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This post was last edited by yunrun on 2020-7-17 at 13:52. As transmitter technology continues to advance, many devices make extensive use of various types of transmitters for monitoring and measuring parameters such as pressure, temperature, rotational speed, and flow rate. A measurement system generally consists of three components: the transmitter, the transmission lines, and the display instruments (those that integrate the transmitter and display instrument together are not covered here). The transmitter is installed on the field equipment during use, while the display instrument based on a microcontroller is located in a control room far away from the equipment site; the distance between the two is usually several dozen to several hundred meters or even more. When calibrating this type of split transmitter, it is difficult to calibrate the transmitter, the transmission lines, and the display instrument as a single unit since they are installed in different locations; therefore, calibration often has to be carried out separately for each component. Both the transmitter and the display instrument can be calibrated using existing measurement standards. However, when it comes to the electrical signal output by the transmitter, it is necessary to analyze the impact of the transmission lines on different types of signals, taking into account the characteristics of signal transmission, to determine whether there is any attenuation in the signal after it travels over long distances, to what extent such attenuation occurs, and how far the signal can be transmitted. Only when both the transmitter and the display instrument have been calibrated successfully, and the effects of the transmission lines have been properly analyzed, can it be determined whether the transmitter is qualified. 1. Working principle of transmitters A transmitter is a component that is capable of detecting non-electrical quantities such as force, temperature, light, sound, and chemical composition, and converting them into electrical quantities such as direct current voltage and current according to certain rules. The function of a transmitter is to convert non-electrical quantities into electrical quantities, thereby enabling easy measurement, transmission, processing, and control. Depending on the output signal, transmitters are mainly divided into DC current signal transmitters and DC voltage signal transmitters. For DC current signals, a 4-20mA current transmission signal is commonly used. The lower limit is set at 4mA rather than 0mA because the transmitter can determine whether there is a fault in the circuit or if the transmitter is damaged by checking whether a current within the normal range can be detected (the minimum value under normal conditions is still 4mA). DC voltage signals typically use a DC voltage of 1-5V, and such voltage signals can be derived from DC current signals of 4-20mA. 2. Analysis of the transmitter wiring diagram: Depending on the wiring method, transmitters can be classified into two-wire, three-wire, and four-wire types. These three types of transmitters not only reflect differences in wiring methods, but also differences in operating principles and layout. ①Two-wire transmitter: A two-wire transmitter is one in which the transmitter is connected to the display instrument using only two wires, which serve both as power lines and signal lines. In the two-wire transmission method, the power supply, load resistor, and transmitter are connected in series. Most of the pressure transmitters and differential pressure transmitters in use in the market today are two-wire type. Two-wire transmitters are widely used due to their strong interference resistance, as well as their ability to save installation space and costs. The wiring diagram for the two-wire transmitter is shown in Figure 1. http://yunrun.com.cn/upload/202006/14/202006141251069496.png Figure 1: Schematic diagram of the wiring for a two-wire transmitter. The power supply for a two-wire transmitter is 24VDC; the output signal is 4-20mA, with a load resistance of 250Ω. The negative terminal of the 24V power supply has the lowest potential, and it serves as the common signal line. For two-wire DC voltage transmitters, if the transmission line is long, the transmitted signal tends to attenuate. Assuming the input impedance of the display instrument is Ri and the resistance of the wires is r, then: In this equation, when r=0, the two sides are equal. That is, when a voltage signal is transmitted, it is related to the resistance of the wires. The larger r is, the greater the signal attenuation; the smaller r is, the less the signal attenuation. Therefore, when voltage signals are transmitted, they are prone to attenuation due to the resistance of the wires. Most early transmitters were of the voltage-output type, that is, they converted the measured signal into a 0-5V voltage output; the signal power was less than 0.05W, and an analog-to-digital conversion circuit was used to convert the signal into a digital form for reading and control by a microcontroller. However, in situations where signals need to be transmitted over long distances or where there is significant interference, the use of voltage-output transmitters is greatly limited. Their weaknesses include poor resistance to interference and accuracy issues caused by line losses. Sometimes, an alternating current component is present in the output DC voltage, which can lead to incorrect readings by microcontrollers and result in control errors; in severe cases, this can even damage the equipment. For two-wire DC current transmitters, the current signal experiences no attenuation and is independent of the wire resistance r. Since the wire is in series with the sampling resistor, the current is the same everywhere ； The current signal is transmitted without attenuation, independent of wire resistance. Special case: When the line impedance is too high, assuming the current output by the current source remains constant, the voltage drop across the line becomes very high. If the current source is unable to generate such a high voltage, it will enter protection mode or reduce its output current; in this case, the measurements will be abnormal. It is precisely because DC voltage signals are not suitable for long-distance transmission that DC voltage transmitters are about to be phased out. Currently, two-wire transmitters mainly use 4-20mA DC current signals for transmission. ②3-wire transmitter: A 3-wire transmitter uses one wire for the positive power supply terminal, one wire for the positive signal output terminal, and one wire shared by both the negative power supply terminal and the negative signal terminal. The wiring diagram of the three-wire transmitter is shown in Figure 2; it is typically powered by 24VDC, outputs a signal of 4-20mA, and has a load resistance of 250Ω. http://yunrun.com.cn/upload/202006/14/202006141253299060.png Figure 2: Schematic diagram of wiring for three-wire transmitters. ③ Four-wire transmitters: In a four-wire transmitter, the power supply and the load resistor are connected to the transmitter separately; that is, the power supply and the transmitter’s output signal are transmitted using two separate wires each. The wiring diagram for the four-wire transmitter is shown in Figure 3. http://yunrun.com.cn/upload/202006/14/202006141255051898.png Figure 3: Schematic diagram of the wiring for a four-wire transmitter. The supply voltage for four-wire transmitters is usually 220VAC, though some are powered by 24VDC. The output signal is 4-20mA, with a load resistance of 250Ω. Currently, some temperature transmitters, level sensors, and humidity sensors still use a four-wire system. 3. Analysis of the impact of transmission lines: Due to shortcomings such as the poor interference resistance of DC voltage transmitters, line losses that affect accuracy, and the unsuitability for long-distance transmission, it has become standard practice to use 4-20mA current as the transmission signal in these transmitters. ①Two-wire DC current transmitters: The transmission distance of two-wire current transmitters depends on the signal excitation voltage, the voltage sensing resistance of the display instrument, the resistance of the wires, and the minimum operating voltage of the transmitter. ②In three-wire and four-wire DC current transmitters, the power lines and signal lines are separate from one another and do not affect each other. As long as the excitation voltage is always greater than or equal to the transmitter’s minimum operating voltage (which is easy to achieve), the transmitter will function properly, and the effect of wire resistance can be disregarded. For two-wire DC current transmitters, the maximum transmission distance for 4-20mA signals can reach several kilometers (in practice, such long wires are rarely needed) ； The larger the cross-sectional area of the wire, the greater the transmission distance. For three-wire and four-wire DC current transmitters, the effect of wires can be ignored. Since the electrical signal output by the transmitter is weak, the transmission line from the transmitter to the display instrument is usually long; especially for voltage signals, long-distance transmission is prone to electromagnetic interference. Based on the above analysis, it can be concluded that: ① When calibrating a DC current transducer, regardless of the wiring method used, the influence of the wires can be ignored (except in extreme cases); it is sufficient to calibrate both the transducer and the display instrument separately. ②If the transmission distance is long, thicker wires can be used to reduce the resistance of the transmission lines, and twisted-pair shielded wires can be chosen to improve interference resistance. ③For DC voltage transmitters, if the transmission line is very long, when calibrating, in addition to calibrating the transmitter and the display instrument, the impact of the transmission line must also be taken into account. Source: Technical Library http://yunrun.com.cn/tech/