I think many expirienced leatherworkers will argue, that instruments to measure thread tension is not needed. You will be able to feel and see when the right tension. And you can quite easily find out what is wrong with the tension, when problems occur.
I am not an expirenced leatherworker, and I have been using a spring dynamometer to measure thread tension. A dynamometer is an instrument to measure force. It has helped me to find problems with thread tension, and for some projects I write D