Why do pressure transmitters use 4-20mA current for transmitting analog signals?

As a company dedicated to the research and production of sensors and automation control technologies, AUTO Instruments always stays at the forefront of the industry, focusing on providing innovative solutions for industrial automation, smart cities, smart water management, and other fields. Today, we will delve into a key technology widely used in industrial automation systems—the 4-20mA current signal transmission.

So, why do we typically choose to use 4-20mA current to transmit analog signals in industrial automation? The answer is actually quite simple, yet profound.

The types of pressure transmitters involved are generally as follows:

   Differential Pressure Transmitters, Absolute Pressure Transmitters, Flange-Mounted Level Transmitters, Radar Level Meters, Ultrasonic Flow Meters, Vortex Flow Meters, Turbine Flow Meters, Electromagnetic Flow Meters, and others.

4-20mA is a commonly used analog current signal and is a standard signal. It means the minimum current is 4mA and the maximum is 20mA. When transmitting signals, because there is resistance in the transmission wires, if a voltage signal were used, a voltage drop would occur along the wire, which would cause errors at the receiving end. Therefore, current signals are generally used as the standard for sensor signal transmission.

Due to the strong anti-interference ability of current signals, their resilience to external factors, and their capacity for long-distance transmission, they are widely used in industrial sensors. To meet the different requirements of industrial control, 4-20mA current sensors generally come in four-wire, three-wire, and two-wire versions.

Advantages of Current Signals (4-20mA Signal):

1. Suitable for long-distance transmission: Current signals are not affected by the resistance of the transmission wire, whereas voltage signals would suffer from voltage drops due to the wire’s resistance, leading to inaccurate measurements. The maximum distance for 4-20mA signal transmission is usually controlled within 100 meters; beyond this, digital signals (e.g., RS485 communication) are recommended.

2. Cost-effective: Current signals typically use two-wire systems, while voltage signals often use three-wire systems. The two-wire system is more cost-efficient as it saves material and reduces overall costs.

3. Stronger anti-interference ability: In environments with complex field conditions, current signals offer better anti-interference performance compared to voltage signals. In cases where interference is an issue, 4-20mA signals are recommended.

4. Overrange output capability: Current signals can exceed the range slightly without losing accuracy. For instance, if the range is 1MPa, a pressure transmitter with a 4-20mA output can go beyond the range and output 24mA. On the other hand, a voltage signal, such as a 0-10V pressure transmitter, cannot output signals higher than the supply voltage (e.g., if the supply voltage is 9V, it cannot output 9V or more).

5. Insensitive to wire material differences: When the wire material differs (e.g., copper, nickel), it can create magnetic fields. In high-accuracy applications, voltage signals may incur errors, but current signals are not affected by these variations.