I would recommend any of the variety that are a balance scale system. This is the type that has sliding weights to bring the scale into a balanced position. These are almost always adjustable and can be calibrated fairly easily.
It is important to keep scales clean, lightly lubricated if exposed to moisture (humid places or places with pools in the building, near locker rooms or showers), and not subjected to abuse (if kids are using the facility, they sometimes play with the scales).
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