METER AQUALAB TDL - Page 40

7 VERIFICATION AND CALIBRATION AquaLab TDL
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
TDL 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 and the standard covers the bottom.
3. Carefully close the lid and move the lever to the Read position.
4. Take two readings. The water activity readings should be
within ±0.005 a
w
of the given value for the verification stan-
dard. See Appendix B for the correct water activity value of
METER’s standards at temperatures other than 25
◦
C.
5. If your AquaLab is reading within ±0.005 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
take two readings. The second water activity reading for the
second verification standard should be within ±0.005 a
w
.
7. If either of the verification standards is not correct, it is prob-
ably due to contamination of the sensor chamber. For cleaning
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