Application of NAMUR output standard in level measurement

　　The sensor signal output plays a very important role in level measurement. It outputs a switch quantity or continuous quantity to the DCS or PLC control room through the signal, and then the control room controls other equipment to control the material to avoid abnormal situations such as material overflow or equipment idling, thereby ensuring the safe and smooth production.

　　At present, in domestic level measurement, sensor signal output generally includes relay output, two-wire output, transistor output, contactless output and NAMUR output. Among them, relay output is the most widely used, and two-wire output is mainly used in intrinsically safe systems for the purpose of intrinsic safety. Transistor output and contactless output are rarely involved, and NAMUR output is the rarest.

　　The NAMUR standard first entered China in 2009. It was originally used in the proximity switch industry, so its working principle is defined by the proximity switch: the sensor needs to provide a DC voltage of about 8V. Depending on the distance of the metal object close to the sensor, a current signal of 1.2mA to 2.1mA will be generated. The typical value of the calibrated switch current is 1.55mA. When the current changes from low to high or equal to 1.75mA, an output signal change (from 0 to 1, or from OFF to ON) will be generated. When the current changes from high to low and is lower than 1.55mA, an output signal change (from 1 to 0, or from ON to OFF) will be generated. In this way, it can be used to check whether the metal object is approaching.

　　From the working principle of NAMUR, it can be seen that it is similar to the two-wire output. It provides power to the sensor through the isolation barrier (usually 8.2VDC, two-wire is 24VDC) and detects its current signal. The NAMUR output detection point is usually ≤1.2mA and ≥2.1mA (different companies set different detection points), and the detection point of the two-wire output is generally 8mA and 16mA. The switch signal is converted through the isolation barrier and finally output to the DCS or PLC control room. The difference between it and the two-wire system is that its current and voltage are smaller, and the power requirement of the safety barrier used is lower, but relatively speaking, its price is much more expensive than that of the two-wire output.

　　At present, the only manufacturers in the world that produce NAMUR signal output sensors are European and American manufacturers, such as VEGA and E+H in Germany, and their products are limited to vibration level measurement products with NAMUR signal output, such as vibration tuning fork level switches and vibrating rod level switches. Other products do not have NAMUR signal output. In China, there are currently no manufacturers producing sensors with NAMUR signal output in terms of level measurement. The main reasons are nothing more than the following two points:

　　1. The NAMUR signal output system is expensive;

　　2. Intrinsically safe two-wire output can completely replace NAMUR output.