Why we take 4 to 20 mA as out put of every instrument like pressure transmitter? Why we cant take 0 to 20mA as output?

why not take0-20mA the reason for this is when0 is considered as minimum value then we will get0 as output when no signal is coming,when the instrument is not working or there is any fault in it we get0.inorder to ensure that the instrument works we choose4-20mA as standard output value.4 corresponds to no signal from the instrument.20mA is the maximum value because below20mA we will be able to avoid schocks and short circuits 

In simple words4ma is called live zero. it is just because we have to know that our signal is low neither our loop is dead.

In addition to the answer of Khalid abdul Rehman,

You can also find the zero down or Up, error from4mA( Live Zero)of the transmitter.

Bonjour Monsieur;

En instrumentation, on utilise4/20 mA ou lieur de0/20 mA, pour éviter le bruit sur le signal analogique.

On prend4 mA pour0 en numérique et20 mA pour le maximum du signal (exemple1204 pour un convertisseur A/N sur12 bits). Bien entendu le courant doit être converti en tension à l'aide d'une résistance de précision, avant de l'appliquer à l'entrée du CAN.

To Distinguish between the cable defect and the lower range value0%

U can say it to measure the the dead zero,because to fulfill the plc or dcs requirement,also i ask many person abt why not zero they told before we using0 ma but calibration results not well.