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VERIFICATION AND CALIBRATION
6. VERIFICATION AND CALIBRATION
It is important to verify the AQUALAB VSA water activity calibration against known
standards to guarantee optimal performance and accuracy. METER recommends
verification or once per shift or daily before use.
METER also recommends annual factory calibration to maintain optimal performance.
6.1 WATER ACTIVITY VERIFICATION
The AQUALAB VSA uses both a capacitance relative humidity sensor and a chilled-mirror dew
point technique to determine water activity. If you set volatiles to yes, the VSA performs water
activity verification on the capacitance sensor only. If you set volatiles to no, the VSA verifies
the dew point sensor. While the instrument does not require a routine full calibration, it is
important to verify for linear offset periodically. The components used by the instrument to
measure water activity are subject to contamination which may affect VSA performance. When
this occurs, it changes the accuracy of the instrument. This is what is called a linear offset.
Therefore, frequent verification assures the VSA is performing correctly. You can check linear
offset by using two different verification standards.
6.1.1 VERIFICATION STANDARDS
Verification standards are specially prepared, unsaturated salt solutions having a specific
molality and water activity value that are accurately measurable. The verification standards
sent with the initial shipment are very accurate and available from METER. Using verification
standards to verify accuracy can greatly reduce preparation errors. Verification standards
come in seven water activity levels: 1.000, 0.984, 0.920, 0.760, 0.500, 0.250, and 0.150 (Table4).
The standards are produced under a strict quality assurance regime.
Table4 Verification standards
Verification Standard at 25 °C
a
w
17.18 mol/kg LiCl 0.150 ±0.005
13.41 mol/kg LiCl 0.250 ±0.003
8.57 mol/kg LiCl 0.500 ±0.003
6.00 mol/kg NaCl 0.760 ±0.003
2.33 mol/kg NaCl 0.920 ±0.003
0.50 mol/kg KCl 0.984 ±0.003
Steam Distilled Water 1.000 ±0.003
NOTE: Safety Data Sheet (SDS) for these standards are available at metergroup.com/food/meter-
safety-data-sheets.
Although distilled water and the 0.984 standard are available as a verification standard,
METER does not recommend using them with the AQUALAB VSA.
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