Sentry ST520 Instruction Manual page 28

c.Using the previous values to set the reference values of
（⊿Ev, ⊿CT, ⊿u'v'）mode.(522)
MODE
Zero the Reference Values of Color Difference
Close the receptor cover and the（Ev, x, y ,Ev, u', v' or EV, CT）
measured values will show zero. Then repeat the section 5-2-1
that using the measured values to zero the reference value.
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