RateOfChange alarm logic

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ariccio
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Joined: 27 Oct 2016, 07:57

RateOfChange alarm logic

Post by ariccio »

Hi UA,

I have a question about the interpretation of the rate of change alarm.

Do i have to assume anything about the alarm configuration?
For example, if the engineering unit is meter per minute, the variable cannot change with a frequency higher than once per minute.
In this case i only have to observe the variable's value every minute and determine the actual rate of change.
Otherwise, if i consider only the variable's value every minute i might be wrong because i do not observe changes within the interval.
Furthermore, if i observe the variable's value at an higher frequency, for instance, of 10 times per minute, i'am estimating an instant rate of change that could be greater than the actual rate of change over the whole interval.

In my opinion the rate of change alarm makes sense only if the engineering unit specifies a time unit equal to the sampling rate of the observed variable.

In the example of the tank level and rate of change per minute, if we only want to consider the level every minute without considering the fluctuations within the interval, the requirements are fullfilled. Otherwise a shorter interval (the time unit) should be set.

Is this the correct interpretation ?

Thanks in advance

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Support Team
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Re: RateOfChange alarm logic

Post by Support Team »

Hello ariccio,

the unit of measurement of a variable and the time interval used for measuring the rate of change are completely independent of each other. It only depends on your underlying system how the rate of change is measured. Otherwise you would have to change the engineering units depending on the requested interval for determining the rate of change.

For example, when measuring the acceleration (rate of change of the car's speed) of a car, you wouldn't measure the speed only every hour because the speed's unit is miles per hour. It only depends on your underlying system / hardware, in which intervals the acceleration is measured. There are no requirements given by the UA specification on how to determine the rate of change, this is implementation specific and not inside the scope of the specification.

The logic used for determining the rate of change is up to your implementation at the data source and does not correlate to sampling intervals or the unit of measurement. Depending on your implementation you observe the value to decide if the rate of change limit is reached and fire the alarm.
Best regards
Unified Automation Support Team

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