HI AGAIN PITOT TUBE.
Cause of your last post it seems you understand the reason for using derated takeoffs. An example will help:
Just figure out you are going to take off at your airfield and OAT is 15ºC, EPR max for this temperature is 1.96. Your maximum operational weight (OAT 15ºC) is 162.000 lbs so you are allowed to take off that heavy. But today your airplane is almost empty and just weighs 114.000 lbs.
In this scenario you can advance throtles full forward to EPR max 1.96 and you will become airbone at the first third of the runway, no problem BUT it does not make any sense to request maximun stress to your engines, if you are leaving 2/3 of the runway unused.
Now we can think of cheating the engine, instead of use the OAT (15ºC= 1.96 EPR) we are going to use NOT a real temperature(OAT) but a fake one (assumed).
IF the runway is long (e.g. 12.000 ft) we could take off with 114.000 lbs if the OAT were 50ºC, and therefore 50ºC could be a good assumed temperature (fake), the engine output will be NOT 1.96 EPR but a lower one 1.82 EPR.
Using this criteria we are saving engine life (not stressing to max) while not compromising safety .
REGARDS