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This article is to provide overview of the EX certified sensors from Rheonics portfolio. This article should not be used as guide to install the sensor in EX environment. Installation of EX sensors should always be done by EX trained personnel according to instructions in Rheonics EX installation manuals. Those manuals are the only approved guide for installation. EX installation manuals supersedes information even if provided by Rheonics personnel.

1. Ex-sensor connection

SRV and SRD sensors can also be delivered in a version that is intrinsically safe.

It means that, as long as the sensors are installed and operated as specified in the EX manual, they are incapable of igniting the explosive atmospheres in which they operate. EX installation manual.

Electronics must be always in the safe zone. Sensor probe can be in all zones as long as the Zener Barriers are present between the sensor probe and sensor electronics

Figure 1. Intrinsically safe installation of Rheonics inline viscometer SRV.

Figure 2. Intrinsically safe installation of Rheonics inline density meter SRD with simultaneous density, viscosity, and temperature measurement.

2. What is an intrinsically safe zone?

A method of analyzing and classifying a potentially hazardous area is necessary in order to protect installations from a potential explosion.

This is to ensure the correct equipment is selected and installed, preventing any safety issues.

An installation can be classified in two ways, depending on where it is located.

In countries that have adopted the IEC (International Electrotechnical Commission) philosophy, this is called "zoning“. The second classification is done by classes, divisions, and groups to determine the level of safety that is required based on NEC 501.

Rheonics sensors fall in the category with level protection EX ia, as they have one of the highest level of protection possible on any EX-certified device by being Intrinsically safe and with energy levels below the lowest requirement. EX certificates for sensors are available on Rheonics and IECEX website to download.

Figure 3. Intricately safe zones categories. 

3. Ex protection – The difference between the Zone and Division model

“The product is suitable for use in Zone 1, as well as for use in Class 1, Div 1.” This statement is heard or read often in descriptions of products for use in hazardous areas. But what is the difference between Zone 1 and Class 1, Div 1? Let’s compare the Zone model with the Division model.

3.1. International view

Sensors and equipment, for example, receive explosion protection in the IEC 60079 series of standards. The ATEX series of standards, which is also the basis for EU directive 2014/34/EU (ATEX), describes, among other things, the types of ignition protection for electrical equipment as well as the evaluation or classification of hazardous areas within plants according to different categories.

3.2. National particularities

There may be deviations from this standard in national regulations. North America, for example, uses different electrical systems for the construction and operation of its infrastructure. Regulations are outlined in the National Electrical Code (NEC) in the US and in the Canadian Electrical Code (CEC) in Canada. One of these differences is the determination and classification of hazardous areas. Below is a simplified comparison of the various designations for gas hazardous areas.

Figure 4. Hazardous area classification with both Class, Div, and Zone methods.

The NEC and CEC have been updated to include more international standards so that both of them now support the Zone model for the classification of hazardous areas, and the CEC even requires it for new installations.

3.3. Rheonics Sensors are available with ATEX & IECex certifications

ATEX (2014/34/EU) Certified

Rheonics’ ATEX certified Intrinsically Safe sensors comply with ATEX Directive 2014/34/EU and are certified for Intrinsic Safety to Ex ia. The ATEX directive specifies minimum and essential requirements related to health and safety to protect workers employed in hazardous atmospheres.

Rheonics’ ATEX certified sensors are recognized for use within Europe and internationally. All ATEX certified parts are marked with “CE” to indicate compliance.


IECEx Certified

Rheonics’ intrinsically safe sensors are certified by IECEx, the International Electrotechnical Commission for certification to standards related to equipment for use in explosive atmospheres.

This is an international certification that ensures safety compliance for use in hazardous areas. Rheonics sensors are certified for Intrinsic Safety to Ex i.

Rheonics offers intrinsically safe sensors certified by ATEX and IECEx for use in hazardous environments. These sensors comply with the essential health and safety requirements relating to the design and construction of equipment and protective systems intended for use in potentially explosive atmospheres.

The intrinsically safe and explosion proof certifications held by Rheonics also allows for customization of an existing sensor, allowing our customers to avoid the time and costs associated with identifying and testing an alternative. Custom sensors can be provided for applications that require one unit up to thousands of units; with lead-times of weeks versus months.

Rheonics SRV & SRD are both ATEX and IECEx certified by TÜV Rheinland.

4. Rheonics Ex sensor marking

All Rheonics Ex sensor has the marking identifying the certification for TÜV Rheinland, ATEX, IECEx.

Figure 5. IECEx Explosion marking description

Figure 6. : ATEX marking description

5. Valid Ex Sensor topologies.

The information below is only provided as an example - Trained Installation personnel should ensure they use the EX Installation Manual to correctly install Rheonics sensor in Hazardous areas with EX classification. Ex Installation Manual.
 These diagrams illustrate how the sensors, their cables, and the associated Zener diode barriers are connected to ensure that the entire system is intrinsically safe. Depending on the configuration of the sensor as well as the choice of temperature measurement connection, a particular connection method can be selected. 

The following rules must be followed for all variants:

  • The cable shield must be connected to one of the sensor-side grounding terminals of the Zener diode barrier associated with the transducer coil circuit. In the following circuit examples, this is Pepperl + Fuchs Z757 Zener diode barrier but other brands with similar characteristics can be used - refer to the EX characteristics table in EX manuals to assess your selection of barrier. The cable shield should be connected to either terminal 2 or 3 of the Zener diode barrier. 

  • The Zener diode barrier(s) must be grounded to a known-reliable earth ground. It is the responsibility of the installer to ensure the quality and reliability of the selected ground point.

  • Zener diode barriers are delivered with installation instructions that include information about grounding methods. Zener diode barriers that are mounted on DIN rails generally have grounding mechanisms that engage with the rails, which themselves must then be tied to a known-reliable grounding point. It is the responsibility of the installer to ensure that the Zener diode barriers are properly and reliably grounded according to the manufacturer’s specifications. Refer to manufacturer datasheet and installation instructions.

  • Input:  240V Maximum AC Single Phase, this is exclusive for Ex sensor configuration and would be the best to protect the Zener barriers used in the configuration, only single phase power supplies should be used.

5.1. Variant 1

Variant 1 shows a Standard sensor with an RTD(Pt1000) temperature element with a 4-wire connection of the RTD(Pt1000) circuit to the electronics unit. 2 Zener diode barriers with 2 channels each are required for the RTD(Pt1000) circuit.

A single 2-channel Zener diode barrier is required for the sensor coil circuit.

This configuration provides the highest temperature accuracy but requires two Zener diode barriers for temperature connection.

The example shown below uses the specified Zener barriers below.


-Ex Rheonics transmitter.

-Ex SRV/SRD probe.

-Ex sensor cable.

-Zener barrier Pepperl+Fuchs Z041.

-Zener barrier Pepperl+Fuchs Z757. 

Figure 7. Variant 1 wiring connection.
5.2. Variant 2

Variant 2 shows a Special sensor with no RTD(Pt1000) temperature element installed.

No Zener diode barriers are required for the RTD(Pt1000). A single 2-channel Zener diode barrier is required for the coil circuit.


-Ex Rheonics transmitter.

-Ex SRV/SRD probe.

-Ex sensor cable.

-Zener barrier Pepperl+Fuchs Z757. 

Figure 8. Variant 2 wiring connection.
5.3. Variant 3

Variant 3 shows a Standard sensor with RTD(Pt1000) installed, with 3-wire connection to the electronics unit.

A single 2-channel Zener diode barrier is required for the RTD(Pt1000) circuit. A single 2-channel Zener diode barrier is required for the coil circuit.

The advantage of this circuit is that one less Zener diode barrier is required for the installation. Although the electronics unit will function with this circuit, the accuracy of the temperature measurement must be verified and possibly recalibrated by the end user.


-Ex Rheonics transmitter.

-Ex SRV/SRD probe.

-Ex sensor cable.

-Zener barrier Pepperl+Fuchs Z041.

-Zener barrier Pepperl+Fuchs Z757. 

Figure 9. Variant 3 wiring connection.
5.4. Variant 4

Variant 4 could be used as an alternative to Variant 1 using a different Barrier manufacturer. 

2 Stahl Zener diode barriers with 1 channel are required for the protection of the Pt 1000 circuit.

A single 2-channel Zener diode barrier is required for the sensor coil circuit.

This configuration provides the highest temperature accuracy but requires two Zener diode barriers for temperature connection.


-Ex Rheonics transmitter.

-Ex SRV/SRD probe.

-Ex sensor cable.

-Zener barrier STAHL 9002/11-120-024-001.

-Zener barrier Pepperl+Fuchs FZ757.

Figure 10. Variant 4, STAHL Zener barriers used with the PT1000.


  1. Pepperl+Fuchs Zener Barriers

  2. Stahl Zener Barriers

  3. EX Rheonics Installation Manual

Recommended articles: 

  1. Powering the Rheonics viscosity and density sensor - SRV, SRD, DVP, DVM, SME 

  2. Is it still possible to measure viscosity without connecting the PT1000 to our SRV? 

  3. Connecting the Rheonics inline viscometer SRV without temperature output 

  4. Rheonics passes surveillance audits for ATEX & IECEx after relocation to new facility » rheonics :: viscometer and density meter 


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