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PLC Raw Count Calculation for Pressure Transmitter

Every instrument has at least one input and at least one output. For instruments responding linearly, the correspondence between input and output is proportional:

Pressure Transmitter Formula

A practical example of this is a pressure transmitter, in this case one with an input range of 0 to 1023 PSI and an output of 4-20 mA

Pressure Transmitter Calculations

If you happened to measure an output current of 14.7 mA from this pressure transmitter

x = { ( 14.7 – 4 ) / 16 } * 1023 = 684.13 PSI

It would be a simple matter for you to calculate the corresponding input pressure to be 684.13 PSI.

Now consider a pressure transmitter is connected to the PLC system. The pressure transmitter is connected to the Analog input card of the PLC system. The AI card have inbuilt Analog to Digital Converter (ADC). The ADC converts the input 4-20mA value coming from pressure transmitter into equivalent Raw Counts.

The AI card finally sends this Raw counts value to CPU card. The CPU have the pre-defined Pressure transmitter values like its Range (LRV & URV), units etc. The CPU calculates the Process variable (pressure value from pressure transmitter) using these Raw Count from AI card & pre-defined LRV, URV, units etc… and displays the process variable on operator workstations/HMI.

Also Read : Formula to Convert 4-20mA into Process Variable

Take for example this analog-to-digital converter, with a 10-bit output (a “count” range of 0 to 1023) and a 4-20 mA input:

PLC ADC Calculation for Transmitter

Calculate the corresponding “count” output of this ADC circuit given a 6.82 mA input signal.

Here we are calculating the ADC output in terms of count. So Input to the AI card is “x” and output is “y” as shown in below fig.

PLC Raw Count Calculation formula for Pressure Transmitter

y = { ( 6.82 – 4 ) / 16 } * 1023  = 180 count

In a PLC system, CPU converts this count into equivalent Process variable and displays on the operator workstations.

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6 comments

RAGU RAJAN July 18, 2017 at 2:26 pm

Dear Mr. S.Bharadwaj Reddy
I like to say ‘Thank You’ for your wonderful contribution to the people who are in search of knowledge like me for instrumentation and especially about PLC as I m from a Mechanical background; as well inspires me to contribute back to the community.
Regards
Rajan.

Reply
Lukkuman hakkim July 19, 2017 at 5:48 am

Hello sir,

very useful article for me ,i got clear idea from this article .thank you very much.

But one thing, better to check the formula mentioned in the field pressure transmitter to PLC ADC count convertion. in this formula x,y interchangely mentioned.

Reply
S Bharadwaj Reddy July 19, 2017 at 6:08 am

Hi Lukkuman hakkim,

Thank you for the update. Corrected.

Reply
Lukkuman hakkim July 19, 2017 at 6:16 am

thank you so much.

Reply
kingsley August 7, 2017 at 9:30 am

Thank you sir for your kindness! you are a great man!!

please how can i scale parameters in logix 500 and logix 5000 controllers

I need your help on these

Thanks very much

Reply
Nakkiran August 17, 2017 at 5:50 am

Hi Mr.bharadwa reddy first of all thanks for your updation ..

Can you update what is different between Instrument earthing and electrical earthing ,why it is need separate for earth pits.

Reply

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