All Oasis cylinder style humidifiers need Humigel Crystals to prevent water leakage. We recommend that players replace the crystals once a year. The OH-4+ will fill 14 more Oasis humidifiers than the OH-4 refill kit (20 refills per kit). This is an excellent alternative to the OH-4 (6 refills per kit) kit for players with many instruments to humidify and already have purchased the OH-4 refill kit that includes the measuring spoon, syringe and instruction. The OH-4+ kit only has bulk crystals and no measuring spoon, syringe and instruction.