1. Catalytically heated sensors (GX-SE, GX-CFC, GX-B...)

A tin oxide plate heated to just over 300°C forms the upper part of a voltage divider. When gas molecules collide, the resistance decreases and the sensor voltage increases. During the heating phase, the sensor voltage fluctuates significantly, which is why the warning devices ignore all incoming voltages for the first 3-5 minutes. During this time, a higher current is also required. Devices with a display indicate "preheating."

2. NDIR infrared sensors (CO2 traffic lights, GX-D...)
A non-dispersive infrared sensor detects carbon dioxide (CO2) using an optical process. CO2 has the property of obscuring infrared light of a very specific wavelength (~4 μm). Inside the sensor, an infrared LED shines through a glass filter, and this light then passes through the measuring chamber to an IR brightness sensor. The less light reaches this sensor, the more CO2 is present in the measuring chamber, which is connected to the outside air via a moisture-repellent membrane. The dual-beam sensors used by Schabus also measure the light output of the IR LED to compensate for measurement errors caused by aging light sources. A powerful μController controls the process and outputs either a sensor voltage corresponding to the CO2 content, pulse width modulation (GX-D250), or directly to the UART, which the various warning devices evaluate, display, and respond with an acoustic alarm and/or relay switching.

3. Electrochemical sensors (GX-C1pro, GX-C...)
An electrochemical sensor detects carbon monoxide (CO) through a chemical reaction with pure water. The sensor consists primarily of a water tank, which is connected to the outside air via an activated carbon disc and a tiny hole. The reaction of CO with H2O produces CO2, hydrogen, and two free electrons. The number of electrons is therefore a direct measure of the CO concentration and can be measured amperometrically. The electron current is in the low nA range, approximately 1.5 nA / ppm CO. Such a sensor cannot be connected directly to a warning device; instead, the measuring electronics must be located very close to the sensor and be designed for extreme sensitivity and precision. Operational amplifiers ensure conversion into a calibrated voltage, which is then evaluated via an ADC in a 32-bit μcontroller. Elektrotechnik Schabus had the success of this complex development of the measuring cell tested by TÜV Süd according to DIN 50291. The stability and precision of the system were certified and all CO warning devices offered carry this measuring cell with the electrochemical sensor.

Let's start with town gas, which no longer exists. It was produced from coal gasification and contained a relatively high proportion of toxic carbon monoxide (see page 74). Town gas existed until around the end of the 1970s, and in West Berlin until the mid-1990s. It was gradually converted to natural gas, which was less toxic. This required the combustion plants to be modified; different seals and valves were needed. However, the name "town gas" is still present among the population, which is why we still refer to our sensor for combustible gases as the town and natural gas sensor (SE). Natural gas, the gas that our municipal utilities and gas suppliers supply us today for heating, hot water, and cooking, is a naturally occurring gas that is mainly a byproduct of oil production, but also comes from pure natural gas fields that do not produce oil. The main component of natural gas is the highly flammable gas methane, which makes up up to 90% by volume. Other substances include butane and propane, various traces of sulfur compounds, ethane, CO2, noble gases, nitrogen, and water vapor. Once extracted, natural gas is purified of toxic and unusable substances such as water, hydrogen sulfide, and carbon dioxide and fed into our gas supply system, but not without first adding the sulfur compounds thioether or alkanethiol to give the gas its typical odor, which we naturally perceive as gas. Without these additives, natural gas would have no odor at all. Every flammable gas that is sold must be mixed with these substances to create an odor. The best gas sensor is therefore already right on our face: our nose. Fortunately, our nose is not always located exactly where gas could accidentally escape. At the various connections in our gas line, at the transfer point, at the gas tap, at the meter, and directly at the heater, stove, or boiler. This is where the "urban and natural gas warning detectors" from Elektrotechnik Schabus come into play – usually in the so-called boiler and utility rooms (HWR), but also in the kitchen directly at the gas stove. They immediately detect any gas leaks and warn of a pipe defect with a loud, piercing tone. If necessary, they switch off a connected magnetic shut-off valve so that no more gas can flow in. Since natural gas consists mostly of the very light methane, it is lighter than air and immediately evaporates upwards when it escapes. A GX-SE sensor must therefore be installed at the top of the room to immediately detect the gas. Not right at the top, however, but about 30 cm below the ceiling, because there is what is known as dead space in the corners. Air that is trapped in the corners and edges of the ceiling cannot escape and displaces the gas. Gas from cylinders (butane/propane) is heavier than air, so the sensor is placed 15-30 cm above the floor.

There is a term used to describe the "lower explosion limit", abbreviated to LEL and expressed as a percentage. A gas-air mixture only becomes explosive when it reaches 100%. It is important to understand that it is not just the amount of gas released that is crucial (e.g., CO, which is easily expressed in ppm), but that other variables also play a role. Be it temperature, humidity, or oxygen content, because every combustion requires oxygen, otherwise nothing will burn. If the humidity is higher, there is less oxygen; if the temperature is higher, there are fewer particles in the room that can react with each other. These three variables are taken into account by our SE sensors and converted into a voltage that is recorded by the warning devices. One could issue an immediate warning, i.e. an alarm, if only one molecule is detected, or more realistically, at, say, only 3% LEL, but no customer would accept such behavior in the long run. Up to 5% LEL, a supposed false alarm occurs more frequently than one would associate with a gas line defect. The sensors could do that, but who wants a warning when open cans of paint and varnish are giving off gases, or when someone walks past the sensor with freshly painted nails or freshly applied perfume? Solvents, along with many other common household substances, are very similar to the hydrocarbons in town and natural gas and are detected just as well by the sensors. Some of the many DIN standards that deal with the detection of natural gas in residential areas recommend a warning when the 20% LEL limit is reached at the latest. Since our sensors also detect liquid gas (LPG with a high proportion of butane and propane), we have agreed on an early warning level of 12% LEL. Always in time to avoid danger, but sufficiently tolerant to avoid frequent false alarms. And of course within the standard.

Where does the carbon monoxide come from? Who mixed it into my gas? What happens in our bodies when we inhale carbon monoxide?

Carbon monoxide is not supplied. It is produced during any combustion where there is not enough oxygen available. Each gas molecule (e.g. CH4 = methane) needs two oxygen molecules (O2) for complete combustion. This then produces two water molecules (H2O) and a carbon dioxide molecule (CO2), which is not nearly as dangerous as a carbon monoxide molecule (CO). Gas is strong and desperately wants to burn. If there is not enough oxygen available, two gas molecules share one oxygen molecule, producing carbon monoxide as well as hydrogen.

CH4 + 2 O2 -----> CO2 + 2 H2O (complete)
or
2 CH4 + O2 -----> 2 CO + 4 H2 (incomplete)

There needs to be enough oxygen at the combustion site and not just anywhere in the room. This explains why CO is produced in almost every combustion device (boiler, heater, etc.). A nozzle clogged with dust is enough. Or a house that has been subsequently tightly insulated. This is easy to see in visible combustion, when you see a yellow component in the flame. Complete combustion with sufficient oxygen always appears blue, although the yellow component is not always easy to see. By the way: methane gas is only mentioned here as an example; of course, this also applies to all other combustion processes, such as butane, propane, oil, paper, cardboard, wood and pellets. All combustion requires sufficient oxygen!

Every cell in our body also burns oxygen to function properly. To do this, we breathe in oxygen, which binds to the hemoglobin (red blood cells) in the alveoli and is transported to the cells by the bloodstream. This is where combustion takes place: the oxygen molecule is taken from the blood cell and the carbon dioxide molecule CO2 resulting from the (complete) combustion is attached back to the blood cell for removal, which then transports it to the lungs for exhalation. However, when we breathe in CO in the air, things get critical. The hemoglobin only recognizes the oxygen particle O in the CO and binds to it around 300 times more strongly than pure oxygen. The cell cannot do anything with CO and sends it back to the lungs for exhalation. There, however, no exchange takes place because there is already an oxygen molecule O strongly attached to the hemoglobin, so we cannot simply breathe the CO out again. This only occurs after an average of about 20 minutes. The CO accumulates in the blood with each breath, while at the same time, there are fewer and fewer blood cells that can absorb oxygen. This is what makes carbon monoxide toxic. A lack of oxygen stops the cells from functioning, especially those of the central nervous system, the heart, and the brain. You become tired, fall asleep, and, in the worst case, die from suffocation despite breathing. In acute CO poisoning, only pure oxygen helps, ideally in a pressure chamber.