Globally, with the widespread application and increasing sophistication of technology, the focus on critical process control equipment is shifting. Sensors are ubiquitous in our lives. From our furniture to the cars that transport us from one place to another, to automated equipment and control systems that monitor industrial processes, sensors are everywhere. However, like most components, they are part of a whole, and the components within systems are becoming increasingly complex. When used in hazardous environments, a comprehensive understanding of the application and rigorous selection are required. Karmjit Sidhu, Vice President of Business Development at American Sensor Technologies, commented: "For hazardous environments, there are many factors to consider, making the safe use of sensors in these environments a difficult but essential task." Art Pietrzyk, a T-certified functional safety specialist at Rockwell Automation, added: "We are reducing the risk factor, but danger is still danger. What has changed is the safety measures we take. Sophisticated instrumentation and more uniform standards bring new opportunities for safety." Sensors for hazardous environments encompass a variety of devices used in many different ways across many applications. However, they share some commonalities, most notably the need for the selection and installation of protective systems and standards to ensure their safe use. [align=center]Figure 1: For safe operation, sensors used in hazardous areas must be designed to be compatible with the environment in which they are used. A key point in the selection and installation process is considering how components are integrated into the system, as these components rarely operate independently.[/align] Classic Approach Most devices can operate safely if selected and used correctly. The key lies in understanding the specific application. A Class I, Div. 1 device for a hazardous environment may not be suitable for another of the same class, such as a toxic environment. Sidhu warns: “This is an abuse and a misunderstanding that can cause problems. End users need to know exactly what they have chosen and in what environment it is intended for. Be clear about what is inside the tank, and don’t rely on what is written on the enclosure. Your application may not require certified equipment, but as a precaution, it is fine to use certified equipment.” For safe operation, sensors used in hazardous areas must be specifically designed for the environment in which they are used. Generally, there are three approaches: selecting intrinsically safe equipment, using explosion-proof enclosures, or using a pressurized isolation system. (Depending on the application, sensors used in these hazardous areas cannot rely solely on this information; they must also be studied and supplemented according to the specific environment.) Intrinsically safe (IS) sensors are designed with energy limits insufficient to ignite the corresponding level of hazardous gas. Compliance checks ensure that IS-level devices have been tested and that the amount of energy required to trigger an action has been determined. IS devices typically use an "external device" called a safety barrier to limit current, thereby limiting the energy in the circuit under fault conditions. A passive safety barrier provides protection by preventing overvoltage and limiting current. If a short circuit occurs on a 4-20mA signal line, the safety barrier will prevent ignition. “Every intrinsically safe device must be powered by a safety barrier,” says Ed Herceg, Chief Applications Engineer at Macro Sensors. “But not all safety barriers are created equal. There are many types of safety barriers, and you must match the appropriate barrier to the overall parameters of your device to ensure you’ve chosen the right one. Otherwise, you may not be protected. However, to this day, I have heard of and believe that no explosion or fire in a hazardous area has been caused by the failure of an intrinsically safe device.” In Class I, Div. 2 hazardous areas, non-flammable equipment, instruments, and field wiring are sometimes used in place of intrinsically safe devices. Sensors can be housed in explosion-proof enclosures that can withstand the expected blast pressure and confine the explosion within the enclosure, preventing propagation. Explosion-proof technology is well-established, and when an intrinsically safe version of the required sensor is unavailable, explosion-proof is a viable option, provided it is installed correctly. Positive pressure isolation systems can also be used to prevent sensors and other equipment from exploding in hazardous areas. There are many types of positive pressure isolation systems, but the most crucial aspect is that these systems introduce a non-flammable gas (inert gas), such as nitrogen or carbon dioxide, through conduits, components, and equipment to prevent flammable gases from entering. An alternative approach is to seal the system so that there is simply no space for flammable gases. Les Schaevitz, president of Everight Precision, says, “Generally speaking, sensors in hazardous areas either house their electrical components within an explosion-proof enclosure or are designed to be intrinsically safe. Typically, placing intrinsically safe devices in hazardous areas is cheaper than using an explosion-proof enclosure, although both methods are somewhat expensive. Besides price, intrinsically safe devices (with safety barriers) are generally more popular because they eliminate the possibility of an explosion. Many users don't want any potential problems, even if they occur within an explosion-proof enclosure.” [align=center] Figure 2: A controlled dust explosion conducted in a laboratory of FM Approvals demonstrates what happens when flammable gases and dust ignite. FM Approvals, a member of the FM Global Group, certifies sensors in hazardous areas to prevent such accidents. Standards Users purchasing sensors for hazardous areas must choose devices bearing the accreditation mark of a local conformity testing agency. In North America, sensors for hazardous areas must be tested and classified by independent laboratories such as UL (Underwriters Laboratories), FM (Factory Mutual Insurance Company), and CSA (Canadian Standards Association), using standards such as the National Electrical Code (NFPA 70) of the Fire Protection Association of America. The NFPA conforms to most operating conditions in the United States and classifies hazardous areas into three categories: vapors and gases (Class I); dust, such as coal and flour (Class II); and fibers (Class III). Division is used to differentiate the frequency of hazardous environments: those that occur under normal operating conditions (Div. 1); those that do not usually occur (Div. 2). Further refinement (Group) is used to differentiate specific hazardous media (hydrogen, acetylene, etc.). In Europe, CENELEC (European Committee for Electrotechnical Standardization) is responsible for managing standards. However, recently, this work has been taken over by the ATEX Directive (Apparatus for Equipment in Potentially Explosive Atmospheres). Japan has its own standards organization, and many regions in Asia also have their own standards. The IEC (International Electrotechnical Commission) publishes international standards in real time, dedicated to "the conformity assessment of international standards and all electrical, electronic, and related technologies in government, commerce, and society." As a result, many standards coexist in the global economic environment, which creates difficulties for sensor selection, application, design, and sales. Herceg of Macro Sensors explains: "One problem is that your product can be sold in the US and Canada, but not in the European or Brazilian markets. This has a certain driving force for standard changes. Today, a user's product is certified, but tomorrow they find it cannot be sold in many regions because the standard has changed." Leslie, Production Manager of Safety Switches and Explosion-proof Switches at Honeywell Sensing and Control... Neill said, “IEC is the most likely to achieve global standardization. Although the ATEX directive has dominated Europe in the past few years, the IEC recently issued the IECEx standard, which aims to unify certification worldwide. Each different standardization system is like an umbrella, and these umbrellas have overlapping areas. If you want, IECEx will cover all these small umbrellas with one big umbrella. However, I believe that so far only one country is willing to use the IECEx standard as the only national standard, and that is Australia.” Figure 3: There are many types of hazardous area sensors, used in various occasions, with various certifications, and rated globally. Some of them are: (1) the AST4600 explosion-proof sensor from American Sensor Technologies, which is CSA certified and can withstand large vibrations, extreme temperatures and hydrogen explosions; (2) the HLR750 series LVDT (current variable differential sensor) from Macro Sensors, which can be used in steam turbines and hydraulic turbines and can also be rated by UL/ULC as Class 1, Division 2, Group A, B, C, D and Class 1, Zone 2, Group IIC operates in hazardous environments; (3) The pressure sensor profile shows the design scheme to prevent hazardous media from penetrating the sensor diaphragm. Schaevics of the integrated Everight company added: “The status quo of global standards may be adjusted by regional certification systems around the world. But it is best if such adjustments are made in the direction of integration, because industrial development is globalized. We believe that the ATEX model is moving in the direction of integration because it has issued an ‘international’ standard. Moreover, the European Community member states that have developed the ATEX standard have long had close economic and cultural ties and look forward to further integration with other countries around the world. Of course, no country is as unified as Europe.” Currently, the EU recognizes ATEX certification and does not accept other certification systems, although many people believe that UL, FM and CSA certifications are even more stringent than ATEX certification. Similar to the US Class/Division system, the ATEX area division system includes Zone 0 where the ignition hazard of flammable gas or vapor is present continuously or for a long time under normal working conditions; Zone 1 where the hazard is present intermittently; and Zone 2 where the hazard only occurs in fault or abnormal conditions. Tim, a member of the FM Global Organization and the Technical Team Manager of Hazardous Areas for FM Certification, Adam pointed out, “Over the past ten or twenty years, acceptance of the zone system has been slow, with three zones and two divisions corresponding to each. Based on this, FM considers both zone requirements and the North American class/division system during certification.” Adam found that recent customer feedback reflected a need for global standardization as manufacturers begin selling worldwide. He said, “We started taking this need into account, and we certify for the US and Canada. We also opened an office in the UK, where we can use the ATEX standard for certification, so we can also certify products intended for sale in the European market. We always keep in mind the importance of market timeliness for our clients and complete certification as quickly as possible.” Most equivalent standards have become parallel options under the same theme, supporting the globalization of standards. Bob, Head of Marketing Strategy at Honeywell Sensing and Control, said… Nickels stated, “Some products can only be sold in Europe, and UL/CSA would never approve them for use in the US, and vice versa. We have both ATEX and UL/CSA versions of the same switch. This is a nightmare for inventory, and applying for these certifications incurs huge costs. If we want to develop a new product, we have to decide where we want to sell it. Design engineers need to work on all the relevant requirements, and after development, they must pass the certification procedures required by the target market region.” This is the main reason for the company's demand for standardization. Nickels insisted, “The physical structure of the device hasn't changed; the only change is in the methods of description and testing. If we reach a consensus on this, then all those cascaded, parallel, cumbersome experiments are unnecessary. Once the basic physical requirements are met, suppliers can sell their products anywhere. In the long run, I think the industry will be looking towards global standards.” Change The global sales of sensors for hazardous areas should be audited, graded, and certified according to their sales regions. The chart shows the UL-CSA certification system (using Honeywell LSX limit switches as an example) and the ATEX certification system (using Honeywell…) The differences (taking BX products as an example). These devices are all the same type of switch, but they differ in that one has a clamping device on the left side of the BX device, requiring the use of special tools to open the switch housing, and a grounding device needs to be installed on the top of the switch for visual inspection. Unlike in the United States, European sites are not required to be connected to a fixed iron conductor, so European mining sites are not considered to be automatically grounded. The BX and LSX lines of explosion-proof limit switches are used for valve body control in petrochemical and offshore drilling equipment in North America and Europe. Future Outlook Globalization and standardization will be affected by technological advancements. FM's Adam points out: "Changes in sensors don't just happen to the equipment itself, but also bring changes to the control loop. Equipment can do more. Correspondingly, we must consider more. We need to ensure that these improvements do not bring more dangers." Sensing technology is a mature technology, Honeywell's Nickels acknowledged: "The working principle hasn't changed, but the peripheral components and support elements are more advanced. 30 to 40 years ago, we didn't have solid-state devices; every sensing function was achieved through electromechanical contacts or similar mechanisms. Now we have communication networks and systems designed for hazardous areas. These reduce system costs and allow for the use of more sensors within a plant or system." Camilo Aladro, Product Marketing Manager at Rockwell Automation, responded to Nickels: "25 years ago, switching functions were primarily performed by electromechanical devices. New standards like IEC 61508 (Functional Safety of Electrical/Electronic/Programmable Electronic Systems Related to Safety) emerged to accommodate new, processor-based, agile devices. Because microprocessors operate at microvoltages, they are almost inherently intrinsically safe, at least in the computational section. Furthermore, the voltage in microprocessors has decreased from 5V to 3.3V or 1.8V." The danger is never completely gone. Aladro continued, “Hazardous gases always need to be handled with caution. We need to focus on how we approach hazardous areas to reduce the risk. Intelligent devices are emerging to do this. Sensors are being designed or redesigned to be intelligent, with capabilities such as calibration. As a result, intelligent devices can self-check to ensure they are functioning correctly.” Sensors in hazardous areas are key components of safety control systems. Pietrzyk of Rockwell summarized, “Today we use performance-based process standards, not instruction-based standards. Performance-based standards begin with hazard analysis/assessment, and then the hazard level is determined. Besides ensuring personal safety, safety itself can also be seen as a business, because if your equipment doesn't malfunction, you can profit.” Doug Rutz, general manager of QComp Technologies, Greenville, WI, a company specializing in safety sensing in hazardous areas , said that these instrument cabinets and corresponding sensors control Class I, Div Two large gas compressors are located in the hazardous area. The OEM equipment can compress various flammable gases for multiple applications. He explained, “The 316 stainless steel panel is exposed, with only a simple snow cap for protection; it was actually designed for installation in Montana, where the cabinet is heated internally and exposed to only -30°F from the outside.” The cabinet panel is IP65 (NEMA 4X) rated because the equipment is exposed to wind, snow, rain, and dust. While Precision Digital’s instruments are designed for non-flammable Class I, Div. 2 environments, the HMI, requiring a custom 316 stainless steel cover, is flammable. For safety, the 75-cu-ft area behind the panel is protected with compressed air. Rutz said, “We love this equipment because one unit can meet all our requirements through setup. The circulation indicator can be reset without removing it from the panel. If the customer wants to change the range, it’s equally easy to set it up from the front panel. We use one instrument to indicate pressure, pumping, temperature, and motor current.”