The  most  popular  form of signal transmission  used in modern  industrial  instrumentation  systems is the 4 to 20 milliamp DC standard. This is an analog signal standard, meaning that the electric current is used  to proportionately represent measurements or command  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:

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:

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  to 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:

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.

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