Actually the "bag test" test is not as far off as you may think.

When I was a Respiratory Therapy Student one of the ways used in one Hospital to check the Exhaled Volume from an Intermittent Positive Pressure Breathing Machine was to hook a Plastic Bag that had liter graduations on it to the machine outlet and leave it there for X amount of time and see how much volume went into the bag.

I know that if I occlude the Crankcase Breather tubing it takes about 7 seconds before my Enigne starts to stumble and try to die.

I think the main issues with blow by is what your options are if you believe you have too much.
Most say as long as it starts and runs OK keep driving it.

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84 300D, 82 Volvo 244Gl Diesel