Formulas to calculate ma from PV, LRV and URV

The formula for calculating equivalent current (ma) from known process variable (PV), Lower range value (LRV) and Upper range value (URV).

Formulas to calculate ma from PV

The formula is:

PV - LRV  =  mA - LRV 

    SPAN               SPAN

SPAN = URV - LRV 

Where

PV          =  Process Variable

LRV       =  Low Range Value

URV      = Upper Range Value

ma         = milli ampere

Consider left side of formula for Process variable, LRV of transmitter lower range, Span is the difference between LRV & URV of transmitter ranges.

Consider right side of formula for current (ma), LRV of standard current range .i.e is 4ma, Span is the difference between LRV (4ma) & URV (20ma) of standard current range           .i.e. 20-4 = 16 

Using this formula we can calculate ma from pv and as well as pv from ma.

Calculate Current (4-20ma) from Process variable PV

For example: The temperature transmitter range is 0 to 50 deg c and  known current ma is 12 then how to calculate the PV of the transmitter ?

Known values :

LRV =  0

URV =  50

mA = 12mA

Standard mA LRV and URV

LRV = 4mA

URV = 20mA

Span = URV - LRV 

Span = 20 - 4

Span = 16

Required :

PV = ?

Formula :

 PV - LRV  =  mA - LRV 

    SPAN               SPAN

SPAN = URV - LRV 

Solution:

Put Values in Formula

   PV - 0  =           mA - 4 

    SPAN                 16

SPAN = URV - LRV 

SPAN = 50 - 0 

SPAN = 50

   PV - 0  =     12 - 4 
      50               16

   PV - 0  =        8 

      50               16

   PV - 0  =       0.5

     50               

   PV - 0  =       0.5  * 50

   PV  =       0.5  * 50

   PV  =    25

Calculate Current (4-20ma) from Process variable PV