Sentry ST520 Instruction Manual page 8

 When using is complete, close the receptor cover.

Be aware the accumulation of irradiations would cause the
sensor continue to age. Please keep away the light source
and cover with the cap when the unit is not in use.
 If the meter will not be used for a long time, remove the
battery and close the receptor cover.
 Never use organic solvents to clean the meter.（such as
thinner, benzene, etc.）
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