Centum VP R5.03 - report measure deviation

Hello everyone!
I have a problem with generated reports with offset time. I'm monitoring the SUM value (trend) of an flow instrument, so that is a cumulative value (not taken directly from an instrument, but in trend history data). Reports are generating normally at wanted time, but the offset SUM value of measured flow is not nearly ok. I set up a resistor to AI card terminal (where the instrument is connected, so the value of measuring is constant). All i'm getting is a non linear deviation of measured data for that field instrument (for example every 2 hours)...
I'm not getting anywhere with this, so please help anyone...
When you expect a number, and there's a problem you have to use numbers to investigate the problem. You have provided no numbers.

Is the value a total, like liters? or 1,000's of liters?
What value/number do you get that does not meet your expectations? What number did you expect?
Why would a fixed resistor provide a constant value?
WIth this resistor installed can you read and record a constant flow rate?
That constant simulated flow rate has what value/number?
What does the non-linear deviation look like? What deviates from what, number-wise?
The forum accepts graphic images, please post a clear image of this non-linear sum.
Where is the summation done calculation? What kind of device? At what interval is the rate summed?
Can you view the real time summation and knowing the constant rate value, confirm whether the fixed value is actually added to the previous total at each interval? If not, why not?