TESTO 815 Instruction Manual page 27

Absolute pressure dependency
testo 815 is calibrated by default for measurements at a height of
0 m above sea level. Measurements at other heights give rise to
measurement errors which can be corrected using the table
below. Subtract the appropriate offset value from the value
measured (e.g. - 0.1 dB for measurements at a height of 500 m
above sea level). You can avoid this measurement error by
calibrating the meter in the corresponding height prior to every
measurement. Please refer to the Instruction Manual for the
calibrator.
Height in m above sea level
0 - 250
>250 - 850
>850 - 1450
>1450 - 2000
Wind cap
Generally the wind cap supplied should be attached during
measurements outdoors and when air movements occur. Wind
noises in the microphone cause measurement errors since the
wanted signal (of the noise source) and the wind noise add up
together.
Overmodulation and undermodulation
With every measurement cycle, the sound level meter checks
whether the measured sound level is in the validity range of the
respective measurement range. Deviations are indicated by
"Over" and "Under" on the display. However, the criteria for
overmodulation and undermodulation are different.
Overmodulation is signalled if the maximum value (peak value e.g.
short sound pulse, bang) during the last measurement cycle was
too high. This value may be considerably higher than the actual
value of the sound level displayed. Therefore, it can happen that
"Over" is signalled although a sound level within the normal
framework of the respective measurement range is shown. By
contrast "Under" is geared to the measured actual value and is
therefore set when the lower limit of the measurement range is
reached.
Pressure p in mbar
1013 - 984
983 - 915
914 - 853
852 - 795
5. Operation 27
Offset in dB
0.0
-0.1
-0.2
-0.3