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    "title": "3 ml glass cartridges explained: the 300-unit standard, seals and storage",
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    "tldr": "A 3 ml glass pen cartridge holds 300 units on the U-100 scale. How its borosilicate barrel and two seals work, and why stopper length decides the fit.",
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            "value": "The two seals",
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            "q": "How many units does a 3 ml glass cartridge hold on a U-100 scale?",
            "a": "On the U-100 scale, where 100 units equals 1 ml, a 3 ml cartridge holds 300 units. This is derived from 300 units × 0.01 ml per unit = 3 ml."
        },
        {
            "q": "Are standard 3 ml cartridges compatible with multiple cartridge pens?",
            "a": "Yes. The outer dimensions of 3 ml cartridges are standardized across brands, so one cartridge format fits a wide range of pens—including the Gansulin metal pen—eliminating brand lock-in."
        },
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            "q": "How should sealed 3 ml glass cartridges be stored?",
            "a": "Store below 75°F (24°C) in a clean, dry place away from direct sunlight and heat sources. Keep cartridges in their individual pouches until needed; discard any whose pouch is torn, wet, or damaged."
        }
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