Ladder Logic and Methane Detectors

[djinak]

I am having troubles getting my PLC to do what I want. Here is what I want to do...I have a facility with 4-20 mA methane detectors that are hooked up to a PLC and monitored by WonderWare on a PC. What I want to happen is:

1. At 25% LEL (1.25% methane to air)-->send an alarm to the PC and dial out messages with Viking autodialers
2. At 50% LEL (2.5% methane to air)-->ESD (emergency shutdown) some valves as well as send an alarm and dial out as above.

Currently, within the ladder logic, I have it programmed:
1. >/= 25% < 50% -----> Alarm and Dial out
2. >/= 50% ----------> ESD and Alarm and Dial out

This doesn't work the way I think it should. I am pretty confident in my values and programming. No matter what the methane detectors are reading, the PLC only acts as if there is 25% LEL. As methane levels rise, the PLC should ignor the 25%-50% levels and react to the >50% actions. It seems like the PLC gets stuck on the 25% LEL actions and never gets to react to the >50% level.

I'd appreciate any suggestions or just discuss with someone who has programmed something similar...Thanks