Hello,

I'm having problems with calculating the contents of a tank with a chemical product.
I read a 4...20mA signal in the PLC.
When the signal is 20 mA I have a content of 9589 liters. 
When the signal is 4 mA I have 653 liters.

What I do is first convert the PIW value into a double and then into a float.
I divide that value with 27660. This is the value I have when the signal is 20 mA.
Then I multiply that value with the maximum 9589 liters.

But doing this I have a correct value when I have 20 mA, but not when I have 4 mA. Then the value is 0.

I cannot add 653 because then the maximum value doesn't correspond anymore.

This is my code in STL : 

Can some one help me out a little bit? Maybe I'm doing this totally wrong?

any other tips?

edit : I use Simatic manager 5.7

thanks in advance

Hello.

Would it be good enough if I said.. 

y = m * x + k

X represents the PIW value and Y represents the volume. It may help you if you draw a straight line  on paper and  fill out the numbers that correspond to 4mA and 20mA.

BR

As an amendment to qwazee's answer.

Linear_equation, look for Two-point Form paragraph.