METER GROUP AquaLab 4TE - Page 48

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SERVICE
To use a verification standard, remove the twist top and pour the contents into an AQUALAB
sample cup. Information about the standard value and molality can be found printed on
the outside of the plastic vial. If for some reason METER verification standards are not
available,contact Customer Support for instructions on how to create a verification standard
salt solution.
In TEV models, the capacitance sensor can exhibit hysteresis. When a high water activity
standard or sample is measured, this may influence future readings. After measuring a high
water activity sample, allow 1 h for the sensor to dry. Additionally, desiccant or activated
carbon can be used to accelerate the drying process.
NOTE: To avoid inaccurate water activity readings, verification standards should be used once immediately after
opening. Do not store standards in sample cups for repeated use.
4.1.1.2. VERIFICATION OF CALIBRATION
Verification of the calibration should be performed regularly to ensure the AQUALAB is
operating within specification. METER recommends verification be run once per shift or
prior to operation. The procedure for verification involves reading a sample of known water
activity. METER recommends using a standard close to that of the sample being measured.
If operating in volatiles mode using the capacitance sensor or measuring a broad range of
water activity values, METER recommends bracket the sample range. Capacitance sensors
can show significantly more drift when compared to a chilled mirror and can exhibit some
nonlinear behavior. Verifications should not be performed with only water standards.
Checking the water activity of a standard solution checks for the possibility of unit
contamination or shifts in the linear offset from other causes.
The following steps explain how to verify for linear offset of the AQUALAB (Figure59).
1. If measuring samples over a wide range of water activity values, select two standards
that best bracket the region. If measuring a relatively narrow region, select a sample that
most closely matches the sample range being measured.
For example, if testing for water activity readings ranging between 0.621 and 0.713, use
the 8.57 mol/kg LiCl (0.50 a
w
) standard for the rst verication and the 6.00 mol/kg,
NaCl (0.76 a
w
) for the second verication.
NOTE: The AQUALAB needs to warm up for approximately 15 min to make accurate readings.
2. Empty a vial of the lower water activity verication standard into a sample cup.
3. Place it in the AQUALAB testing chamber. Make sure that the standard is as close to the
instrument temperature as possible. See Section2.2.2 for detailed instructions.
NOTE: Make sure the rim and outside of the sample cup are clean.
4. Carefully close the lid and move the lever to the READ position.
5. Take two readings.