Temperature Controls.

I need to turn a temperature element off at a high set point, let the tank drop to a lower set point before turning it back on. Although this is a insulated tank.

AB MicroLogix 1400.

Limit Test doesn't seem to do what I need, it would change the moment the temp shifted one degree back into the range. I'm looking for a 15 degree allowable fluctuation (or whatever the testing team sets through a HMI screen)

Greater Than / Less Than on the same rung also would seem to not work, as that would function the same as Limit Test.

N7 Integers will be used for the limit numbers.

Thoughts or suggestions on how to pull this off?

I would use two LES instructions in series to OTE the heating element.
The first LES would be testing the temperature against the lower setpoint. Put a seal in contact around this with an XIC of the heating element output. The second LES would test against the high setpoint, which will drop out the sealing when the temperature exceeds the high limit.
KISS

HTH

Cheers,
Dustin

Bmacattack33 beat me to it, but here is a pretty picture for you.

Your set points and addresses may differ.

Brilliant guys, Thanks! So simple. Kept thinking I had to use LES+GRT. Shoulda seen I could use a second LES and latch around the lower LES.

Be aware that the output bit is non-retentive, and will be turned off in the case of a processor restart. You will then have to wait for the temperature to drop below the lower setpoint before it comes on again.

It's probably not important in your simple tank heating situation, but can be in other applications, for which I would use

LES Temp Lo-Setpoint OTL Heating Control
GRT Temp Hi-Setpoint OTU Heating Control

daba said:

Be aware that the output bit is non-retentive, and will be turned off in the case of a processor restart. You will then have to wait for the temperature to drop below the lower setpoint before it comes on again.

It's probably not important in your simple tank heating situation, but can be in other applications, for which I would use

LES Temp Lo-Setpoint OTL Heating Control
GRT Temp Hi-Setpoint OTU Heating Control

Click to expand...

That's fine for the current application as it would have to drop below the low setpoint to turn on in automatic mode and stop modes anyway. Only Manual overrides the limits, the element cannot heat beyond 180 on its own so it will stop there if there is a PLC malfunction. The temp range will be set around the nominal process requirements. It's preferred that this system comes back on with everything Off.