I have researched how you should measure time under tension when exercising instead of counting reps. However, I do not always have a timer with me and I was wondering if it would still be effective if I just did the number of reps I would normally achieve while exercising with a timer.
If you do the same number of reps with the same tempo , yes it will be similarly effective
If you are not measuring time with a stop watch but with the reps , then the time under tension is reps * time taken for each rep , where time taken for each rep is the tempo.
If you don't have a timer either stay at a constant tempo or just count seconds like kids on the play ground instead of the reps.