Applications for Explosion-Proof Sensors

Sensors can be installed in all parts of an industrial application. Some applications such as food processing, mining, spray painting, and chemical processing require special enclosures or barriers for the sensors to protect them from explosions and contact with chemicals and acids. This can be accomplished by making the enclosure for the sensor so that it prevents the atmosphere from coming into contact with the sensor. In this type of application, the sensor is fully enclosed and sealed so that the explosive gases or chemicals can't come into contact with the area where the sensor produces the source of ignition. Another way to enhance the seal is to pressurize the area inside the enclosure so that if small leaks do occur, the harmless gas that is used to pressurize the enclosure will leak out rather than allowing the explosive gas to leak in.

Another way to provide protection against explosion is to limit the amount of power the sensor will have. In order to have an explosion, fuel, oxygen, and a source of ignition must all be present. Since it can be assumed that the fuel and oxygen are always present in the area where explosion-proof sensors are needed, the goal is to limit the amount of energy available to the sensor or the part of the circuit that is located in the hazardous location. One way to limit the power to the area of the sensor that is in the exposed atmosphere is to use zener diodes to limit voltage, and resistors to limit the amount of current . ill. 1 shows the block diagram for this type of circuit. From the circuit notice that the zeners are located in the circuit between the power supply and the sensor so that the voltage to the sensor is limited. In the circuit in the bottom part of the diagram notice that the zener-limiting circuit is used with a signal conditioner which would include the sensor. Each of these circuits meets ratings limiting the source of ignition so that these devices can be used in hazardous locations. These circuits can also be used with virtually any sensor.

Above: ill. 1 Electrical diagram that shows zener diodes and resistors used to limit voltage and current to provide explosion-proof sensors and transmitters for use in hazardous locations.

The sensors that must come into contact with acids or other chemicals are provided with enclosures that are made of substances such as stainless steel to resist corrosive damage. These enclosures are also sealed to keep out the vapors and fumes that are present in these types of atmosphere. It's usually a common practice to pressurize the enclosures for these sensors in a similar manner as the explosion-proof enclosures. In other applications such as food processing, the equipment and sensors must be thoroughly washed down twice a day, so the sensor enclosures must protect against high levels of humidity and be able to withstand the direct spray from the wash down. These enclosures may be classified by the National Electric Manufacturers Association (NEMA). The table below shows the NEMA standards for enclosures. From this list, notice that the explosion-proof enclosures must have a 7, 8, or 9 rating, and if they are to be rated for wash down, they must be rated 4, 4x, or 6.

Above: Chart for the National Electric Manufacturers Association (NEMA) classification for enclosures.

Summary of Overview