Internet PLC Forum

DLUN,
At the risk of not knowing what application you are using for monitoring the 30 thermocouples, or why it is important to check them every second, I would like to add my 1 1/2 cents worth.
   As you probably know, thermocouples are subject to what is known as "thermal inertia" meaning it takes time for the thermocouple to react to a change in temperature.  Especially when a thermocouple is cooling down. Heat goes to cold. Whether cold is room temperature, or any other reference point, it dictates how fast a thermocouple reacts to a cooling change, as opposed to a heating change.  I.E. it takes longer for a thermocouple to cool, because the change to warmer takes energy, whereas the cooling process takes no energy.  Cold can be defined as an infinite heat sink (Earth, Factory, test environ or Space) and heat as a localized event, in terms of a Delta T, ambient vs. desired process variable (fish sticks cook better at 400 F than at ambient) in respect to the limitations, (Earth, Factory, test environ or Space).
  To add to the equation, the distance of the heating (or cooling)
body or element, from the thermocouple's aspect, is inversely proportional to the square of the distance from said body or element.  The placement and location of the thermocouple needs to be taken into consideration when evaluating raw data.
   In short, it may be beneficial to extend the sample times, of the thermocouples, to 2 1/2 to 5 seconds. It might result in a more representive temperature change during the measured process and it will make the asssociated PID loops ( if used) easier to manage.  And, it will reduce the load on the communication network (RS-485).
   I hope I have not wasted anybody's time reading this......

Dave

Try this in your {dCusF} to experiment with:

C=2
P=1
if C>P
setioTripRelay
endif

Create a relay named TripRelay first.
You will need to provide a latch contact for it to stay latched.

Good Luck, Dave

Management is likely to reconsider if numbers are used.  In support of what Support is saying:
15Hz = 15 cycles per second
15Hz X 60 seconds = 900 cycles per minute
900 X 1441 minutes (One Day) = 1,296,826

At 10,000,000 cycles of expected relay life in a 24/7 environment, this equals 7.7 days (10,000,000/1,296,826)  before failure of the relay, either mechanically or electrically.

Another factor to keep in mind is Relay Harmonic Chatter.  This is caused by the reed/contact assembly that all electro/mechanical relays share.  At the speeds you intend to run these you will end up with spurious operation due to RHC and Magnetic Flux Collapse of the relay coil, (satuation) at best.  Contact failure due to micro-pitting of the contact surfaces will probably occur well before the 10 million cycle life of a "normally" used relay.

It should be readily apparent that an electro/mech relay cannot be used in the application described with any confidence.
Best of luck......Dave

Support, you were right, it was something in my program.  I simplified the program to a single input firing the {dCusf} with DM[1]=[n].  Worked perfect.  Good call........Dave

Thanks to the moderator!  I WAS missing something obvious....