So both tests are fundamentally measuring the exact same physical principals, the only difference being that obviously a PI would be more comprehensive?
Is the DAR called that simply because much of the current increase in that test is also capacitive, so it's not primarily measuring polarization and leakage?
There is no difference between absorbtion current and polarization current by IEEE43 terminology. Specificaly IEEE Std 43-2000(R2006) Definition 3.1 treats them equivalently:
"3.1 absorption (polarization) current: A current resulting from molecular polarizing and electron drift, which decays with time of voltage application at a decreasing rate from a comparatively high initial value to nearly zero, and depends on the type and condition of the bonding material used in the insulation system."
So personally, I'd be be inclined to think there is not a lot of significance to those particular two choices of names for those particular two tests.
Am I correct in thinking that's the importance of comparing the 30 second and 60 second readings: You're trying to null out the capacitive charging that occurs in the first 30?
Let's start with absorption: I'd be inclined to think (from the IEEE43 figures) that the PI has a fairly straightforward meaning:
PI = (Iabs1min + Iresistive) / Iresistive.
Where Iabs1min = 1-minute absorption current.
The capacitive is not included here because it is pretty much gone by 1 minute.
You could also rewrite this as:
(Iabs1min / Iresistive) = PI - 1 which gives imo a nice simple definition.
The DAR is not as straightforward. For one thing, absorption current is present on both measurements (30 sec and 60 sec) and it is of course not the same for both. So already we are working with three values (I resistive, Iabs1min, Iabs30sec where in PI we only had two). If Icap is not completely gone at 30 seconds, it's even messier. So there is not as simple an interpretation for DA as for PI. But I think the values we use as limits tend to be based on empirical-based thumbrules (rather than theory) anyway, so it's not a big problem.