7 VERIFICATION AND CALIBRATION AquaLab
Linear offset should be checked against two known verification stan-
dards daily, either once per shift or before each use. Linear offset
should never be verified solely against USP Purified Water, since
it does not give an accurate representation of the linear offset. For
batch processing, the instrument should be checked regularly against
a known standard of similar water activity. It is also a good idea to
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 alerts you to the possibility
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. See Section 8
for detailed instructions.
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 dew point and
±0.015 for capacitance when testing the verification standard.
See Appendix B for the correct water activity value of Decagon’s
standards at temperatures other than 25
◦
C.
5. If your AquaLab reads within ±0.003 a
w
of the verification
standard for dew point and ±0.015 for capacitance, choose a
second verification standard that would border the range of
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