Mettler Toledo WXTS3DU, XP26, XP105DR, XP205DR, XP26DR Micro-Weighing Guide
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Micro-Weighing Guide
Maximize Efficiency
in Micro-Weighing
Improve Quality
of Results
Ensure Safer
Operations
10 Steps
to Successful Micro-Weighing
2
10 Steps to Successful Micro-Weighing
Practical Weighing Guide
1. Which microbalance is right for me? 4
2. Where should I place the microbalance? 6
3. Why is calibration of the microbalance important? 7
4. How can I ensure accurate results every day? 8
5. How can I deal with drifting weight results? 9
6. How can I optimize working with small samples? 10
7. How can I clean my microbalance more easily? 11
8. How can I make the weighing process more efficient? 13
9. What data capture / data transfer options do I have? 14
10. Other helpful tips to optimize micro-weighing 15
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Micro-Weighing Guide
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le
D
1. Which Microbalance is Right for Me?
Consider which criteria are relevant in choosing the right balance for your needs.
Every measurement on ANY balance is subject to uncertainty. Understanding this uncertainty is the key to
ensuring accurate weighing results and avoiding errors in downstream processing. For a microbalance, the
uncertainty is almost entirely due to repeatability (precision) when weighing small samples (i.e. at the low end
of the weighing range of a specific balance).
It is not the readability that determines the accuracy of a weighing instrument, but rather its repeatability,
ordepending on it, its minimum weight capability.
To find the appropriate microbalance for your needs you should define the smallest amount you want to weigh
and how accurately you need to weigh it (i.e. at what tolerance). From this basic information, the accuracy of the
required microbalance can be determined. Your weighing consultant can calculate this for you and verify that it
meets your daily needs:
Lab Technician
Sulfamidin Prep
Measurement uncertainty [%]
Accuracy limit:
Minimum weight
Methods
03:07
pm
Sulfamidin
B2
Protocol
g
Methods
0.0009999
Sampl
I
Delete protocol Edit method Complete task Add result
Process tolerance
of customer [%]
Smallest
net weight
Lab Technician
Sulfamidin Prep
Samp
I
Delete protocol Edit method Complete task Add result
Methods
Methods
0.005000
Sulfamidin
B2
03:07
pm
Protocol
g
Weight [g] Max.
Fig. 1: Typical measurement uncertainty curve of
atypical ultra-microbalance. At the accuracy limit
the relative measurement uncertainty increases with
decreasing sample weights above the user defined
tolerances. The yellow area represents a safety margin between user formulated smallest net weight
and accuracy limit.
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Example
• Smallest sample typically weighed in (defined by user).
(smallest net weight = 2 mg)
• Tolerance required = 2 mg ± 10 μg [= 0.5%]
Formula applied to fulfill user requirement
Smallest net weight ≥ minimum weight * safety factor
Minimum weight = Repeatability of the balance * k / Required tolerance [%]
• For a safety factor of 2, select a balance with a repeatability ≤ 2.5 μg
Explanation of terms:
Safety factor: A safety margin to protect against variations in the weighing process, environmental conditions or
user influence. A safety factor of 2 is recommended for typical processes. For higher risk processes, a higher
safety factor is recommended.
Expansion factor (k): Typically an expansion factor k = 2 is used.
(For a Gaussian distribution, 95% of the results statistically fall within the k = 2 range;
99.7% of the results fall within k = 3).
Recommendation
A microbalance is advised for use in this application. An analytical balance has a repeatability of 7 µg and
is not accurate enough for this task, and an ultra-microbalance with a repeatability of 0.15 µg would allow
weighing much smaller amounts of sample as required and might not be needed. The XPR2 microbalance with
arepeatability of 0.5 μg fulfills the requirements perfectly.
Special Tip
• Ask your METTLER TOLEDO consultant about GWP® Recommendation which helps you select the right
weighing instrument based on your weighing requirements: www.mt.com/gwp-recommendation.
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