What is the instrument of { Two wire, three wire system, four wire}

Heidi to see professional guidance, instrument in two wire, three wire system, the distinction between the four wire system and the advantages and disadvantages of the analysis and application of two wire two wires is both the power cord and is commonly used in line 4 ~ 20 ma signal transmission. Three wire system is to point to a line as the main power supply, a line as the main signal, a thread for the power line and signal line of the public line; Generally used for 1 ~ 5 v signal transmission. Four wire system refers to the power supply two wires, signal two wires. Power supply and signal is separated from work. On the basis of three wire system, signal lines have their own land, not the power cord to. His current is part of the total current, and because compared with the power to total and three-wire system it can be difficult to calculate the current. Two wire, three wire system, the merits and demerits of the four wire? Two wire connection of the method is simple, is only for general power small of a sensor, such as: pressure, differential pressure change and temperature change, capacitive liquid level meter, rf admittance, electromagnetic flowmeter, vortex flowmeter, etc. In electricity by two wire sensor itself, is will affect the loading capacity. The advantage is the limit of thermal resistance to three wire connection, in order to dispel the bridging conductor resistance provoked measurement deviation. This is due to the measurement of thermal resistance of the individual is unbalanced bridge circuit. Thermal resistance as a bridge bridge arm resistance, the connecting wire ( From the thermal resistance is controlled by the) Also become a department of bridge arm resistance, resistance is unknown and the department with the circumstance temperature change, form the measurement deviation. Take three wire system, will lead a received the power supply end of the bridge, the other two root distinguish from thermal bridge arm resistance site and adjacent bridge arm, madly off wire line resistance measurement deviation. Four wire system has the advantage of being separate power supply and power, so there is no power on the machine power and signal connection, is suitable for high power sensors, such as ultrasound ( Because of its in order to increase the anti-interference ability, so the power of launch will be very big, so this money to four line in the model selection of the products, the second line generally weak anti-interference ability) , will not be able to make 2 line, can only be 4 lines, work power 2, output of 2. In the instrument work, everyone knows that there are two wire sensors, but in the big family of sensors in addition to the two wire sensor, and three wire system, four wire sensors; Is not only the sensor, as transmitter, liquid level meter, flow meter and so on all have different connection mode, so the different connection mode, what is the difference between in we use? Based on the sensor, heat resistance and the transmitter is given priority to, explain for everybody, different connection mode, what's the difference! Two wire the difference between the sensor and three-wire system sensor? Two wire and three-wire system ( A root is the power cord, two signal lines, one of the root GND) And four wire ( Two positive and negative power cord, two signal lines) Compared to the advantage of two wire system is: ( 1) Susceptible to parasitic thermocouples and pressure drop along the wire resistance and the influence of temperature drift, very cheap finer wire are available; Can save a lot of cables and installation costs; （ 2) When the current source output resistance is big enough, the magnetic field coupling induction in the conductor loop voltage, will not have a significant impact, because of the interference sources caused by the current minimum, general use twisted-pair cable can reduce interference; Two wire and 3 wire must be shielded wire, shielding wire shielding layer should be properly earthed; （ 3) Capacitive interference resistance leads to receiver error, for 4 ~ 20 ma two wire loop, receiver resistance is usually 250 & Omega; （ Sampling Uout = 1 ~ 5 v) This little resistance to the not enough to have a significant error, so, can allow the wire length longer farther than voltage telemetry system; （ 4) Each single reading device or recording device can be in wire length varies among different channels for exchange, not due to wire length range precision difference, realize the decentralized collection, the advantage of the decentralized collection is: decentralized collection, centralized control. （ 5) Add 4 ma used to zero level, the judgment open circuit and short circuit or sensor damage ( 0 ma state) It is very convenient. （ 6) In two lines of outlet is very easy to add one or two lightning protection against surge device, is advantageous to the lightning protection explosion-proof safety. No line resistance compensation two wire, wiring simple, but to bring in additional error lead resistance.

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