The Institute for Mechanical Engineering of the Westphalian College is currently working on a hydrometer, that enables the calibration of flow rate instruments for hydrogen. The team around Prof. Dr. Karl Klug aims for an accuracy tolerance of 2%.
The project is scheduled to be finalised this year. A whole report can be found in the 2018/02 edition of the Trikon magazine, published by the Westphalian College.
The article by Barbara Laaser can be downloaded here, courtesy of the Westphalian College (WH): Hydrometer.