4 to 20 mA Calculator For DCS / PLC Programmer and E&I Engineer

4 to 20 mA Calculator For DCS / PLC Programmer and E&I Engineer

4 to 20 mA Calculator 

4-20 mA signals are widely used  in  field Instrument ranging from ( Flow Meter , Pressure Transmitter , Level Sensors ,  Temperature Transmitter (RTDS), etc .

The Current Signal( 4 to 20mA)  , use   to represent range of  the actual instrument measurement value ( eg:  Temperature , Flow rate   , Pressure  etc )

( 4 to 20mA)  Signal from the Instruments, will be  send to an analog Card  reside in a PLC / DCS Controller.

In the PLC/ DCS Controller, the Current signal will be convert back to its Actual value  ( eg . Temperature / Flowrate  etc ) by multiplying it with a scale Constant .

The User then will be able to view the actual value of the field instruments  from his / her HMI or SCADA Interface.

Who need to Know how to Scale the Analog Factor 

1. PLC Programmer
2. DCS Engineer
3. E & I Engineer

Common Raw Value Setting From Well Known PLC / DCS  Vendor

How To Calculate 

4 to 20 mA Calculator For DCS / PLC Programmer and E&I Engineer,How i came out with the Formulae  to write this Calculator ?

The Scenario

(1)   E&I engineer went  on site to measure  the  current (mA)  of a Temperature Transmitter . The Value  recorded  was  15 mA

(2)  He then  check the Temperature Transmitter Specification Sheet . The Minimum Range is 10°C , while the Maximum range is 200°C

(3)He  got the Measured Current , and  want to know what is the Calculated Temperature  ?

The Findings  as below

Signal Range in Current : 4mA to 20mA

Instrument Range : 10°C to 200°C

Measured Current Value :  15mA

Instruments Current Value :      ??°C   ( This is what he wanna Find Out )

Step 1 

What is the Minimum Value in mA  for 0°C ?

Answer : Minimum mA Value = 4mA

Step 2 

What is the mA Current Range between 4mA to 20mA?

Answer : mA Range  Value = 20mA – 4mA =  16mA

Step 3

What is the Instrument  Range between 10°C to 200°C?

Answer : Instrument Range  Value = 200°C – 10°C = 190°C

Step 4

Write out the Formulae

[(Measured Current – Minimum  mA Value ) / (mA Range  Value ) x (Instrument Range  Value)] + Minimum mA Value 

Step 5

[(15mA – 4mA ) / (16mA) x ( 190°C)] + 4 mA 

Note: In Reality  you will not get exact 4mA in your Instrument , Check your instrument Spec Sheet  to identify the plus minus tolerance

What if i got  current below 4mA ?  Your instrument might be in Faulty mode , check your instrument spec sheet as well.

How To Calculate Raw Data For PLC Scaling

The Scenario

(1) The Process Engineer requested , if  Current Temperature reach  16o°C , an Alarm should be triggered.

(2) The PLC Engineer   need  to  modify the existing logic.

(3) He is using Allen Bradley Analog Card  Minimum Raw Value  : 6241  Maximum Raw Value : 31206

(4) He  check the Temperature Transmitter Specification Sheet . The Minimum Range is 10°C , while the Maximum range is 200°C

(5) He  need to get the Raw Value ? for temperature 16o°C

The Findings  as below

 Range ( Raw Analog Value) : 6241 to 31206

Instrument Range : 10°C to 200°C

Triggering Instrument Value : 160°C

Triggering Raw Value :      ??°  ( This is what you wanna Find Out )

Step 1 

What is the Minimum Raw Value ?

Answer : Minimum Raw Value = 6241

Step 2 

What is the Raw Value Range?

Answer : Raw Range  Value = 31206 -6241 =24965

Step 3

What is the Instrument  Range between 10°C to 200°C?

Answer : Instrument Range  Value = 200°C – 10°C = 190°C

Step 4

Write out the Formulae

160°C = [ (Raw Value –    Minimum Raw Value ) / Raw Range  Value    ]  x Instrument Range  Value

Raw Value = [ (160°C/Instrument Range  Value)*Raw Range  Value ] + Minimum Raw Value

Raw Value = [ (160°C/190°C)*24965 ] + 6241

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