dual analog inputs

I was wanting to pick the forum's collective brains here to see what other people have done. Using a ControlLogix processor.

I have a pair of flow meters both measuring the same fluid. They are paired for redundancy. Obviously if either one full scale either high or low you can tell the meter is bad and alarm accordingly. But how do you determine if the meter is gunked up or not reading correctly? I've looked on google a bit and am considering using a sort of standard deviation calculation. If the deviation gets too high I know there is a problem. There is also the method of simply taking a difference of the two. If that gets too high there is a problem. Has anyone come across this conundrum and, if so, what have you typically used in this scenario?

On an alarm, I would allow the operator to choose which meter to follow until the faulty meter could be looked at after the process is completed.

When looking at deviation, be mindful of the position in the line and the delay in getting to flow. If you know the rate of rise/drop in flow you should be able to account for it in the deviation between the values.

I wouldn't trust an operator to know which reading to rely on unless it stopped measuring or went full scale.

Which condition is the worst for your process? Too much flow or two little flow? If too much then take the highest value of the two and alarm when difference is greater than xx.x.
If too little, use the lowest value. Same difference alarm.

As Ken said read in both signals, use the Highest or the Lowest depending on your needs and alarm if the difference is to large

This is always hard with only two. With three instruments, a voting system is much easier to implement.

For systems where we have only two, i do as others have said and calculate difference between. Alarm when discrepancy over a threshold for a period of time.

Give the operator some controls for meter selection.

Auto Mode: select meter that isn't out of range, or is safest selection (higher, lower, depends on the process)
Select Meter 1: force use of meter one
Select Meter 2: force use of Meter two