There are five common signal output types for sensors in level switches: relay output, two-wire output, transistor output, contactless output and NAMUR output. Among them, relay output is the most widely used, transistor output and contactless output are rarely involved, and two-wire output and NAMUR output are mainly used in intrinsically safe systems for the purpose of intrinsic safety. So what is the difference between two-wire output and NAMUR output in terms of application?
The two-wire system is a communication and power supply method relative to the four-wire system (two power supply lines and two communication lines). The power supply line and the signal line are combined into one, and the two lines realize communication and power supply. The two-wire instrument is a line without a power supply, that is, they do not have an independent working power supply. The power supply needs to be introduced externally, usually for the safety barrier to power the sensor, and the signal it transmits is a passive signal. The two-wire system generally uses 4~20mA DC current to transmit signals. The upper limit of 20mA is due to the explosion-proof requirements: the spark energy caused by the 20mA current on and off is not enough to ignite the gas. The reason why the lower limit is not 0mA is to detect line breakage: it will not be lower than 4mA during normal operation. When the transmission line is disconnected due to a fault, the loop current drops to 0. 2mA is often used as the line break alarm value, and 8mA and 16mA are used as the level alarm values.
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. Its working principle is: the sensor needs to provide a DC voltage of about 8V. According to the distance of the metal object approaching the sensor, a 1.2mA to 2.1mA current signal 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 PLAC 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, its price is much more expensive than that of the two-wire output.
At present, in China, the two-wire output is more widely used in the intrinsic safety system, and the NAMUR output is less used. The reasons are nothing more than the following two points:
1. The NAMUR signal output system is expensive;
2. The intrinsically safe two-wire output can completely replace the NAMUR output, and its price is cheaper.
