using the calculated data function in pi datalink you might have noticed an
additional field for the conversion factor it is a multiplier used to change a number from one unit
of measure to another since the data archive is not aware of engineering units it assumes
that rate points are in terms of units per day typically this assumption is inaccurate as many
points are measured in terms of units per second units per minute or units per hour
when using the total function in calculated data a conversion factor must be used to correct pi's
assumption that the data is in units per day the conversion factor is equal to 1 when the source
data are in units per day to make it more clear let's have a look at an example we have a flow
rate measured in liters per minute and stored in the data archive we are interested in calculating
total liters for an 8 hour period as we can see in the illustration the flow rate is 3 liters per
minute for 3 minutes then 5 liters per minute for 2 minutes and 1 liters per minute for 3
minutes total flow is the area below the flow rate line so the area of the 3 rectangles added up the
expected total flow would therefore be 22 liters data archive however assumes the unit of measure
of flow rate is liters per day without applying the conversion factor the total flow calculated
by data archive is highly inaccurate so to get the correct number the total computed by the data
archive must be multiplied by the conversion factor to get 22 liters and don't worry when
making the conversions you can use the conversion factor calculator from the drop down we can make
a selection based on the units of measure of the process variable the required conversion factor
number will be then calculated and placed in the conversion factor field let's practice what we
have learned we want to calculate the total flow and the average flow rate of mixing tank 1 for
yesterday to know what conversion factor to use we need to bring the unit of measure of flow rate for
mixing tank 1. let's begin by creating a template in excel we need a field for the root path
data item start time
end time unit of measure
and finally don't forget the total conversion factor let's also add columns for average
total
and percent good
let's now go to search and navigate to mixing tank 1 and find the flow rate attribute
remember to set the slider to name only and click ok for the start time type y which translates
to yesterday at midnight and t for the end time which translates to today at midnight
to get the unit of measure go to properties
select our data item and choose unit of measure from the drop down list and click ok
to find out what's the conversion factor we need to use let's open up the calculated data function
and use the conversion factor calculator now we know that our flow rate is in units per minute
so let's select it here to get the correct number now we can type it in into our template
now we are ready to calculate the average so let's go to the calculated data function
select our data item root path start time end time and the average for the calculation mode
now let's calculate the total of flow rate for this time range so again choose the
calculated data function now select our data item root path start and end time
don't forget to select the conversion factor and also add the percent good
this concludes our video on calculating the total in pi data link thanks for watching