7 VERIFICATION AND CALIBRATION AquaLab
check the offset with a standard of similar water activity when the
general water activity range of your sample is changing. Checking
the water activity of a standard solution will alert you to the possi-
bility of unit contamination or shifts in the linear offset from other
causes.
Follow steps 1 through 8 to verify for linear offset of your AquaLab.
(Refer to Figure 3: the Verification Standard Flowchart for a quick
overview.)
1. Choose a verification standard that is close to the water activ-
ity of the sample you are measuring.
Note: The AquaLab needs to warm up for approximately 15
minutes to make accurate readings.
2. Empty a vial of solution into a sample cup and place it in the
AquaLab testing chamber. Make sure that your standard is as
close to the instrument temperature as possible.
Note: Make sure the rim and outside of the sample cup are
clean.
3. Carefully close the lid and move the lever to the Read position.
4. Take two readings. The water activity readings should be
within ±0.003 a
w
of the given value for the verification stan-
dard. See Appendix B for the correct water activity value of
Decagon’s standards at temperatures other than 25
◦
C.
5. If your AquaLab is reading within ±0.003 a
w
of the verification
standard, choose a second verification standard that would bor-
der the range of water activity you plan to test. For example,
if you plan to test for water activity readings ranging between
0.713 and 0.621 you should use the 8.57 mol/kg LiCl (0.50 a
w
)
standard for your first verification and the 6.00 mol/kg, NaCl
(0.76 a
w
) for the second verification.
6. Prepare a sample cup of the second verification standard and
make two readings. The second water activity reading for the
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