4 to 20 mA Calculator
Click the Button to Calculate
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
- 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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