Difference between Two - wire Heating Resistance and Three - wire Heating Resistance
Two-wire heating resistance we all know, but in the thermal resistance of the family in addition to two-wire heating resistor, there are three-wire, four-wire heating resistor, then the two-wire heating resistance and the three-line system What is the difference What?
Two-wire system with three-wire system (a positive power line, two signal lines, one of which is a total of GND) and four-wire (two positive and negative power lines, two signal lines) :
(1) less susceptible to parasitic thermocouple and voltage drop along the wire resistance and temperature drift effects can be used very cheap and more fine wire; can save a lot of cable and installation costs;
(2) When the output resistance of the current source is large enough, the voltage in the wire loop is induced by the magnetic field coupling, so that the current caused by the interference source is extremely small, and the twisted pair can be used to reduce the interference. Line and three-wire system must use shielded wire, shielded cable shield to properly ground.
(3) Capacitive interference can lead to receiver resistance related error, for 4 ~ 20mA two-wire loop, the receiver resistance is usually 250Ω (sampling Uout = 1 ~ 5V) This resistance is small enough to produce significant errors, The allowable length of the wire is longer and longer than the voltage telemetry system;
(4) each single show device or recording device can be in the length of the wire between the different channels for the exchange, not because of the length of the line caused by differences in precision, to achieve decentralized acquisition, decentralized acquisition of the benefits is: Decentralized collection, centralized control.
(5) 4mA is used for zero level, making it easy to determine open circuit and short circuit or sensor damage (0mA state).
(6) in the two-line output is very easy to add one or two lightning protection devices, is conducive to safe lightning protection.
Two-wire system without line resistance compensation, wiring is simple, but to bring additional resistance to the lead resistance. So do not apply to the manufacture of A-class precision thermal resistance, and in use when the lead and wire should not be too long.
Three-wire circuit resistance compensation, can eliminate the impact of lead resistance, measurement accuracy is higher than 2-wire system. As the process of detection components, the most widely used.
The two-wire heating resistor resistance change value and the connecting wire resistance value together constitute the output value of the sensor, the additional error caused by the wire resistance to the actual measurement value is high, for the measurement accuracy is not high occasions, and the length of the wire should not Too long
Three-wire system: the requirements of the three lead cross-sectional area and length are the same, measuring platinum resistance circuit is generally unbalanced bridge, platinum resistance as a bridge arm resistance, the wire connected to the power supply side of the bridge, The other two were connected to the platinum resistance of the bridge arm and adjacent to the bridge arm, when the bridge balance, the wire resistance changes have no effect on the measurement results, thus eliminating the wire line resistance measurement error , But must be a full-arm bridge, otherwise it is impossible to completely eliminate the impact of wire resistance. The use of three-wire system will greatly reduce the additional resistance of the wire resistance, the industry generally use three-wire connection.
The above is the difference between the two-wire heating resistor and the three-line system, we understand it? Should be able to find the advantages between the two, we can choose when the application of the use.
