METER AQUALAB TDL, Руководство пользователя

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38
SERVICE
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.
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 TDL ( Figure 47 ).
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 (0.500
a
w
) standard for the first verification and the 6.00 mol/kg
NaCl (0.760
a
w
) for the second verification.
NOTE: The AQUALAB TDL needs to warm up for approximately 15 min to make accurate readings.
2. Empty a vial of the lower water activity verification standard into a sample cup.
3. Place it in the AQUALAB TDL testing chamber. Make sure that the standard is as close to
the instrument temperature as possible ( Section 2.2.2) .
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.
The water activity readings should be within ±0.005
a
w
of the given value for the
verification standard. The standards will read at 25 °C. If other temperatures are
required, refer to
Table 4 .
Table 4 Temperature correction of verification standards
Temperature
(°C)
Water
0.50 mol/kg
KCL
2.33 mol/kg
NaCl
6.00 mol/kg
NaCl
8.57 mol/kg
LiCl
13.41 mol/kg
LiCl
17.18 mol/kg
LiCl
15 1.000 0.984 0.923 0.761 0.492 0.238 0.140
20 1.000 0.984 0.922 0.760 0.496 0.245 0.145
25 1.000 0.984 0.920 0.760 0.500 0.250 0.150
30 1.000 0.984 0.920 0.760 0.504 0.255 0.155
35 1.000 0.984 0.920 0.760 0.508 0.261 0.160
40 1.000 0.984 0.921 0.760 0.512 0.266 0.165
50 1.000 0.984 0.894 0.740 0.517 0.275 0.172