Warning systems gas | smoke

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 if necessary a connected solenoid shut-off valve, 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 can not 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.