Delivery method — what the difference means.
Two of the three machines use gas diffusion. One uses hydrogenated water delivery. This is a meaningful technical distinction.
Gas diffusion — Echo Revive and S69
Molecular hydrogen gas is produced by the electrolysis cell and bubbled directly into the bath water via a diffuser. The gas dissolves into the bath water during the session. The rate at which the bath reaches target concentration depends on the gas flow rate and the volume of water being saturated.
The Echo Revive delivers gas at 850 ml/min. The S69 delivers gas continuously — flow rate not published. Both dissolve hydrogen gas into your existing bath water.
Hydrogenated water delivery — WZ-1
The WZ-1 produces hydrogen-rich water at >2,000 ppb inside the machine and delivers it into the bath at 3,500 ml/min. Hydrogen is dissolved into the water inside the machine before it enters the bath. The bath receives a continuous feed of pre-saturated hydrogen water at 3,500 ml/min throughout the session.
At 3,500 ml/min the WZ-1 delivers more than four times the flow volume of the Echo Revive's 850 ml/min gas diffusion rate. Higher flow means the bath reaches target concentration faster and hydrogen is replenished continuously as it outgasses from the bath surface.
Neither delivery method is definitively superior — they are different engineering approaches to the same goal. The flow rate differential is a factual specification that buyers can use to inform their decision.