Time Constants
Fast or F: The time constant for F is 125 milliseconds. This is the most used time constant.
Slow or S: The time constant for S is 1 second.
Impulse or I: The time constant for I is 35 milliseconds. This time constant is followed by a max hold function that allows the level to drop with 3 dB per second. This time constant is suited for characterizing noise with rapid level changes.
The time constants Fast and Slow are now defined in the IEC61672 standard, and the proper names are simply F and S. The I time constant is defined in IEC60651.