Ideally, to drive a controller the required current must be just above 0 mA. However, practically we consider readings only taken from 4 mA as the valid data sets. Now, my question is why do we take 4 mA and not 3 mA or 2 mA? Is there any particular reason or is it a randomly chosen point for the sake of an ideal graph?
In a 4-20mA current loop (which appears to be what you are talking about), the minimum 4mA current is set not for any measurement reasons per se, but to provide a guaranteed operating current for the electronics at the far end of the loop. This allows them to operate with no additional power supply at the far end, saving the extra wiring that would be needed. Often the transmitter will be a pressure sensor, or optical gate, or thermometer.
The 4mA is a compromise between low power consumption for the system, and enough power for the sensor to operate. There is no more magic behind the exact figure of 4mA than (say) 240v for mains voltage. It is a reasonable value, which over the course of time has been found useful, so has been supported by many different players, and become a standard.
In addition to @Neil_UK's answer, the following extract from Wikipedia's Current loop article may help. (Emphasis mine.)
For industrial process control instruments, analog 4–20 mA current loops are commonly used for analog signaling, with 4 mA representing the lowest end of the range and 20 mA the highest. The key advantages of the current loop are that the accuracy of the signal is not affected by voltage drop in the interconnecting wiring, and that the loop can supply operating power to the device. Even if there is significant electrical resistance in the line, the current loop transmitter will maintain the proper current, up to its maximum voltage capability. The live-zero represented by 4 mA allows the receiving instrument to detect some failures of the loop, and also allows transmitter devices to be powered by the same current loop (called two-wire transmitters). Such instruments are used to measure pressure, temperature, level, flow, pH or other process variables. A current loop can also be used to control a valve positioner or other output actuator. An analog current loop can be converted to a voltage input with a precision resistor. Since input terminals of instruments may have one side of the current loop input tied to the chassis ground (earth), analog isolators may be required when connecting several instruments in series.
Note that the live-zero feature can be used in a number of ways:
- Deliberately sending a current of, for example, 3 mA to indicated a sensor fault. This allows the receiving end to differentiate between zero measurement, sensor disconnection and sensor fault.
- If the received signal goes below 4 mA the actuator can move to a preset safe position.
I believe that the 4-20mA loop standard long predates electronics that can operate from 4mA, so I think it was simply an arbitrary choice based on older pneumatic control systems that use 3-15 PSI etc. as the signal (note the same ratio). 10-50mA was also used in some cases.
The choice of 20% of full scale as the live zero is just an arbitrary pragmatic engineering choice.
Of course the live zero allows the receiver to distiguish between a broken wire (or out of range) and 0, just as it can detect >20mA as out of range on the high side.
the protocol 4-20mA is think for comunication through long distance. When the current travel long distance, is very important not loss voltage, because that we use a protocol based in current and not voltage, because if there is long distance, the voltage signal change but not the current. And the protocol begin in 4mA because we need distinct the situacion when there is not connection and the situation when the voltage signal is zero.
With a 4-20mA current loop it is possible to have signal loss detection.
For example, when the cable is cut the signal is 0 mA.
This won't work with 0-20 mA or 0-10V signals, since both 0 mA or 0 V are valid.
You can often find these signals in non-stationary systems where cable wear can be a problem. Or high reliability or safety systems.
A current sensor needs to use 4mA for a Zero reading rather than 0mA. If 0mA were used for the zero value, there would be no way to detect a sensor malfunction vs. a broken wire.
This is a very old technique to read sensors and at that time the chips which are used to read pressures or any other data used to consume 3mA. This was a two-wire protocol so the current has to be greater than 3mA to make it work. The second question is: why didn't they start with 5,10,20 or something else.
In this protocol the values change linear, so they have to have something like this 4-20mA or 5-25mA or 10-30mA or 10-40mA something to make calculation real easy.
The human body can take up to 30mA current above that it could damage the heart, so they had to keep it below 30mA.
There are two choices, either 4-20mA or 5-25mA. There could be two reasons. One being the case of 5-25mA the 25mA was still really close to the max limit of 30mA, so they went with 4-20mA standard. https://ncd.io/how-to-read-4-20ma-current-loop-sensors/