What is NAMUR Output in a Level Sensor？

In recent years, there have been frequent inquiries about whether we have NAMUR output interface tuning fork level switch products. Currently, NAMUR products are not widely used in China, leading to some unfamiliarity. Therefore, it is necessary to briefly introduce the concept of NAMUR output signals.

NAMUR output is a specific current output signal used to indicate the switching status, adhering to particular electrical standards that ensure signal compatibility and reliability. NAMUR, which stands for “Normenarbeitsgemeinschaft für Mess- und Regeltechnik in der Chemischen Industrie” (Standardization Association for Measurement and Control in the Chemical Industry), is a standard established by the German committee for measurement and control. This standard was initially proposed by Pepperl+Fuchs (P+F) and is applicable to safety barrier manufacturers as well as PLC and DCS I/O module manufacturers. Originally a German standard (DIN19234), it later became a European standard (EN 50227, DIN EN 60947-5-6). The corresponding Chinese national standard is GB/T14048.15-2006/IEC60947-5-6:1999, titled Low-voltage switchgear and controlgear Part 5-6: Control circuit devices and switching elements — DC interface for proximity sensors and switching amplifiers: NAMUR.

NAMUR Standard and Working Principle

The NAMUR standard was initially used in the proximity switch industry, where GB/T14048.15-2006 specifies the current change for switching state ΔI1 to be within the range of 1.2mA to 2.1mA, with a DC power supply (8.2±0.1)V. A NAMUR interface proximity switch requires an approximately 8V DC power supply (typically 8.2V), which is provided by an isolated safety barrier. When powered, a NAMUR proximity switch generates a small milliamp current in the circuit when no metallic object is nearby. For instance, with a NAMUR normally open proximity switch, the safety barrier signal isolator detects the input signal current. If the detected signal is greater than 2.1mA, it indicates a nearby metallic object. If the signal is below 1.2mA, it indicates no nearby object. The safety barrier then converts this to a corresponding relay signal or transistor switch signal.

From the working principle of NAMUR, it is evident that a NAMUR proximity switch checks the proximity of a metal object through current changes and outputs a current signal to the DCS or PLC for control. Its principle is similar to a two-wire 4-20mA system, but NAMUR operates at lower current and voltage. Due to its low-voltage power supply and low current output design, NAMUR output sensors are intrinsically safe explosion-proof instruments, typically used in conjunction with isolated safety barriers. These sensors can be applied in hazardous areas requiring explosion-proof measures. The safety barrier provides an 8V DC power supply (usually 8.2V) to the sensor and monitors its current signal. The detection points are generally ≤1.2mA and ≥2.1mA (varying among different manufacturers), and the safety barrier converts this switch signal to relay or transistor switch signals, ultimately outputting it to the DCS or PLC control room.

Comparison with Two-wire 4-20mA Systems

Since NAMUR output is based on the two-wire system, both are applied in intrinsically safe explosion-proof areas. The difference between NAMUR and two-wire systems lies in the lower current and voltage levels of NAMUR, requiring safety barriers with lower power specifications. However, NAMUR output tends to be more expensive than two-wire systems. Compared to two-wire systems, NAMUR output is advantageous in environments with higher explosion-proof requirements. In recent years, NAMUR output signals have begun to be applied in level measurement instruments. The Chinese brand, Jiwei, has introduced the Ring-11 tuning fork level switch with a NAMUR signal output interface, meeting customer needs. The following sections will provide a more detailed explanation of the Ring-11’s NAMUR output interface to enhance understanding of NAMUR signals.

Ring-11 Tuning Fork Level Switch with NAMUR Interface

The Ring-11 tuning fork level switch can operate in low-level dry-run protection mode and high-level overflow protection mode. The functionality of its NAMUR interface output is outlined in Table 1. From Table 1, it can be observed that this tuning fork switch has both low-level and high-level working modes. The power supply voltage is 8.2±0.1VDC, and the device features a single red LED indicator with the following output current and function specifications:

• Red LED On: Output current ≥ 2.1mA (High-level alarm, non-contact state / Low-level alarm, contact state)
• Red LED Off: Output current ≤ 1.0mA (High-level alarm, contact state / Low-level alarm, non-contact state)
• Red LED Flashing: Output current ≤ 1.0mA (Fault alarm)

Figure 1 illustrates the typical connection diagram of the Ring-11 tuning fork level switch’s NAMUR interface with field control equipment and a safety barrier. Figure 2 shows the panel layout of the Ring-11 NAMUR output model. For safety reasons, wiring should be performed only when the device is powered off. For explosion-proof models, wiring must comply with relevant explosion-proof wiring requirements. The product’s housing is made of aluminum alloy, and when used in Zone 0, it is necessary to prevent ignition hazards due to impact or friction. The housing contains non-metallic materials, posing a potential electrostatic charge hazard, so friction should be avoided. When cleaning, use a damp cloth.

The NAMUR output version of the Ring-11 tuning fork switch, produced by Jiwei, is designed with low voltage and low current to comply with intrinsic safety explosion-proof standards. It has obtained intrinsic safety explosion-proof certification and is compatible with safety barriers, making it suitable for hazardous areas requiring explosion protection. This model has been widely used in industries such as water treatment, chemical processing, environmental protection, food, and pharmaceuticals for liquid level measurement in tanks, vessels, and various pipelines.