S7 value in delay

We have enzyme tank with very little flow and Step7 project. so we need to check if it present or not. if not - send a dicrete signal. want to compare signal with itself with delay in 3 min.

Hllo TFMD;

How do you deternine that

if it present or not

Click to expand...

?

If there is a value at the analog input of the flowmeter of the enzyme tank after the 3 minutes delay? Any value? Or a value that is close to the value measured 3 minutes ago? How close a value?

Basically, if this is the way you wanted to do it, the way I would look at it would be:
1)at program initialization (OB100), move act_value (of the measured flow)to a register old_value; this will initialize the oomparison values.
2)Then in OB1 (or any block called from OB1);
Start a 3 minutes timer (S_ODT, for example); when it expires,
-compare the new act-value to old_value;
-take a decision based on a comparison of act_value and old_value; send an alarm contact if need be
-move act_value to old_value; ready for the next comparison
-Re-start the timer for another 3 minutes

This would work on any platform; until you give us more details, no use getting deper in the program.
Hope this helps,
Daniel Chartier

Really, before someone jumps in and explains the lot to you, try to solve this task by yourself.
It is a typical beginners task, and is perfect for learning how to program.
If you cannot get it to work, then post your code and we shall help you figure out what goes wrong.

edit:
Now that Daniel went in and posted a suggestion, I have to comment that I dont interpret TMFDs task that way.
To me it sounds like he needs an alarm if the conditions are met for more than 3 minutes. And the conditions are that an intial condition is ON (pump on, or valve open f.ex.) AND the flow is less than a certain minimum value.

thnx
give try