solutions.
June 7, 2017.
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