1. Demand the volume behind the ppb figure
Dissolved hydrogen concentration depends entirely on how much water it is dissolved into. A concentration quoted without a bath volume, a water temperature and a run time is not a comparable specification.
Ask for all four together. A vendor who measures properly will have them.
2. Look at how the hydrogen gets into the water
Fine bubbles dissolve; coarse bubbles vent. Circulation distributes; a static diffuser concentrates. The combination of bubble size and flow loop is the real engineering content of a bath system, and it is what determines whether a reading at one end of the tub is true of the whole tub.
Ask whether the system keeps circulating during the soak or only primes beforehand.
3. Check priming time against your routine
Every system needs time to raise a full bath to its working concentration. If that period is long, it needs to fit your evening rather than fight it.
In commercial settings priming time compounds: it sets how many guests an hour a room can serve.
4. Plan the install honestly
A portable unit beside the bath is the simplest case. Plumbed installations, hot tubs and resort rooms bring in water treatment, electrical isolation, service access and drainage. Decide which case you are in before comparing prices.
Service access is the detail most often discovered too late. Leave room to reach the unit.
5. Size the duty cycle to the venue
A home system running one bath a day and a spa system running eight are different machines even when the output figure matches. Continuous-duty components, filtration and cleaning schedules separate them.
As everywhere on this site: certification and engineering data describe hardware and manufacturing quality, not medical or therapeutic status.