In an era long before mechanical precision, quality control was much less of an exact science than it is today. Without robots, computer programs, etc, it was up to people to do the best they could in ensuring that a product’s quality was consistent, time after time.
Gunpowder was just one of the many items that needed to be made to the same exacting specifications from batch to batch. If quality control was lacking and a sub-par batch made it out the door, it might be too weak and not ignite properly. Conversely, it might be too strong and cause a catastrophic failure in the firearm shooting it. Either result is bad for business.
This is where the eprouvette comes into play. Its operation is incredibly simple, but it was also incredibly effective. A charge of powder was loaded into the barrel and the flashpan. When the flint ignited the powder charge, the steel cover on the end of the barrel was pushed away from the muzzle by the pressure of the explosion. This moved a numbered gear to a certain notch. The number at said notch was noted and the powder mixture was deemed correct, weak, or strong.
Repeated testing ensured that each batch of powder met requirements. The method wasn’t perfect, but it worked.