The diagrams indicating, simultaneously, the pressures and the relative position of the piston in the engine cylinder are known as indicator diagrams.
Purpose of indicator card
Which indicator diagram/ card is taken first?
Compression card is taken first to check the timing of the indicator came otherwise if the timing on cam is wrong, our results will be wrong.
FIG.: Compression diagram
Compression diagram- It is only a line on the indicator diagram and gives the compression pressure and a timing check on the indicator cam.
It is taken in phase with fuel cut-off and at reduced RPM.
We know that compression card is a single line. So, any deviation from the line shows the fault with instrument. So, we have to correct the timing of the indicator cam.