I've had several recent discussions about the importance of knowing the purity level in the hydrogen cooling system of a large generator. The level needs to stay high (more than 90%) so that it cools sufficiently and so that it remains safe and not explosive...too much air is a bad thing. The recommendation for this application is to use a specific gravity meter (SGN) along with a coriolis meter. The SGM provides real-time info about the purity of the system while the flowmeter provides indication that hydrogen is flowing through the system.
Since most new generators are on the larger end of the scale, hydrogen cooling is more and more prevalent. Using these recommendations will help ensure the availability of your generator while keeping plant workers safe.