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Elektrotechnik Schabus - our F A Q 's

Here we try as best as possible to answer the typical, frequently asked questions about our products.

Questions about the topic:

Gas | Gas Detectors & Sensors

  • - How do gas detectors work?

    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:

    0,0 ... 0,1 VCable break / sensor failureno sensor that works halfway gives off such low voltage
    0,1 ... 0,3 VSensor errorThere is something wrong with the sensor, but it is not a broken cable
    0,3 ... 2,0 VMonitoring modeIdle, e.g. a GX-SE sensor is factory set to 0.8 V
    2,0 ... 2,5 VPre-alarmthe sensor has reacted to "something", it is a prewarning to the alarm
    2,5 ... 5,0 VMain alarmthe sensor has definitely detected "something", now full alarm

    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.

  • - How do the gas sensors work?

    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.

  • - Where did the carbon monoxide come from, who mixed that into my gas?

    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)
    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!

  • - What happens in our body when we breathe in CO - carbon monoxide?

    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.

  • - Urban and natural gas, what is it?

    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.

  • - At what point does escaping gas become dangerous?

    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.

CO-2 traffic light

  • - How much electricity does the CO2 traffic light actually consume?

    Due to the automatic dimming of the LED lights of our traffic lights at night, the actual power consumption is extremely low and thus also helps to save energy:

    0.03 kWh in 24 hours*

    (Important: the traffic light should not be disconnected from the power supply at night because the automatic self-calibration is designed for 7 days of continuous operation in order to determine better, more realistic standard values!)

    *current measurement as of 08.11.2022

  • - On which side and at what height should the traffic light be mounted?

    The traffic light is not mounted directly next to the windows on a wall. The height of the traffic light should be high enough to be visible from as many places as possible and at most so that the touch field for resetting the acoustic signal can still be touched comfortably with the hand.

  • - How many square meters does a traffic light cover?

    CO2 from breathing air is distributed quite homogeneously in the room. In classrooms, one traffic light is sufficient, a standard gymnasium should be equipped with 2 traffic lights and a Pax400 auditorium perhaps with 3 or 4 traffic lights. This also depends on the room geometry and cannot be expressed in square metres.

  • - How long does it take for the device to be ready for operation after it has been switched on?

    [embedyt][/embedyt]In less than 90 seconds the first meaningful evaluation with immediately displayed result takes place.

  • - Does the device stay on continuously for 24h or is it to be switched on every time the room is used?


    The traffic light is suitable for both possibilities; if it is only switched on when the room is in use, one can assume a longer service life.

  • - Does a self-calibration take place?

    To ensure consistent correct Co2 measurement results, make sure that the Co2 traffic light is switched on every 7 days (from year of manufacture 2021), or every 24 hours (before year of manufacture 2021) is exposed to fresh air for at least 30 minutes. Simply ventilating the room in which the measuring device is installed is sufficient for this. This leads to a constant self-calibration, thus to the full and accurate functioning of the device.

  • - The instructions say: "Inspection after 5 years". How does that work?

    Basically, you can check the sensor yourself. Simply blow on it calmly for about one minute and observe the colour changes on the traffic light. You can also send us the traffic light for a free factory inspection, during which we may readjust the sensor. If we need to replace the sensor, you will receive a discounted replacement offer, just as with all other gas detectors from our company. Regular or daily draught-ventilation also helps the traffic light to a higher and longer reliability.

  • Changing the battery on the window contact - how to do it?

    First, the cover must be removed from the transmitter to the FDS 100 or FDS 200. To do this, press the cover together slightly on the right and left at the height of the LED and pull it off upwards:

    To remove the old battery from the device, place the transmitter in front of you so that the antenna (black cable) is at the bottom right. Use the supplied screwdriver to drive vertically into the cavity between the battery and the battery holder and carefully pry the battery upwards.

    The new battery is simply pressed into the battery holder from above.

  • Receiver in the hood rattles - what to do?

    You have already changed the battery, the small LED on the window contact transmitter also lights up green - but still the exhaust hood does not work and the receiver in the exhaust hood clatters?

    Then it often helps to switch the fuse of the exhaust hood off and on again once. This hard reset often works wonders (similar to the PC).

  • - After changing the battery, the exhaust hood no longer works - what should I do?

    When changing the battery, make sure to insert the CR2032 button cell with the writing facing up.

    If the battery is inserted the right way around and the exhaust hood still does not work, please try the following steps:

    Close the window - wait briefly - open the window again - wait briefly again - then close the window again and open it again.

    Now the transmitter (at the window) and receiver (at the exhaust hood) should have calibrated again and work properly

Starting current limiter

  • - How exactly does this work with the ASB?

    Electric motors make an axis rotate with magnetic fields. The magnetic field is generated with copper coils when current flows through the wire. Before the magnetic field has been established after switching on, the copper wire "looks" like a short circuit to the fuse. It almost is one, after all, there is a conductive connection between the two terminals and the fuse "flies out" because enormous currents flow in that brief moment, see the tripping diagram. The ASB connects a power resistor in front of the copper coil, which "consumes" the excess current as heat in a flash and thus limits the current. After 0.7 seconds, the magnetic field has long since built up, the motor turns and consumes the current itself, and only then does a relay in parallel with the power resistor switch the power supply fully on. The ASB itself has only the function of a normal line when the machine is running and the power resistor can cool down again.

  • - My compressor has only 2000 watts. Why does not this even work with the ASB 120?

    Electric motors that start at idle draw a very high current for a very short time when they are switched on. If this is limited by the ASB, the motor starts anyway. If the motor starts under load because it has to move the pistons of the compressor against the pressure in the boiler, it cannot start with the limited current. If the ASB now switches on fully after 0.7 seconds, the full starting current is still drawn. The limitation had no effect and the fuse "blows". The ASB does not help here.

  • - My inverter brings 2300 watts, the Flex needs only 1000 watts. Why should that not work with the ASB?

    An inverter turns a DC voltage, e.g. a 12-volt car battery, into an AC voltage of 230 volts and can supply about 10 amps at the rated power. An electronic system regulates current and voltage, the regulation takes a certain amount of time and this is not available for the rapid load change. At the moment of switching on, the ASB limits the high starting current, but it switches on fully after 0.7 seconds and a lot of current is still needed quickly. Since no inverter has large capacitors that could supply large currents very quickly and the connected car battery is too sluggish, the voltage at the inverter collapses. Either the fuse on the inverter "blows" or the internal protective circuits simply switch off the inverter.

  • - Does an ABS 120 work with an inverter generator?

    Yes - if the inverter is a pure sine wave inverter.

    No, if the inverter has a square wave output.