Multiplying by a factor instead of scaling

I have a pressure sensor with an output signal 4-20 mA and a pressure range 0-250 bar.

I'm going to use this sensor in a PLC based pressure control system. The user is going to set a required value to pressure by an HMI panel and the current pressure value will be displayed on the same panel screen.

My question is can I directly multiply by the factor of the sensor which is 12.5 in this case instead of using a scaling PLC function to display the pressure reading in bars?

You could do that if the analog value in your PLC was a floating point number between 4 and 20. But that would give you the wrong answer at 4 mA, 50 bar instead of zero.


That's usually not the case, though. More often, the PLC value is an integer, maybe 0 to 4095, maybe 4000 to 20000. Since you haven't included any details about your PLC, I can't say much more than that.

Steve Bailey said:

You could do that if the analog value in your PLC was a floating point number between 4 and 20. But that would give you the wrong answer at 4 mA, 50 bar instead of zero.


That's usually not the case, though. More often, the PLC value is an integer, maybe 0 to 4095, maybe 4000 to 20000. Since you haven't included any details about your PLC, I can't say much more than that.

Click to expand...

Thanks Steve. By the way, I didn't state the PLC type because I thought that no one would know its brand. It's a product of a Brazilian company named "altus".

Slightly OT - with the incredible range of experiences of those who frequent this forum it is always likely that someone will have experience with your equipment, no matter how obscure you think it is.

It depends on whether or not your I/O card data is zero at 4 mA.

In the general form of the scaling equation if the data value at 4 mA = zero and the engineering units at 4 mA = 0 then the terms cancel out and it becomes a simple multiplication as you suggest.

Tom Jenkins said:

It depends on whether or not your I/O card data is zero at 4 mA.

In the general form of the scaling equation if the data value at 4 mA = zero and the engineering units at 4 mA = 0 then the terms cancel out and it becomes a simple multiplication as you suggest.

Click to expand...

But could I subtract 4 mA corresponding register value from the reading before I make the multiplication and so the value will be zero at 4 mA signal?

Shohadawy said:

But could I subtract 4 mA corresponding register value from the reading before I make the multiplication and so the value will be zero at 4 mA signal?

Click to expand...

What you are now describing is exactly all that a scaling PLC function is doing in one instruction,

Greetings Shohadawy ...

you might find the “Scaling” articles from my “Sample Lessons” page helpful ... note that this comes in TWO parts ... the techniques that you’re asking about are all covered – and diagrams are provided ...

I hope that this helps ...

This kind of question comes up a lot. Here is the general form of any scaling function:

For any unit conversion on any PLC you can use four function math with the following, whichasumes integer math:

DataOffset = Data in input register at 4 mA (or zero analog signal of whatever range - for example 0 VDC on a 0-10 VDC transmitter)
DataSPan = Data at 20 mA - Data @ 4 mA
FACTOR = arbitrary factor (multiple of 10) needed to get proper resolution and accuracy
EU = Engineering Units x FACTOR
EUOffset = Engineering Units @ 4 mA x FACTOR
EUSpan = (Engineering Units @ 20 mA - Engineering
Units @ 4 mA) x FACTOR
DATA = Actual data reading in input register
EU = ((EUSpan x (DATA - DataOffset))/DataSpan) + EUOffset
ProcessValue = EU / FACTOR

For Example, 0-60 psig from 4-20 mA with resolution of 0.1 psig:

FACTOR = 10
Data @ 4 mA = 6240
Data @ 20 mA = 31208
DataOffset = 6240
DataSpan = 31208 - 6240 = 24968
EUOffset = 0 x 10 = 0
EUSpan = (60 - 0) x 10 = 600
DATA = 18975
EU = ((600 x (18975 - 6240)) / 24968) + 0 = 306
Process Value = 306 / 10 = 30.6 psig

Gil47 said:

What you are now describing is exactly all that a scaling PLC function is doing in one instruction,

Click to expand...

I know that, but if your PLC does not have the scaling function you must find a solution

Shohadawy said:

I have a pressure sensor with an output signal 4-20 mA and a pressure range 0-250 bar.

I'm going to use this sensor in a PLC based pressure control system. The user is going to set a required value to pressure by an HMI panel and the current pressure value will be displayed on the same panel screen.

My question is can I directly multiply by the factor of the sensor which is 12.5 in this case instead of using a scaling PLC function to display the pressure reading in bars?

Click to expand...

When I first read your post, I read it that you were avoiding using a scaling function that was in your PLC ,
Now, with your last answer I discover that you are needing to create the answer that a scaling function would give you.

So can you answer the questions below for us please.

The value 12.5 that you quote is it 12.5 mA ?

When you send 4 mA to the Plc what value do you see in the PLC ?

When you send 20 mA to the PLC what number do you see in the PLC ?

lol - awesome! We are talking about linear scaling, right?

Gil47 said:

What you are now describing is exactly all that a scaling PLC function is doing in one instruction,

Click to expand...