7 VERIFICATION AND CALIBRATION AquaLab TDL
7 Verification and Calibration
It is important to verify AquaLab water activity calibration against
known standards to guarantee optimal performance and accuracy.
METER recommends verification daily, once per shift or before each
use. METER also recommends annual factory calibration to main-
tain optimal performance.
Note: To avoid inaccurate water activity readings, verification stan-
dards should be used once immediately after opening and not stored
in sample cups for repeated use.
7.1 Water Activity Verification
AquaLab uses the TDL technique to determine water activity. Be-
cause this is a primary measurement of relative humidity, no calibra-
tion is necessary; but we recommend periodic verification for linear
offset. The components used by the instrument to measure water
activity are subject to contamination which may affect the AquaLab
performance. When this occurs, it changes the accuracy of the in-
strument. This is what is called a “linear offset.” Therefore, frequent
verification assures you that your AquaLab is performing correctly.
Linear offset is checked by using two different verification standards.
Verification Standards
Verification standards are specially prepared unsaturated salt solu-
tions having a specific molality and water activity value which are
accurately measurable. The verification standards that were sent
with your initial shipment are very accurate and readily available
from METER. Using verification standards to verify accuracy can
greatly reduce preparation errors. For these reasons, we recommend
using standards available through METER for the most accurate
verification of your AquaLab performance. Performance Verifica-
tion Standards come in six water activity levels: 1.000, 0.984, 0.920,
0.760, 0.500, 0.250, and 0.150 a
w
. The standards are produced un-
der a strict quality assurance regime. Please contact METER to
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