HP 5890 Series II Plus Reference Manual page 56

Temperature Control
Oven temperature calibration
3. CALIB DELTA is displayed until
temperature recalibration occurs. Note that, after calibration, the
displayed oven temperature value should match closely the measured
value.
Any delta value within the range -10.00 through +10.00 C may be
entered. If a value outside this range is entered, the message
CORRECTION TOO HIGH is displayed.
Assuming the battery protecting HP 5890 memory is operational, a new
calibration constant remains in effect even if the instrument is switched
off, or disconnected from its power source, or if power fails.
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is pressed; then oven
ENTER