Tell us more about the PLC5 you have /10,11,12,15,20,25,30,40,60,80 also which analog input card do you have? If configured properly you shouldn't have to mess with a BCD
conversion.
You could then use math instructions to scale your analog input to engineering units.
If to can give us the full scale in engineering units e.g. 4 to 20ma equals 0-100 m3\day someone could help.
Post the program if you can. Zip it first.