I am trying to define a current range. when it is on is it pulling the correct amount of current. Trying to detect open heaters on the hmi.
If it shorts and blows a fuse i have alarms for that but if it's open i don't know several heaters in each zone so the other heaters will keep the zone up to temp but will still produce cold spots which Effect quality.
Sounds like to me that what you want are CT's (Current Transformers) to monitor the current of the heaters.
Put a resistor in series with the CT and read the voltage across the resistor back into the PLC (via an analog input) to monitor your current.
The "uptown" method would be to use one analog input per CT. Me being cheap and all, I would multiplex the heaters into one analog input and then just use relays and a "round-robin" routine to read each of the heaters.
Keep in mind that the CT always has to have a complete circuit (in other words, switch the analog inputs, not the the CT's themselves).
Opening up a CT under load will cause the CT to burn up.
One place to look for CT's...
http://www.magnelab.com/products.php
We had Ube injection molders where I used to work, and the
$%@&'ing mercury-wetted relays would weld shut causing the barrel heaters to cook (as in catch fire).
We had logic that would monitor the barrel temperature via a thermocouple, but by the time that the alarm was registered the barrel would have gotten so hot that it would take about and hour or two to cool off enough to resume production.
I wanted to install CTs but we didn't have the money. I was even willing to wind my own (all I cared about was on/off, not amplitude), but about that time I took the severance package from GM...
What made matters worse was one of the electricians was trying to
Do The Right Thing by taking the bad mercury relay and replacing the one pole that had welded shut (he really did mean well). He didn't realize that the remaing two weren't far behind. So when the "rebuilt" relay would be put back into service, it would be a very short time before one of the remaining poles became welded shut.
My "bottom dollar" plan was to use one CT that was sized large enough to handle all of the heaters being on at the same time and periodically shut off all of the heaters. If I detected current flow during that time, then I knew that at least one of the mercury relays had welded shut. Then I would turn each of the heaters on one at a time and monitor the current. If the current didn't increase when I turned on a relay, then I knew that it was the one with the bad contacts.
Edit:Um, did I forget to mention that you will have to convert the voltage coming off of the resistor to DC?