Totalizer of the sum is different form the sum of the totalizers

[Francesco]

Hello,

I have three signals (A,B,C) that represent three different feeds to a reactor unit. I want to know the total mass fed to the reactor in 24 hours therefore I am using the totalizer in the scorecard metrics. I also built a fourth signal (D=A+B+C) that is simply the sum of the three signals mentioned above.

At this point I computed the totalizer for the four signals expecting to be totalizer(D) = totalizer(A) + totalizer(B) + totalizer(C)  but the result is different, sometimes the difference can also be around 70/80 Kg.

 

In the image above, from above the signals are totalizer(A), totalizer(B), totalizer(C) and totalizer(D). For the first point there is a difference of +19 Kg, for the second point  +59.4 Kg and for the third point -16.6 Kg.

Is this a computation error? Which would be the most precise option: compute the three totalizer and add the up or add up the three signals and compute the totalizer of the sum? 

Thank you,

Francesco

[John Cox]

Hello Francesco,

I doubt this is a computation error. There could be interpolation effects going on. To figure out why you are seeing a discrepancy, it would be best if you can attend an upcoming Seeq Office Hours time slot (see this link: Office Hours), where a Seeq Analytics Engineer can look at your calculations as you share your screen. 

 

John

[Francesco]

Hello,

the issue was solved successfully. The problem was given by invalid values present in the signals used to compute the totalizers.

Issue was resolved by replacing invalid values in the signals with zeros using the function replaceNotValid(0).

You can also use the function resampleHold() to replace invalid samples with the last valid sample.

 

Francesco

[John Cox]

Thanks for the update Francesco! Glad the invalid values have been replaced.