How to use an outside spring-joint caliper
Step 1 - Open legs of caliper
Turn the adjusting nut to widen the legs of the caliper.
Step 2 - Position object between legs
Put the object you are measuring in between the measuring faces of the caliper.
Step 3 - Close legs around object
Turn the adjusting nut again to close the legs around the widest part of the object.
Tighten the caliper so that the measuring faces are able to slide past the outside diameter with the slightest bit of resistance.
You should be able to carefully remove the object from the caliper without opening the legs.
Step 4 - Remove object and transfer measurement
Remove the object you are measuring from in between the measuring faces.
Put a measuring tool such as a ruler against the caliper.
To take the measurement, set one leg at the end of the scale and read the location of the other leg on the scale.