I've been telling them to not bother taking the TD for years now, and those taking it with an IR thermometer - yes, you are right
and I will add
.
Taking TD with an IR thermometer is like taking the temperature of the water in your vehicle's engine by measuring the temperature of the exhaust pipe at the rear.
But, the HI schools keep cranking out HIs who *have been told that a TD between 15-22 means the system is operating properly*, and now I find out that something called NATE for HVAC techs is saying that a "temp drop should be between 18 and 21 degrees 17 or 22 is ok", and that is even worse than HI schools telling HIs the TD means anything - those guys (according to the site) are supposed to be the best HVAC guys.
Have you heard of NATE before (I had to look it up on the web)?