A sensing element inside a wrapping compares the concn. of a chosen vapour with a reference value appropriate to the conditions and indicates the result of the comparison. The wrapping is impermeable w.r.t. liq. but permeable w.r.t. vapour. A representative sample of vapour set free at the phase boundary can be absorbed and transferred to the sensing element. The indicator component is pref. of CoCl2 and polyvinylpyrrolidone-vinyl acetate copolymer. The device can indicate moisture content in nappies and surgical dressings, NH3 in soil, and can by detection of NH3, be used to indicate decay in foodstuffs such as fish. When used to indicate the need to water pot plants, the device is unaffected by excess water with which it temporarily comes into contact.
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