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 with it, 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. A higher current is also required during this time. Devices with a display show "preheating".

2. NDIR Infrared Sensors (CO2 traffic lights, GX-D...)
A non-dispersive infrared sensor detects carbon dioxide (CO2) using an optical method. CO2 has the property of attenuating infrared light of a very specific wavelength (~4 μm). Inside the sensor, an infrared LED shines through a glass filter, and this light is then directed through the measuring chamber onto an IR luminance sensor. The less light reaching this sensor, the more CO2 is present in the measuring chamber, which is connected to the outside air via a moisture-resistant membrane. The dual-beam sensors used by Schabus additionally measure the light output of the IR LED to compensate for measurement errors caused by aged light sources. A powerful microcontroller controls the process and outputs either a sensor voltage corresponding to the CO2 concentration, a pulse width modulation (GX-D250), or direct UART, which the various warning devices evaluate, display, and respond with an audible alarm and/or relay switching.

3. Electrochemical sensors (GX-C1pro, GX-C...)
An electrochemical sensor detects carbon monoxide (CO) via a chemical reaction with pure water. The sensor consists primarily of its 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 thus provides a direct measure of the CO concentration and can be measured amperometrically. The electron current is in the lower nanoampere range, approximately 1.5 nA/ppm CO. Therefore, such a sensor cannot be directly connected to a warning device; instead, the measuring electronics must be located very close to the sensor and be extremely sensitive and precise. Operational amplifiers convert the signal into a calibrated voltage, which is then processed via an analog-to-digital converter (ADC) in a 32-bit microcontroller. Elektrotechnik Schabus had the success of this elaborate development of the measuring cell tested by TÜV Süd according to DIN 50291; the system was certified for stability and precision, and all CO warning devices offered use this measuring cell with the electrochemical sensor.

Let's start with town gas, which no longer exists in that form. It was produced from coal gasification and contained a relatively high proportion of toxic carbon monoxide (see page 74). Town gas was available until around the end of the 1970s, and in West Berlin until the mid-1990s. It was gradually replaced by the less toxic natural gas. This required modifications to combustion plants, including the use of different seals and valves. The name "town gas" is still commonly used, which is why we still refer to our sensor for flammable gases as a town and natural gas sensor (SE). Natural gas, the gas that our municipal utilities and gas suppliers provide today for heating, hot water, and cooking, is a naturally occurring gas that is primarily a byproduct of oil production, but also originates 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 to 90% of its volume. Other substances besides butane and propane include 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 before being fed into our gas supply system. However, it is not before being mixed with the sulfur compounds thioethers or alcanethiols to give the gas its characteristic odor, which we naturally perceive as the smell of gas. Without these additives, natural gas would be odorless. Every flammable gas sold must be mixed with these substances to create its odor. We therefore already carry the best gas sensor right in our faces: our noses. Fortunately, our noses are not always located precisely where gas could unintentionally escape. This is especially true at the various connections in our gas line, at the transfer point, at the gas valve, at the meter, and directly at the heating system, stove, or boiler. Here, usually in the so-called utility rooms (HWR), but also in the kitchen directly at the gas stove, the "city and natural gas warning detectors" from Elektrotechnik Schabus come into play. They immediately detect gas leaks and warn of a pipe defect with a loud, piercing alarm. If necessary, they shut off a connected magnetic shut-off valve to prevent further gas flow. Since natural gas consists mostly of the very light methane, it is lighter than air and immediately rises when it escapes. Therefore, a GX-SE sensor must be installed high up in the room to detect the gas immediately. However, it shouldn't be placed at the very top, but rather about 30 cm below the ceiling, because there is a so-called dead space in the corners. Air 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's a term called the "lower explosive limit," abbreviated LEL and expressed as a percentage. A gas-air mixture only becomes explosive when it reaches 100%. It's important to understand that the pure amount of gas released isn't the deciding factor, as with CO, which is easily expressed in ppm. Other factors also play a role. These include temperature, humidity, and oxygen content, because combustion absolutely requires oxygen; otherwise, it won't burn. Higher humidity means less oxygen is present, and higher temperature means fewer particles in the room that can react with each other. Our SE sensors take these three factors into account and convert them into a voltage that is detected by the warning devices. Now, if even a single molecule is detected, or more realistically, if... z.BOnly 3% LEL (Lower Explosive Limit) triggers a warning, i.e., an alarm, 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 expect from a gas line defect. The sensors are technically capable of detecting these levels, but who wants a warning when open paint and varnish cans are off-gassing or someone walks past the sensor with freshly painted nails or freshly applied perfume? Solvents, among many other common household substances, are very similar to the hydrocarbons in town gas and natural gas and are detected just as effectively by the sensors. Several of the numerous DIN standards dealing with the detection of natural gas in residential areas recommend a warning at the latest when the 20% LEL limit is reached. Since our sensors also detect liquefied petroleum 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 prevent 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 CO (carbon monoxide)?

Carbon monoxide is not supplied. It is produced in any combustion process where insufficient oxygen is available. Every gas molecule (z.BMethane (CH4) requires two oxygen molecules (O2) for complete combustion. This produces two water molecules (H2O) and one carbon dioxide molecule (CO2), which is not nearly as dangerous as a carbon monoxide molecule (CO). Gas is potent and readily combusts. If insufficient oxygen is available, two gas molecules share one oxygen molecule, producing carbon monoxide along with hydrogen.

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

Sufficient oxygen must be present at the point of combustion, not just scattered throughout the room. This easily explains why CO is produced in virtually every combustion appliance (boiler, heating system, etc.). A nozzle clogged with dust is enough to cause this. Or a house that has been retrofitted with thick insulation. This is easily recognizable during visual inspection of combustion if you see a yellow component in the flame. Complete combustion with sufficient oxygen always appears blue, although a yellow component isn't always easy to spot. Incidentally, methane gas is just one example here; 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 uses oxygen to function properly. We inhale oxygen, which binds to hemoglobin (red blood cells) in the alveoli of the lungs and is transported to the cells via the bloodstream. Here, the combustion process takes place: the oxygen molecule is removed from the red blood cell, and the carbon dioxide molecule (CO2) resulting from this (complete) combustion is reattached to the red blood cell for transport to the lungs for exhalation. However, when we inhale CO2 in the air mixture, it becomes problematic. Hemoglobin only recognizes the oxygen atom (O) in the CO2 and binds to it approximately 300 times more strongly than pure oxygen. The cell cannot utilize CO2 and sends it back to the lungs for exhalation. However, no exchange occurs there because an oxygen atom (O) is already strongly bound to the hemoglobin; we simply do not exhale the CO2. This typically happens after about 20 minutes; CO accumulates in the blood with each breath, while at the same time there are fewer and fewer red blood cells that can still absorb oxygen. This is what makes carbon monoxide toxic. A lack of oxygen stops the cells from functioning. v.a...the central nervous system, the heart, and the brain are affected; one becomes tired, falls asleep, and in the worst case, dies from suffocation despite breathing. In cases of acute carbon monoxide poisoning, only pure oxygen helps, ideally administered in a hyperbaric chamber.