Where When and Why 4 to 20 mA analog Signals are used?

RIDHAM SHAH

Where When and Why 4 to 20 mA analog Signals are used?

The learning objective of this topic to understand three important ‘W’; Where When and Why 4 to 20 mA signals are used. Additionally, we will cover the advantages and the disadvantages of the signals.

When 4 to 20 mA analog Signals are used?

to understand well the answer to the question we need to go to the history of signals. In the olden days, industries were equipped fully with mechanical-operated devices like pneumatic sensors and controllers. Pneumatic means pressure. pneumatic devices used pressure as a medium for the measurement and to control applications in the industries and pneumatic devices generated output signals in the range of three to 15 pounds per square inch. The three pounds per square inch represents a 0% of the measured process variable. The 15 pounds per square inch represents 100% of the measured process variable. pneumatic devices are bulkier needed compressed air for operation, had higher maintenance costs, and take more time for troubleshooting and repair in the 1950s. With the evolution of electronics, Analog transmitters started replacing pneumatic transmitters. The analog transmitters generate an output signal in the range of 4 to 20 milliamp.

Where 4 to 20 mA analog Signals are used?

4 to 20 milliamps can be designed for intrinsically safe approval for use in hazardous areas, which allows troubleshooting of the circuit while it is in live and powered condition. So to prevent unexpected accidents, according to the ISO standard it is compulsory to use 4 to 20 milliamp signals to solve the safety measured problems, these signals are used in the hazardous area which is, nowadays, an essential part of every industry.

Why 4 to 20 mA analog Signals are used?

Any process parameters such as temperature flow ratio, density is seals measured by a sensor and it is converted into an electrical signal in the form of voltage. The voltage on the sensor is sent to the transmitter from that it converts into a standard current output signal and that is for 20 milliamps to signal. most of the applications globally use for 0 to 20 milliamps or to 5 voltage. So, why basically we should use 20 milliamps signals. We all know that a process parameter is being sent as a photon in billions of signals from failing to control them. So, this is how the person paid is related with code condominiums you could see over here, but 4 milliamps we have 0 % process parameters, for 8 milliamps to 25, for 12 milliamps 50%, for 16 milliamps sounded a 75% and finally 20 milliamps would be 100 % of the process parameters

Now you must have a question why 4 milliamps is used? Why not 5 milliamps is not used? or Why not 6 milliamps? The first reason is, in the olden days the power consumption of analog devices was pretty high. The analog transmitters require a minimum of three milliamperes current for their operation. So we have to consider the starting range from above the 3 milliamps pairs. The second reason is 20% bias. The engineers followed the 20% bias as one of the factors for selecting the 4 milliamps pairs. Calculate the 20% bias for the standard 3 to 15 pounds per square inch pneumatic signal, 20% of 15 pounds per square inch is 3 pounds per square inch. So engineers followed the same rule for the current signal 20% of 20 milliamps which gets 4 milliamps.

Now you have the question why do we use the maximum limit of 20 milliamp? Why not 30 milliamps or 35 milliamps? The reason is, in the olden days 30 milliamperes is considered a dangerous threshold limit for the human heart. So, we have to select a value below the 30 milliamperes a ratio of one to five. This is one of the major reasons for the selection of 4to 20 milliamperes current as a standard signal range. We have a very big advantage with a ratio of one to five. The industrial transition from pneumatic devices to Analog Devices is not simple. Every industry was using pneumatic devices that operated with 3 to 15 pounds per square inch signals. We need to design the analog equipment to handle the existing 3 to 15 pounds per square inch signals and eventually replace the pneumatic devices with analog devices wherever possible. So design engineers decided to use a ratio of one to five which helps them to design the equipment with better accuracy and linearity. Initially, for a short period of time, they have chosen 10 to 50 milliamperes as the signal range, this range is using 10 milliamperes to represent 0% process variable and 50 milliamperes to represent 100% Process Variable. As discussed previously, the highest signal range must be below 30 milliamperes. So, eventually, they stopped using the 10 to 15 milliamperes range. Next, they have chosen for 20 milliamperes current to the standard signal range. It has an advantage of a ratio of one to five 4 milliamps higher than the analog transmitter’s minimum current and 20 milliamperes is lower than the dangerous threshold limit of 30 milliamperes.


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