MSA Altair 4 QuickCheck Mode D'emploi page 14

The gas tolerance level indicates the accuracy of the instrument
reading, verified by the bump check (based on a +0% accurate gas
cylinder) as follows:
LEL, CO, and H 2 S Units
GAS
TOLERANCE
LEVEL
20%
30%
40%
50%
O 2 Unit
GAS
TOLERANCE
LEVEL
20%
30%
40%
50%
As a reference, a full calibration sets the instrument response to the
accuracy of the calibration cylinder.
2-4
ACCURACY
80%
70%
60%
50%
ACCURACY
87%
80%
73%
70%