What should the temperature be as measured in the duct for a central air conditioning system?

I am thinking this is one way to test a system (how cold the air is).

Thanks!

Look for a 18 degree give or take TD once the interior is cooled down. That is total temp differential from the return air to the supply air. The give or take on this depends upon the humidity.

NOTE the temperature you are looking for will vary depending upon the return air temp. This is why you always use TD. See your other post.