Temperature measurement in hazardous environments with potentially explosive atmospheres, presents a specific challenge. In order to prevent fire and/or an explosion, the risk of ignition must be excluded from the system entirely, using various explosion protection techniques. These methods aim to ensure that sources of ignition (e.g. sparks and heat) are excluded or separated from the flammable material. Most equipment for use in a hazardous environment has to be certified to standards set out in the ATEX directive 2014/34/EU.
How to select temperature sensors for potentially explosive (Ex) atmospheres?
The choice of temperature sensor requires consideration of the type of hazardous atmosphere it is going to be used in as well as the required ATEX certification for that zone.
Firstly, it is important to consider the chemical composition of the application atmosphere so as to choose the correct sheath material for the temperature sensors and cables. This will ensure longevity of the components and minimize the effects of the hazardous atmosphere (e.g.corrosion) that could lead to device errors or failures.
A priority in potentially explosive atmospheres is that equipment should not malfunction (creating sparks or heat) such that an ignition of volatile substances occurs. There are three main approaches to explosion protection: pressurization, explosion proofing and intrinsic safety. Temperature sensing assemblies can be certified for all three approaches.
Pressurization
Temperature sensors can also be used in assemblies which have been pressurized and purged of all potentially explosive matter. In this case, the only particular consideration, aside from ATEX certification, is that the chosen temperature sensor has the correct sheath to ensure that the gas used in pressurization does not cause corrosion.
Explosion proofing
Thermocouples and RTD’s can be included in explosion proof assemblies. This is where robust instrument enclosures are used to contain potential explosions. ATEX approved connection heads are also an option for the explosion proof housing of temperature sensors.
Intrinsically safe circuits
Thermocouples and RTD temperature sensors, such as those supplied by Kamet, generally meet the ‘simple apparatus’ rule. This rule exempts low power and passive devices from needing ATEX certification. As such, mineral insulated (MI) temperature sensors are considered intrinsically safe and are therefore approved for building into intrinsically safe circuits. Most thermocouples and RTD’s even qualify as being nonincendive, which is an even lower risk class of intrinsically safe devices that are not even required to be connected to an intrinsically safe barrier system. Such a barrier system guards against higher energy surges in fault conditions
Cables
Mineral insulated (MI) cables offer a simple and cost-effective solution for connecting thermocouples and RTD’s to intrinsically safe circuits. By combining an MI cable with an Ex d rated gland, the temperature sensor assembly conforms to the ATEX requirements of intrinsically safe circuits in zones 1 and 2. This is thanks to the unique properties of MI cable, including their tolerance of high temperatures as well as their fire resistance. Furthermore, thanks to the highly compacted magnesium oxide powder layer, they are incapable of transmitting potentially explosive gas, dust or liquids through the cable.
Transmitters
The transmitters that link to the temperature sensors must be intrinsically safe if being used in a hazardous zone. They need to be tested for the possibility of them storing electrical energy and releasing a spark during a malfunction. If during the testing process the potential for stored energy is found to be sufficiently low, the transmitter will be approved as intrinsically safe. In general most transmitters will pass the intrinsic safety certification test.
Kamet and intrinsically safe temperature sensing components
Kamet offers a wide range of thermocouples and temperature sensors. In many applications in hazardous environments, these components meet the ‘simple apparatus’ rule and may even be considered nonincendive. Our stator slot RTD’s can be supplied in an ATEX certified version. The rest of our thermocouples and RTD’s do not come with ATEX certification and official approval for use in explosion proof systems needs to be arranged by the customer. However, as explained in this article, our sensors are compliant for most intrinsically safe circuit applications
In addition, Kamet can supply the following ATEX approved components associated with temperature sensing assemblies:
- Limatherm connection heads
- Cable glands
- Transmitters
- Complete Okazaki sensors
Lastly, all our mineral insulated cable is ATEX ready and supplied by our manufacturing partners, ARi and Okazaki.
Our team is available to answer any of the questions you might have regarding the choice of temperature sensor for potentially explosive atmospheres.