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How far can the 4-20mA signal of the temperature and humidity transmitter be transmitted?
In recent years, with the maturity of technology and the development of society, the demand for intelligence in industrial and agricultural production and daily life has also increased. In order to improve the intelligence level in the use of these industries, the 'humidity' indicator and Like the 'temperature' indicator, it has been introduced into monitoring applications in all walks of life, and temperature and humidity transmitters are increasingly being used in various industries. Temperature and humidity transmitter refers to a device or device that can convert temperature and humidity into electrical signals that can be easily measured and processed. This device that converts physical quantities into electrical signals is called a transmitter. The most widely used in the industry is to use 4-20mA current to transmit analog quantities. Why use 4-20mA current? The reason for using the current signal is that it is not easy to be interfered. And the internal resistance of the current source is infinite, and the resistance of the wire in series in the loop does not affect the accuracy, and it can transmit hundreds of meters on the ordinary twisted pair wire. The upper limit is 20mA because of the explosion-proof requirement: the spark energy caused by the on-off of the 20mA current is not enough to detonate the gas. The reason why the lower limit is not set to 0mA is to be able to detect disconnection: it will not be lower than 4mA during normal operation. When the transmission line is broken due to a fault, the loop current drops to 0. 2mA is often used as the disconnection alarm value. There are two reasons. One reason is to avoid interference, another reason is that the 4-20mA uses a two-wire system, that is, the two wires are the signal wire and the power wire at the same time, and 4mA is to provide the static working current of the circuit to the transmitter Used. How far can the 4-20mA current signal of the temperature and humidity transmitter be transmitted? Interference factors:①It is related to the level of excitation voltage;②It is related to the minimum working voltage allowed by the transmitter;③It is related to the voltage-taking resistor used by the board device to collect current The size is related;④It is related to the size of the wire resistance. Through these four related quantities, the theoretical transmission distance of the 4-20mA current signal can be calculated. Among them, Uo is the supply voltage of the transmitter, and it must be ensured that Uo≥Umin at full load (current Iu003d20mA). That is:. According to this formula, the maximum wire resistance can be calculated when the transmitter is at the lowest operating voltage. Assumption: Known: Ueu003d24V, Iu003d20mA, RLu003d250Ω, Uminu003d12V. Calculate the maximum value of r to be 175Ω:According to the calculation formula of wire resistance:, among them:ρ——resistivity (copper resistivityu003d0.017, aluminum resistivityu003d0.029) L——Line length (unit: meter)S——Line section (unit: square millimeter) Note: The resistance value is proportional to the length and inversely proportional to the cross-sectional area. The longer the wire, the greater the resistance, and the thicker the wire, the lower the resistance. Take copper wire as an example, ρu003d 0.017 Ω·mm2/m, that is, the resistance of a copper wire with a cross-sectional area of u200bu200b1mm2 and a length of 1m is 0.017Ω. Then the wire length of 175Ω corresponding to 1mm2 is 175/0.017u003d10294 (m). Therefore, in theory, 4-20mA signal transmission can reach tens of thousands of meters (depending on factors such as different excitation voltages and the lowest working voltage of the transmitter). Then why is the DC voltage signal not suitable for long-distance transmission? Described in the standard 'GB/T 3369.2-2008 Analog Signals for Process Control Systems Part 2: DC Voltage Signals': 'In contrast to the analog DC current signals specified in GB/T 3369.1-2008, this part of the The specified analog DC voltage signal should not be used for long-distance transmission'. The main reason is that the voltage signal will be attenuated after long-distance transmission, and it is susceptible to interference.
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