Why 4 20 ma is used

The disadvantages of using mA for an active sensor input loop are that current loops can transmit only one process signal, which requires multiple loops when there are numerous process variables that need to be transmitted. Furthermore, using multiple loops can lead to ground loops problems if the individual loops are not isolated.

One other advantage of mA current loops is safety. Also, the mA current loop is intrinsically safe for hazardous areas that may include dangerous levels of dust or vapor because the low power consumption does not cause combustion if normal operating or fault conditions are in play. A loop transmitter measures a process variable, and regulates the loop current between 0. Loop transmitters can measure nearly any process variable, for example temperature , pressure, level, or flow.

In the example below a loop transmitter measures a temperature sensor. A process controller is also wired into the loop in order to measure loop current. The loop wire itself has some resistance as well, which should be considered when calculating a loop budget more on that further down.

Current is common in a series circuit, so the loop current regulated by the transmitter and the current that is measured by the controller are identical. To ensure accurate process measurements, it is important to minimize error due to electromagnetic interference EMI. Electromagnetic interference EMI is interference caused by the electromagnetic fields generated by electrical or electronic devices during their operation. Current signals are inherently more immune to EMI than voltage signals, especially over longer distances.

This is one of the big advantages of using current instead of voltage to get process measurements to a control system. Other reasons for why 4…20 mA is a widely established standard for use in process control are:.

In the previous example, the transmitter regulates current flow on a loop that is powered by 24 V, and has one other device, the controller, wired into the loop. To do this out, we calculate a loop budget. The voltage from the sensor is sent to the transmitter to convert in a standard form of the current output signal. Live Zero i.

It is easy to detect that the signal wire is not broken. If the range zero were truly zero mA, then there would be no current, and therefore no power to drive the field instrument. Field instrument would require 3 or 4 wire devices. Additional wiring costs come into account.

Another reason for choosing 4 mA is when the transmitter was first introduced, it was working on pneumatic pressure signal for transmission. Later, electronics chips were introduced those require a minimum of 3 mA of current to function. Different type of standard signals are used in Industrial Instrumentation.

The measuring device has ohm resistor. As for current signals,they are converted to voltage signals like this. If instrument is faulty,it can be easily identified.