Be sure to actually count every collar! The field method of marking the first 10 collars on a cigarette pack, and quickly extending the measurement to the liquid level can result in significant errors! The sound wave actually speeds up as it descends, thus joint reflections become slightly closer together. The simplest and most accurate (and most expensive) method of counting is to use a purpose designed 11-point scissor caliper (marked 0–10). Nevertheless, mark each 10 joint kicks with a pen for easy checking, adding, and #welltest quality control.
To turn #AWS joint counts into depth measurements one requires a wellbore description with the number of tubing joints and length (i.e. 168 joints of 60 mm 6.85 kg/m J55 EUE landed at 1587.6 m). Divide the length by the joint count to get an average m/joint measurement (i.e. 9.45 m/jt for the example). An average casing joint length can be more difficult to obtain or figure out from a wellbore schematic.