Rate of change

I have chlorine gas on site. Along with chlorine sensors to alert me of a leak. I would like to write in a rate of change in my plc that can alert me to the fact that the chlorine is leaving the
cilinder fast than it is being feed. I have to cilinders on a scale
so I know the weight.
What I want to do is if the weight is decreasing at a rapid rate
per hour set a bit.

The easiest way is to use a timer. After it times out:
Check the weight
Subtract previous weight from current weight
Divide by time interval. The result is rate of change
Compare actual rate of change to feed rate and max rate of
change allowed
Sound alarm if required
Transfer existing weight to register for previous rate
Reset the timer

If this is a ControlLogix there is an analog Alarm (ALM) function block that can do the rate of change alarming.

Darren

Dash said...
"If this is a ControlLogix there is an analog Alarm (ALM) function block that can do the rate of change alarming."

Likewise for TI... for many years. Welcome aboard ControlLogix!

This topic came up on eng-tips a few weeks ago.


Here is a place that didn't use a rate of change alarm, as well as several other operating problems. They stored gasoline:

http://www.buncefieldinvestigation.gov.uk/images/index.htm