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Why Pressure Transducer Use 4-20mA as Output Signal

The pressure transducer select 4-20mA as the output signal mainly considering the comprehensive reasons of the three aspects. The first one is safety, the second is using a current source to transmit signals better than the voltage source; the third is the choice of current starting and ending point. The 4-20 mA signal system is an analog signal standard developed by the International Electrotechnical Commission (IEC) for process control systems. As product technology is increasingly innovated, the output signals of pressure transducers are increasingly diversified to suit different production needs. Despite it, most pressure transducers have 4-20mA output signals and other types of signals for users to choose.

I. The first is security considerations from the application site

The focus of production safety is based on safety spark-type explosion-proof instruments. Based on the premise of controlling the energy of the instrument, reduce the static and dynamic power consumption for maintaining the normal operation of the instrument to a minimum. For pressure transducers that output a 4-20 mA standard signal, the supply voltage is typically 24 VDC. The main reason why the pressure transducer uses DC voltage is that it can not use a large capacity capacitor. Since in the explosion-proof occasion, if the filter capacitance of the meter exceeds 50μF, there will be great danger. If the pressure transducer use only a small capacity capacitor and inductor, just consider the distributed capacitance and inductance of the connecting wire between the transducer and the control room meter. For example, a 2mm2 wire has a distributed capacitance of about 0.05μF/km. For a single wire connection, the inductance is about 0.4mH/km. These values are much lower than the value of the detonated hydrogen, which is obviously very advantageous for explosion protection.

II. Pressure transducer transmits signal with current source better than voltage source

Usually, the distance between the production site and the control room is far. Some systems use a voltage source for remote signal transmission. When the resistance of the connecting wire is large, there is a partial pressure between the wire resistance and the input resistance of the receiving meter. It will produce a large error. If the current source is used for signal transmission, as long as the transmission loop does not branch, the current in the loop does not change with the length of the wire. It ensures the accuracy of the transfer.
In fact, not only pressure transducers, most instruments or meters in industrial production use 4-20mA current for signal transmission.
In the industrial field, using an instrumentation amplifier to perform signal modulation and long-line transmission will cause the following problems.
  1. Since the transmitted signal is a voltage signal, the transmission line is subject to noise interference.
  2. The distributed resistance of the transmission line produces a voltage drop.
  3. How to provide the operating voltage for the instrumentation amplifier at the production site is also a problem.
In order to solve the above problems and avoid the influence of related noise, the system instrument use 4-20mA current to transmit signals. Since the current is not sensitive to noise.

III. Reasons for maximum signal current is 20 mA

The maximum signal current of 20mA is mainly considering on safety, practicality, power consumption and cost considerations. The 30V voltage and 30mA current that cause the spark are the average lower limit of the igniting dangerous gas. In order to be safe, and refer to other traditional settings, many instruments are set to 24V power supply. At the same time, limit the current less than 30mA. In order to leave room for the signal, the signal upper limit is set to 20 mA.

IV. The reason for the signal starting point selects 4 mA

In order to reduce the complexity of the wiring, the two-wire system of the pressure sensor is much simpler than the multi-wire system. The two-wire that is, the power supply and the load are connected in series, and has a common point. At the measurement site, only two wires are used for signal communication and power supply between the pressure transducer and the control room meter. Two-wire system requires both signal transmission and power supply to the sensor. A certain energy supply is still required when the signal value is zero. At 24V, the energy supplied by the 4mA current is the lower limit of energy supply that most manufacturers can accept when setting standards at that time. At the same time, the electrical zero point of the meter is 4mA, which does not coincide with the mechanical zero point. This live zero is good for identifying faults such as power outages and disconnections. As a result, the 4-20 mA current standard is determined.

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