Temperature measurement can be a hot issue

Companies that use temperature measurement products - including thermocouples and RTDs - that are installed in potentially explosive atmospheres need to ensure that the sensor assembly (sensor, enclosure and terminal glands) meets the overall area classification requirements for each installation. In Europe, this is covered by the ATEX Directives, including the ATEX harmonised standards.

Directive 94/9/EC covers equipment that is intended for use in potentially explosive atmospheres. The CE mark on equipment such as this represents the manufacturer’s declaration of compliance to this directive and to the EN standards that are harmonised to it. All equipment should be supplied with a declaration of conformity, stating compliance with these standards and with product certification provided by a notified body, such as BASEEFA or SIRA in the UK.

If the chosen certification classification is Exia for use in zone 0, 1 or 2, there is no need for product certification in the case of thermocouples or RTD sensors, but care must be taken to ensure the correct installation with an appropriate barrier, that the IP rating of the termination is correct and that there is appropriate terminal clearance.

Of more concern is when thermocouples or RTD sensors are designated Exd (flameproof). In this case, product certification is 100 per cent required. Care should be taken when using a terminal housing - the sensor should be terminated in a suitable Exd-certified enclosure. A standard temperature assembly normally requires a spring-loaded sensor to ensure good contact is made with the tip of the thermowell. The base of the housing is therefore fitted with a flame-path-controlled bore or collar that provides a flame-path seal into the base of the housing, enabling free movement of the sensor via spring loading.

This can lead to certification problems; although the enclosure rating is not affected, the inclusion of the sensor introduces a possible fault condition, which requires additional certification.

Okazaki has overcome this by certifying the complete assembly as Exde IIC T6, providing Exd certification for the enclosure and Exe (increased safety) certification for the sensor. This means that the user can install the assembly in a Zone 1 hazardous area without thermocouple or RTD IS safety barriers. If a head-mounted, hockey-puck-style transmitter is installed, no extra certification is needed for the electronics module.

For many years, the EN50014 standards have been the backbone of ATEX compliance. However, by the end of 2008 most of the series was superseded by standards from the EN60079 series and has been withdrawn. This presents a potential problem for holders of ATEX EC-type examination certificates to these withdrawn standards.

Other scenarios also require careful consideration. For example, if companies install the latest Hart and digital fieldbus transmitters, care should be taken to ensure that approval of the complete assembly is certified, not just the transmitter. First, the integral transmitter and housing should carry its own certification and, second, the temperature sensor coupled to this transmitter should also carry certification or be glanded to the unit using an approved terminal gland.

To simplify installation, Okazaki has developed its FPN (flameproof nipple) range of thermocouple or RTD sensors. These units, which are pressure tested to 1,000psi, can be fitted to other manufacturers’ temperature transmitters while maintaining the spring-loading feature and an IP66/67 rating, without requiring any compression glands that would restrict movement of the sensor. By working with BASEEFA, Okazaki has achieved Exde IIC T6 certification for the FPN range.

When terminating mineral-insulated (MI) thermocouples and RTD sensors into suitably certified Exd or Exe enclosures, engineers also need to ensure that the terminal glands carry the appropriate certification. The glands themselves need to be approved for use with sensors rather than just for cables because, unlike cables, a temperature sensor is not terminated at each end and so can have different fault conditions.

Chris Chant is business development manager, Okazaki Manufacturing.