The «RELAY» is an electrical switch that enables another electrical circuit to be switched on and off. Relays are used in many areas. Devices are controlled with the received status signals. For example, it is the job of relays to stop the refrigerator working after it has cooled enough and to restart when the temperature increases. The contacts open and close with the signal it receives from the temperature sensors. We said that relay is used in many areas. It can be expressed differently in every field, but they all describe the function of relays. Input, input, output, output, contact are the expressions we will hear most often. Relays are mostly used as outputs. They give output to input modules. Ignition is the key you see in the figure below. It is used normally open or normally closed. It is used in systems where normally closed contact will not cut the current and should continue to work when it does not receive a signal. (For example, the boiler should operate unless a gas leak is detected, NC contact is used in the device that detects this gas, when it detects gas, the contact opens and cuts the current)

4-20mA is a range of current values. It is used as an information output signal in level measurement systems such as temperature, humidity, gas leakage amount. It is mostly referred to in automation systems.

A current of 30mA and above can stop the human heart Semiconductor devices work with 3mA current 3-30mA range is not linear Alternatives 4-20mA and 5-25 mA It was preferred because it is easier to calculate the 0, 25, 50, 75 and 100% equivalents of the measured values and to make the calculations with multiples of 2.

Detecting the changes in any process is the first step in directing that process. If we want to use these changes in an electronic environment, it is necessary to make them specific. At this point, the transmitter comes into play. Transmitter literally means converter. They convert values such as temperature, temperature and pressure to voltage (0-10V) or milliampere (0-20mA) levels. The level to be converted is up to the user.

PLC (Programmable Logic Controller) is an automation device used in the control of processes such as production departments in factories or control of machines. Unlike normal computers, PLC has many inputs and outputs (I / O). The biggest advantage is that they are designed to be resistant to electrical noises, temperature differences and mechanical impacts. PLCs of different brands install an operating system of their own. This supervisory system scans the input information at speeds invisible to the naked eye, and works in a way to respond in close to real time with the corresponding exit information. PLCs consist of 4 main parts. ► Central Processing Unit (CPU) ►Memory Unit (RAM, ROM, PROM etc.) ► Input Unit (IN) ► Output Unit (OUT)

Hot surfaces Static Spark Hot gas Open fire Friction Electrical equipment Mechanical equipment

ZONE 0: Places where an explosive atmosphere occurs (and has a high probability of occurrence) under normal operating conditions and that takes a long time when it occurs are included in the scope of ZON 0. ZONE 1: Locations that are less likely to form explosive atmosphere due to normal operation (or not at all), only in malfunction and abnormal operating conditions and where an explosive atmosphere may or may occur accidentally, and also for a short time when it occurs are included in this group. ZONE 2: Locations that do not have the possibility of forming an explosive atmosphere due to normal operation and also have little possibility of forming an explosive atmosphere in cases such as malfunction, accident, repair, maintenance, and in such cases that have a very short duration (possibility of driving) are included in the scope of Zone 2. .

For a gas to burn, it must be in sufficient quantity in the air. The minimum amount of gas required for combustion is called Low Explosion Limit (LEL)

If the gas concentration rises above a certain level in the air, the explosion does not occur because there will not be enough Oxygen in the environment, this is called the Upper Explosion Limit (UEL).

The operation of catalytic sensors is based on the principle of oxidation of ammable gases. Catalytic sensors as shown in the gure, a platinum spindle is wrapped around a ceramic pellet. The surface is covered with a special substance which when exposed to a ammable gas, causes an exothermic oxidation. The lump surface needs to be warmed up for a while before operating. Warming is done with the wire inside. After the gas reaches the sensor, and the cause of the oxidation heat starting to the surface. This causes changes in heat coil the electrical resistance; These changes are perceived as signals from the sensor. Catalytic sensors are built with a compensator inside to prevent false signals that can be caused by environmental factors with their ability to compensate of environmental changes, catalytic sensors offer many advantages in use. linearity, repeatability in the zero and span gas values. (the same values can be measured after calibration), reproducibility (taking the same results from the same model sensors produced from the same factory). Catalytic sensors can be damaged and even lose their sensitivity when they are under high gas concentrations after more a five minutes. Therefore, the sensors must be routinly checked and calibrated at regular basis.

Pellistor sensor has the same operating principle of the catalytic sensor has. There is only compensator part as a different part. The sensor consists of two paired pellistors. The detecting element consist of small "pellets" catalyst loaded ceramic whose resistance changes in the presence of gas. Many of them require to be gently heated in use, so they may be four terminal devices with two connections for a small heating element and two of the sensor itself. To avoid any risk of explosion, the sensitive element is usually enclosed in a wire mesh housing. More robust sensors for use in high risk environments may have solid steel housing with a gas port of sintered metal granules Both of these work in a similar manner of the davy safety lamp; gas may percolate through the permeable mesh, but the passages are too long and narrow to support the propagation of a flame Pellistor sensors have longer life than catalytic sensors and can operate over a wide temperature range. Especially the 0-5 LEL performances are very good in terms of time and accuracy

Today, in technology many IR instruments are available for a wide variety of applications. Many of them oer simple, rugged, and reliable designs. In general, for toxic and combustible gas monitoring applications, IR instruments are among the most user friendly and require the least amount of maintenance. There is virtually an unlimited number of applications for which IR technology can be used. Gases whose molecules consist of two or more dissimilar atoms absorb infrared radiation in a unique manner and are detectable using infrared techniques. Infrared sensors are highly selective and oer a wide range of sensitivities, from parts per million levels to 100 % percent concentrations. This chapter provides general information, with a special emphasis on instruments used for area air quality and safety applications. They are placed at the end of the optical path so that they are not aected by corrosion.

Electrochemical gas sensors are that measuring the concentration of a target gas by oxidizing or reducing the target gas at an electrode and measuring the current result. Electrochemical sensors are usually used for the detection of toxic gases except a few combustible gases. Hydrogen and Carbon monoxide concentrations up to sub-explosion limits and Oxygen volume up to 25%. The sensors contain two or three electrodes, occasionally four, in contact with an electrolyte. The electrodes are typically fabricated by xing a high surface area precious metal on to the porous hydrophobic membrane. The working electrode contacts both the electrolyte and the ambient air to be monitored usually via a porous membrane. The electrolyte most commonly used is a mineral acid, but organic electrolytes are also used for some sensors. The electrodes and housing are usually in a plastic housing which contains a hole gas entry for the gas and electrical contacts. The gas diuses into the sensor, through the back of the porous membrane to the working electrode where it is oxidized or reduced. This electrochemical reaction results in an electric current that passes through the external circuit. In addition to measuring, amplifying and performing other signal processing functions, the external circuit maintains the voltage across the sensor between the working and counter electrodes for a two electrode sensor or between the working and reference electrodes for a three electrode cell. At the counter electrode an equal and opposite reaction occurs, such that if the working electrode is an oxidation, then the counter electrode is a reduction.