Using the reading from the PT100

[olaf777]

Hi all,

I have PT100 (0-100deg C) connected to an EM231 analog input module on a Siemens S7 224. When I check the status with MicroWin I can see the value clearly, but it is reading 11300 or so. It does fluctuate quite a bit up and down. How do I convert this to a usable value, ie. the actual temperature. I am teaching myself PLC programming and am using Ladder programming.

Thanks in advance.
Matthew.

 

[paraffin power]

Hi
Welcome to the forum.

First, check System block->Analog input filters if the value is jumpy.

To scale the value to engineering units, use this formula.

ScaledOut=[(In-LowerLimitIn)/(UpperLimitIn-LowerLimitIn)*(UpperLimitOut-LowerLimitOut)]+LowerLimitOut


Don't think S7-200 has a ready-made block for you to use though I may be wrong.

pp

 

[olaf777]

>First, check System block->Analog input filters if the value is jumpy.

They are all on. Its not drastic but I just thought since the temperature isn't fluctuating, the values shouldn't be.

To scale the value to engineering units, use this formula.

>ScaledOut=[(In-LowerLimitIn)/(UpperLimitIn-LowerLimitIn)*(UpperLimitOut-LowerLimitOut)]+LowerLimitOut

pp [/B][/QUOTE]

So to tranlate this: "Output I'm looking for"=[("value read at input"-"0degrees value")/("100deg value"-"0deg value")*(100-20)]+20? 100degC being my upper limit and 20degC being my low.

Is that correct?

 

[paraffin power]

No.
The received value is dependent on the analogue input's resolution. There is an upper and lower limit for the actual received number. You'll have to find this in the analogue module's documentation, but let's say it's 0 to 4095.

Scale this between 20 and 100 degrees

LowerLimitIn = 0
UpperLimitIn = 4095
LowerLimitOut = 20
UpperLimitOut = 100

pp

 

[Kidblue]

Hope this gives you an idea.




Rob

 

[Ken M]

What version of the EM231 analogue module are you using? There's more than one.
The EM231-0HC22 takes 'standard' voltage and current input signals - 0-10V, 0-5V, +/5V, 0-20mA etc.

There's also the EM231-7PD22 which is designed for direct connection of thermocouple signals.

And finally the EM231-7PB22 for RTD sensors. This last one should return a value to the CPU of the temperature in 1/10ths of a degree. You configure the sensor type and whether it uses °C or °F. So your value of 11300 would equate to 1130.0 °C or °F. Neither one seems likely!

Are you using some kind of signal conditioning to convert the Pt100 signal to current or voltage, or are you using the 'true' RTD module?

More info, please.

Ken