When performing temperature measurement, there are two ways to introduce the measurement signal into the monitoring room or the control room. The first method is to use the output terminal of the sensor to transmit the signal (ohm or mA voltage line number) generated by the on-site RTD or thermocouple as shown in Figure 1. Another method is to install a temperature transmitter near the measuring point. When the transmitter amplifies the sensor signal and transmits it through a twisted pair for temperature measurement, there are two ways to introduce the measurement signal into the monitoring room or control it. room. The first method is to use the output terminal of the sensor to transmit the signal (ohm or mA voltage line number) generated by the on-site RTD or thermocouple as shown in Figure 1. Another method is to install a temperature transmitter near the measuring point. The transmitter amplifies the sensor signal and transmits it to the control room through a twisted pair cable. Use compensation wires or four-wire low-impedance wires to connect to the monitoring room. Wiring with temperature transmitter and double cross-wire. Generally speaking, the direct wiring strategy is relatively cheap and easy to implement. In terms of cost, some people only change the temperature. The transmitter is used in important circuits. Today, when electronic products are becoming more and more mature, the cost of applying temperature transmitters to the simplest measurement is not higher than the cost of direct wiring, and the use of such transmitters can save time and avoid a series of headaches caused by maintenance. problem. Especially when the measuring point is far away from the control room, this kind of wiring with transmitter has more features and advantages. 1 Reduce wiring costs. The use of sensors that are directly wired to the control system requires long-distance wiring. When remote wiring, not only the wire itself is easily damaged, but also the cost is equivalent to the shielded copper of the 4-20mA signal of the ordinary transmitter More than 2 times the wire. In fact, general users are unwilling to change the original sensor to a transmitter because they mistakenly believe that the 4-20mA signal line needs to be reused when switching to a transmitter. This is not the case. The temperature transmitter can be directly used. The wires connected to the sensors, RTDs or thermocouples in the past will be wired, so there is no need to re-distribute any wires in the control room. This means that there is no additional installation time and wire material, but all the advantages of temperature transmitters can be obtained. 2 Protect the signal from field interference. Currently in many industrial environments, RFI (Radio Frequency Interference) and EMI (Electromagnetic Interference) will seriously affect the process measurement signal. Before eliminating RFI and EMI, which cause signal instability, we will first Considering the sources of interference, such as inverters, high-power switching devices, mobile phones, transformers, etc., if we have these devices on our production line, then we should pay attention to it. Using the direct wiring scheme, the signal generated by RTD (ohm) or thermocouple (mV) will be affected by RFI/EMI, and the sensor signal output wire will become the RFI/EMI antenna and introduce interference into the wire. Thereby affecting the transmitted signal. On the contrary, a reasonably designed temperature transmitter first converts the sensor signal into an analog signal (usually 4-20mA or 0-10v), and then transmits it, which can effectively improve the influence. The amplified signal can effectively resist RFI/EMI interference and can be transmitted over a long distance in the effective interference without affecting the measurement accuracy. When the input board adopts a differential input design, the interference can basically cancel each other (such as Shuo Ren STEC2000 Analog quantity acquisition board). 3 Prevent the ground loop from confirming that the selected transmitter is isolated. Generally, domestic temperature transmitters are equipped with photoelectric isolation circuits. The input and output power signals of the transmitter are isolated and protected. The signal is not affected by the ground loop, but it is not affected by the ground loop. It is also very important when grounding thermocouples, because they want to prevent their insulators from breaking and not working. 4 Reduce hardware and storage costs. When using direct wiring, it is necessary to match the sensor type with the input board of the DCS and controller/PLC. The sensor input board is much more expensive than the 4-20mA input board. When a large number of different types of sensors are used in a factory, it is necessary to purchase a large number of different types of boards for backup. This is not only a cost issue, but also There is labor. 5 Strengthen the accuracy and stability The use of temperature transmitters can enhance the measurement accuracy. The measurement readings of DCS and PLC systems are within a wide sensor range. As we all know, a smaller measurement range can obtain higher measurement accuracy, so the temperature transmitter can be calibrated in any interval within the full measurement capability of the sensor. The measurement accuracy is much higher than that of the direct wiring measurement strategy. 6 Simplify the project to prevent miswiring. The use of temperature transmitters can replace many sensor wires and DCS/controller PLC connection channels, which can greatly simplify the engineering design drawings. You only need to indicate the type (twisted pair 4-20mA) when designing. Such a system of one wire and one input board means that future maintenance operations will become simple and easy, and those four-wire resistors and three-wire resistors, high-end-low-end are never considered, and it also reduces the possibility of eliminating loop miswiring. .
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