If you are just starting a career in oil and gas, chemical processing or any industry that involves flammable materials, dust and other potentially ignitable substances, you need to understand the dangers associated with hazardous locations. This includes knowing what to look for when purchasing instruments for challenging and potentially explosive applications.
As a product technical leader at Ashcroft - a recognized authority in pressure and temperature measurement instrumentation - I understand these challenges well. In this article, you will find valuable insights to help you navigate the complexities of monitoring temperature in hazardous location applications. This includes a definition of a hazardous location, the agency standards and approvals required for equipment used in these areas and why temperature instruments, in particular, to be fully certified to ensure the safety and compliance of your operation.
When you are done reading, you will have a foundation of information and be more confident about your instrument choices for these challenging applications. You will also have access to additional resources that will deepen your knowledge of RTDs and thermocouples.
Definition of a hazardous location
In my Understanding the Basics article on this topic, we refer to the International Electrotechnical Commission's (IEC) definition, which generally describes a hazardous location as an area where there is a threat of a fire or explosion. More specifically, if there is a flammable substance present (such as fuel), an ignition source (high temperature or spark) and air (oxygen), there is a high risk for a fire or explosion to occur. Removing any of these elements from the area eliminates the risk of fire or explosion.
Figure 1: Elements of a fire or explosion.
How RTDs and thermocouples are used in high-temperature applications
Resistance temperature detectors (RTDs) and thermocouples play an important role in applications where accurate temperature measurements are critical. Here's a general idea of how they work:
- RTD sensing elements are generally made from a pure metal such as platinum. When the temperature of the process being measured increases or decreases, the resistance of the RTD changes. This enables the RTD to provide accurate temperature readings of the process. Read, What is an RTD to learn more.
- Thermocouples are made from two kinds of metal wires that are joined at one end to form a junction, also known as the hot junction. The other ends of the wire are attached to a reference point, which is called the cold junction. When the hot and cold junctions have different temperatures, a voltage is created that indicates the temperature. Read, What is a Thermocouple to learn more.
While these temperature sensors are designed to ensure accurate temperature readings for safety and efficiency in your process. only those that are fully certified to meet the specific area classification and the method of protection for hazardous locations should be considered for your system.
What approvals are necessary to be fully certified for hazardous locations?
To be considered for use in a hazardous location where the potential for explosion is present, temperature probes should be certified by an approval agency to meet one or more of the following standards:
National Electric Code (NEC)
NEC is a set of standards created by the National Fire Protection Association (NFPA). For the purposes of installing electrical wiring, the NEC 500 uses a class and division system and NEC 505 uses a zone system to clarify the level of risk. See the tables below.
Figure 1: National Electric Code (NEC) 500 system
Figure 2: National Electric Code (NEC) 505 system
International Electrotechnical Commission for Explosive Atmospheres (IECEx) or ATEX
IECEx and ATEX are similar in that they both are used to classify whether equipment can be used in hazardous environments. The IECEx is an internationally recognized voluntary standard. If an instrument or equipment is IECEx certified, it can be used globally without having to be re-tested and re-certified for every country.
ATEX, on the other hand, is a mandatory requirement for instruments and equipment used in the European Union.
Factory Mutual (FM) approvals
An instrument with FM approval means it has gone through comprehensive evaluation at FM's testing facilities.
What makes an RTD and thermocouple fully certified?
Fully certified direct-mount RTD and Thermocouple probes like the Ashcroft® S50 Industrial RTD and the Ashcroft® S50 Industrial Thermocouple are both rated explosion-proof Class 1 Division I. This means the electrical enclosure (head) can contain a defined explosion and the RTD or thermocouple is mounted to the head using a tested and approved flame path. Both the Ashcroft® S50 RTD and Thermocouple probes have an FM approval for this configuration.
Fully certified remote-mount probes like the Ashcroft® S80 Thermocouple and Ashcroft® S81 RTD with remote explosion-proof heads are rated non-incendive Class 1 Division II. These probes are designed with a non-incendive probe for limiting energy to prevent an explosion without using a barrier. The RTD or Thermocouple probe is connected to a remote explosion-proof head using an approved cable gland. The Ashcroft® S80 Thermocouple and Ashcroft® S81 RTD have FM approval for this configuration.
What is not considered a fully certified RTD or Thermocouple?
Often you will see a manufacturer offering an RTD or Thermocouple probe with an optional explosion-proof head. This requires the end user to determine the correct flame path and mounting of the probe to the head to fulfill the requirements of the head manufacturer’s approval.
If the end user didn’t meet all the requirements and an incident were to occur the fault would be with the end user, not the head manufacturer. So, it is best practice to buy a probe that is fully certified by the manufacturer.
Ready to learn more?
Now that you understand why it is critical to use fully certified RTDs and thermocouples in hazardous location applications, you should be in a better position to select the right instruments for your operation. For more information, here are some additional resources for you to check out:
- When to Use an RTD or Thermocouple Temperature Sensor
- Grounded vs. Ungrounded Thermocouple Junctions
- How Much Do Temperature Sensors Cost?
Feel free to contact a product expert if you have questions. In the meantime, you can access our guide on RTD and Thermocouple Probes.
