Basics of 4 to 20 mA analog Signals

The most popular form of signal transmission used in modern industrial instrumentation systems is the 4 to 20 mA DC current signal.

This is an analog signal standard, meaning that the electric current is used to proportionately represent measurements or command signals.

Analog Signals

Typically,  a 4 milliamp  current value represents 0% of scale, a 20 milliamp  current value represents 100% of scale,  and  any  current value  in between  4 and  20 milliamps represents a commensurate percentage in between  0% and  100%.

The following table shows the corresponding current and percentage values for each  25% increment between  0% and  100%. Every  instrument technician tasked with maintaining 4-20 mA instruments commits these values  to memory because  they are referenced  so often:

4 to 20ma vs 0 to 100 percent

For example, if we were to calibrate a 4-20 mA temperature transmitter for a measurement range of 50 to 250 degrees C, we could relate the current and measured  temperature values on a graph like this:

4 to 20 mA vs measured value

3 to 15 PSI and 4 to 20 mA

This is not unlike 3-15 pounds per square inch (PSI)  pneumatic signal standard, where a varying air pressure  signal proportionately represents some process variable.

Both 3-15 PSI  and  4-20 mA signal standards are referred  to as live zero because their ranges begin with a non-zero value.  This “live”  zero provides  a simple means  of discriminating between  a legitimate 0% signal value  and  a failed signal (e.g.  leaking tube or damaged cable)

An important concept with all analog instrumentation is that instruments  sending and receiving analog   signals  must  be  compatibly  ranged   in  order   to  properly   represent  the desired  variable.

For example,  let  us  consider  a  temperature  measurement  system  consisting  of a thermocouple, a temperature transmitter, a 250 ohm  resistor  (to convert  the 4-20 mA analog signal into a 1-5 volt analog signal), and a special voltmeter functioning  as a temperature indicator:

4 to 20 mA Signals

4 TO 20 MA ANALOG CURRENT SIGNALS

Note that how the output range of each sending device matches the input  range of its corresponding receiving device.  If we view this system as a path for information to flow from the thermocouple’s tip to the transmitter to the resistor and finally to the voltmeter/indicator, we see that the analog output range of each device must correspond to the analog input range of the next device, or else the real-world meaning of the analog signal will be lost.

This correspondence does not happen automatically but must be established by the instrument technician building the system. In this case, it would be the technician’s responsibility to properly adjust the range  of the temperature  transmitter, and  also to ensure  the indicator’s  display  scale was  properly  labeled.

Both the thermocouple and  the resistor are  non-adjustable devices,  their input/output characteristics being fixed by physical  laws.

DC current signals are also used in control systems to command  the positioning of a final control element, such as a control valve or a variable-speed motor drive (VSD).

In these cases, the milliamp value does not directly represent a process measurement,  but rather how the degree to which the final control element influences the process.

Typically  (but  not always!), 4 milliamps  commands  a closed (shut) control valve or a stopped motor, while 20 milliamps  commands  a wide-open valve or a motor running  at full speed.

Final  control elements often are equipped  with adjustable ranges so that an accurate correspondence  between  the analog  signal and  the desired  control action  may be ensured.

Process Control Loop

Thus,  most industrial control systems use at least two different 4-20 mA signals:  one to represent the process variable  (PV)  and one to represent the command  signal to the final control element (the “manipulated variable”  or MV):

The  relationship between  these two signals depends  entirely on the response  of the controller. There  is no reason to ever expect  the PV  and  MV current  signals  to be equal  to each  other,  for they represent entirely different variables.

In fact,  if the controller is reverse-acting, it is entirely normal  for the two current signals  to be inversely related:  as the PV  signal  increases  going to a reverse-acting controller, the output signal will decrease.

If the controller is placed  into “manual” mode by a human  operator,  the output signal will have no automatic relation to the PV  signal at all, instead being entirely determined by the operator.

Also Read: Standard 4-20mA Conversion Formula

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12 thoughts on “Basics of 4 to 20 mA analog Signals”

  1. Dear sir,
    please tell me about why we short the two wires of RTD and one common in three core wire RTD.and show the formula for how we can convert the resistance into temperature.

    Reply

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