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4 to 20 mA Field Instrument & Analog Module conversion Calculator

Raw Value Upper Maximum Limit For Example : AB PLC Analog Card 20 mA Raw Count = 31206

Raw Value Lower Minimum Limit For Example : AB PLC Analog Card 4 mA Raw Count = 6241

Instrument High Maximum Limit You Can Input any Field Instrument ( It Can be Flow meter , RTD ,Level Sensor etc )For Example : Emerson Flow Meter Maximum Flow < 55000 M 3 /hr > Instrument Low Minimum Limit You Can Input any Field Instrument ( It Can be Flow meter , RTD ,Level Sensor etc )For Example : Emerson Flow Meter Minimum Flow < 0 M 3 /hr > Field Measured Value in ( Current mA or (Instrument displayed Value) ) Measured Current( mA) Measured Field Instrument Value



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

PLC Programmer DCS Engineer E & I Engineer

Common Raw Value Setting From Well Known PLC / DCS Vendor

Brand Min Raw Count Max Raw Count Allen Bradley 6241 31206 Siemens 5529 27648 Emerson 800 4095 Yokogawa -30000 30000

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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