Calling I the output current and V the Voltage—or other physical variables (PV) —between Vmin (which is usually 0) and Vmax; the conversion formulas are as follows: 

• I [mA] = ( ( (V − Vmin) / ( Vmax − Vmin) ) × 16 ) + 4

• V [Voltage] = ( (I − 4 ) / 16) × ( Vmax − Vmin ) + Vmin

• PV [units] = ( (I − 4 ) / 16) × ( PVmax − PVmin ) + PVmin

Where,

I is the current, V is the voltage, 

PV is the Process Variable

min is the Lower Range Value of the respective signal

max is the Upper Range Value of the respective signal

Note that the values and units of min and max are units must be same.

A Analog output card gives an output voltage of 2.5V and its overall range is 0-5V. calculate the equivalent 4-20mA current value.

I =  ( ( (V − Vmin) / ( Vmax − Vmin) ) × 16 ) + 4

I =  ( ( (2.5 − 0) / ( 5 − 0) ) × 16 ) + 4

I = 12 mA

Example 2 :

A transmitter output range is 13.5mA and its output range is 4-20mA. Calculate equivalent voltage. consider 0-10V is the output voltage range.

V = ( (I − 4 ) / 16) × ( Vmax − Vmin ) + Vmin

V = ((13.5 – 4)/16 x (10 – 0 ) + 0

V = 5.9375 V

Example 3 :

A Flow transmitters output current is 16.5mA and its range is 0-1500 m3/hr. Calculate the equivalent flow rate value.

PV = ( (I − 4 ) / 16) × ( PVmax − PVmin ) + PVmin

PV = ( (16.5 − 4 ) / 16) × ( 1500 − 0 ) + 0

PV = 1171.87 m3/hr