The signal output types commonly used by sensors in level switches generally have the following five types: Relay output, two-wire output, transistor output, non-contact output and NAMUR output, of which the relay output is the most widely used, transistor output and non-contact output are rarely involved, two-wire output and NAMUR output are mainly used in the intrinsic safety system, 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, two communication lines), which combines the power supply line and the signal line into one, and the two lines achieve communication and power supply. Two-wire instruments are not connected to the power line, that is, they do not have an independent working power supply, the power supply needs to be introduced from the outside, usually for the safety gate to supply power to the sensor, the signal transmitted is passive signal. The two-wire system generally uses 4~20mA DC current to transmit the signal, and the upper limit is 20mA because of the requirements of explosion-proof: the spark energy caused by the 20mA current break is not enough to ignite the gas. The reason why the lower limit is not 0mA is to detect the broken line: it will not be lower than 4mA in normal operation, and when the transmission line is broken due to a fault, the loop current drops to 0. 2mA is usually used as the wire break alarm value, 8mA and 16mA as the level alarm value.
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, and a current signal from 1.2mA to 2.1mA will be generated according to the distance of the metal object close to the sensor. The typical value of the calibrated switching current is 1.55mA. When the current is low to high or equal to 1.75MA, an output signal will change (from 0 to 1, or from OFF to ON). When the current goes from high to low below 1.55mA, an output signal changes (from 1 to 0, or from ON to OFF). So it can check for the proximity of metal objects.
As can be seen from the working principle of the NAMUR, it is similar to the two-wire output, providing power to the sensor through the isolation gate (usually 8.2VDC, 24VDC in the two-wire system) and detecting its current signal. The NAMUR output detection point is usually ≤1.2mA and ≥2.1mA (the detection point set by different enterprises is different), the two-wire output detection point is generally 8mA and 16mA, and the switching signal is converted through the isolation grid 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 requirements of the safety gate used are lower, but relatively, its price is much more expensive than the output price of the two-wire system.
At present, in China, the application of intrinsic safety system is more two-wire output, NAMUR output application is less, the reason is nothing more than the following two points:
1. NAMUR signal output system is expensive;
2. the intrinsic safety two-wire system output can completely replace the NAMUR output, and its price is cheaper.
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