Data with an uncertain start point

[PhillipB]

I am working with testing data and there is a complication as to when the test is started.  The data is recorded automatically and relates to engine RPM and oil pressure.  The recording starts as soon as the engine is switched on but the test can actually start at some point later and this varies by several minutes per test.

We can see when the test starts as the RPM of the engine is raised to 1850 RPM, the first occurence of 1850 is when the 0 needs to be.  I need to have this as the 0 point for the test with all other enties prior to this point to be negative and then positive after.

RPM	Pressure	Time - sec	Relative Time
800	991	145	-1
1850	995	146	0
1850	994	147	1
800	991	148	2
800	992	149	3
1850	994	150	4

The blue values are the ones I need to create.

Any help would be much appreciated.

[PhillipB]

Thanks, I think that will do what we need.

[jgeddes · Accepted solution]

In Power Query

Starting with

-add an index column starting from 1

Table.AddIndexColumn(#"Changed Type", "Index", 1, 1, Int64.Type)

-find the lowest value of the index column where the RPM = 1850

Table.AddColumn(#"Added Index", "initialZero", each List.Min(Table.SelectRows(#"Added Index", (x)=>x[RPM]=1850)[Index]) )

-subtract the initialZero column from the Index column

Table.AddColumn(#"Added Custom", "Relative Time", each [Index] - [initialZero])

-remove the Index and intialZero columns

Table.RemoveColumns(#"Added Custom1",{"Index", "initialZero"})

and you should end up with