Apart from the GX-D250, all gas warning devices from Elektrotechnik Schabus work in the same way:
The warning device gives the sensor an operating voltage and some current, the sensor sends the sensor voltage back to the warning device, and the warning device interprets the voltage sent back and reacts to it. So simple, so good. So there are some gas warning devices with specializations, others are more universal in nature, some can interpret more different voltages, others less. On the basis of the new GX-A1+ (successor of the over thousandfold proven GX-A1), which can evaluate most different voltages so far, we want to illustrate this.
With a few exceptions, all sensors get an operating voltage of 5 volts, which means that the sensor voltage cannot drop below 0 volts and cannot rise above 5 volts. Already many years ago the warning levels "pre-alarm" with 2.0 volts and "main alarm" with 2.5 volts were fixed. This has remained so to this day in order to remain as upward and downward compatible as possible, and new sensors are adapted to this. The GX-A1+ evaluates these voltage ranges:

By the way, the simplest version of the gas detector is the GX-HS, it only knows above or below 2.5 volts, whereby it also reports a cable break or a sensor that is not connected at all as an "alarm", at first glance this cannot be distinguished. And how does the GX-D250 do that? It communicates with its external sensors via pulse width modulation. This is the only way the warning device designed for this purpose can display the CO2 concentration accurately to the ppm.

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

A tin oxide plate heated to just over 300°C represents the upper part of a voltage divider. If gas molecules meet, the resistance decreases and the sensor voltage increases. During the heating phase, the sensor voltage oscillates around significant values, which is why the warning devices ignore all incoming voltages in the first 3 - 5 minutes. A higher current is also required during this time. Units with display show "preheating".

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

3. electro-chemical sensors (GX-C1pro, GX-C...)
An electro-chemical sensor detects carbon monoxide CO via a chemical reaction with pure water. The sensor mainly consists 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 therefore represents a direct measure of the CO concentration and can be measured amperometrically. The electron current is in the lower nA range, about 1.5 nA / ppm CO. It is therefore not possible to connect such a sensor directly to a warning device. Instead, the measurement electronics must be located very close to the sensor and be designed to be extremely sensitive and precise. Operational amplifiers provide the conversion to a calibrated voltage, and the evaluation is then carried out via ADC in a 32-bit μController. Elektrotechnik Schabus has had the success of this complex development of the measuring cell checked by TÜV Süd in accordance with DIN 50291, stability and precision were certified for the system and all CO warning devices offered carry this measuring cell with the electrochemical sensor.

Carbon monoxide is not supplied. It is produced in any combustion in which not enough oxygen is available. Every gas molecule (e.g. CH4 = methane) needs two oxygen molecules (O2) for complete combustion, a carbon dioxide molecule (CO2) is then produced in addition to two water molecules (H2O), which is not nearly as dangerous as a carbon monoxide molecule (CO). Gas is strong and desperate to burn. If there is not enough oxygen available, two gas molecules share an oxygen molecule and carbon monoxide is produced in addition to hydrogen.

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

There must be enough oxygen at the place of combustion and not somewhere in the room. This easily explains why CO is produced in probably every combustion device (boiler, heater, ...). A nozzle clogged with dust is enough. Or a retrofitted, tightly insulated house. This is easy to recognise in visual combustion when you see a yellow component in the flame. Complete combustion with sufficient oxygen always shows up blue, although a yellow component in it is not always easy to recognise. By the way: methane gas is only mentioned here as an example, of course this also applies to all other combustions, such as butane, propane, oil, paper, cardboard, wood and pellets. All combustions require sufficient oxygen!

Every cell in our body also burns oxygen in order to function properly. To do this, we breathe in oxygen, which docks with the haemoglobin (red blood cells) in the alveoli and is transported to the cells with the bloodstream. This is where the combustion takes place: The oxygen molecule is taken from the blood corpuscle and the carbon dioxide molecule CO2, which comes from the (complete) combustion, is reattached to the blood corpuscle for removal, which transports it to the lungs for exhalation. But when we breathe in CO in the air mixture, it becomes critical. The haemoglobin only recognises the oxygen particle O in CO and attaches it about 300 times as strongly as pure oxygen. However, the cell cannot do anything with CO and sends it back to the lungs for exhalation. There, however, no exchange takes place, because oxygen O is already strongly attached to haemoglobin, so we do not simply breathe out the CO again. On average, this only happens after about 20 minutes, the CO accumulates in the blood with every breath, and at the same time there are fewer and fewer blood cells that can still absorb oxygen. That is the toxic thing about carbon monoxide. A lack of oxygen stops the work of the cells, especially the CNS, the heart and the brain, you get tired, fall asleep and in the worst case die. By suffocation despite breathing. In the case of acute CO poisoning, only pure oxygen can help, ideally in a pressure chamber.

Let's start with town gas, which no longer exists. It was produced from coal gasification and contained a fairly 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 converted to natural gas, which was not quite so toxic. To do this, the incinerators had to be rebuilt, and different seals and valves were needed. However, the name "town gas" is still present in the population, which is why we still refer to our combustible gas sensor as the town and natural gas sensor (SE). Natural gas, the gas that our municipal utilities and gas suppliers provide us with today for heating, water heating, and cooking, is a naturally occurring gas that is mainly a byproduct of oil production, but also comes from natural gas-only fields that do not yield oil. The main component of natural gas is methane, a highly flammable gas, which makes 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, not without first again adding the sulfur compounds thioether or alkanethiol to give the gas its typical odor, which we perceive quite naturally as the smell of gas. Without these additives, natural gas would have no odor at all. Any combustible gas that is sold must have these substances added to it to create an odor. So we already carry the best gas sensor right in our face: our nose. Now, fortunately, our nose is not always located exactly where gas might unintentionally escape. At the various connections of our gas pipeline, at the transfer point, at the gas tap, at the meter and directly at the heating, stove or boiler. Here, mostly in the so-called heating and 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 determine whether gas is escaping and warn with a loud penetrating tone of a line defect and switch off a connected solenoid shut-off valve if necessary, so that no further gas can flow in. Since natural gas consists largely of methane, which is very light, it is lighter than air and immediately evaporates upward as it escapes. So a GX-SE sensor must be placed at the top of the room to immediately detect the gas. But not at the very top, but about 30 cm below the ceiling, because there is the so-called dead space in the corners. Air that is in the corners and edges on the ceiling cannot escape and displaces the gas. Gas from bottles (butane/propane) is heavier than air, so the sensor is placed 15-30 cm above the floor.

There is the term "lower explosion limit", it is abbreviated to LEL and given as a percentage. A gas-air mixture only becomes explosive when 100% is reached. It is important to know that it is not only the amount of gas that escapes that is decisive, as is the case with CO, which is easily expressed in ppm, but that other variables also play a role. Be it the temperature, the humidity or the oxygen content, because every combustion necessarily needs 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 detected by the warning devices. Now, one could warn immediately if even one molecule is detected or, more realistically, at e.g. only 3% LEL, but no customer would accept such a behaviour in the long run. Up to 5% LEL, a supposed false alarm occurs more frequently than one might associate with a gas pipe defect. The sensors could do that, but who wants a warning when open paint and varnish cans outgas or someone walks past the sensor with freshly painted nails or freshly applied perfume? Solvents, among many other household substances, are in fact very similar to the hydrocarbons in town and natural gas and are just as well detected by the sensors. Some of the many DIN standards dealing with the detection of natural gas in the home recommend a warning at the latest when the 20% LEL limit is reached. Since our sensors detect liquid gas (LPG with a high proportion of butane and propane) just as well, we have agreed on an early warning level of 12% LEL. Always in time, so that it doesn't become dangerous, but sufficiently tolerant to avoid frequent false alarms. And of course within the standard.