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4.13 Suggested Initial Tolerance Limits
Based on our experience with many different makes and models of
pipettes, Artel recommends the values in Table 1 as a starting point for
developing achievable tolerance limits. Refer to Artel Lab Report, Issue 5,
“Setting Tolerance for pipettes in the laboratory” for additional information.
Using Table 1 below, begin by choosing either the “Relative Error” or
“Absolute Error” tolerance limit values. These tolerance limits reflect what
is reasonably achievable in a working laboratory. They presume that the
pipette is calibrated and functioning properly, is used with good quality
tips, and is tested by a skilled operator. When these criteria are met, most
makes and models of pipettes should test within these tolerance limits
unless they are mechanically defective.
For fixed-volume pipettes, the nominal value is the fixed volume. For
variable-volume pipettes, the nominal value is the largest user-selectable
volume setting; e.g., a 10 to 100 µL pipette has a nominal volume of 100
µL.
The absolute error for the nominal volume applies to every selectable
pipette volume; e.g., a 100 µL nominal volume yields limits of ±2.0 µL
inaccuracy (mean value) and ±1.0 imprecision (STD) for all volumes. The
relative error varies throughout the pipette range; e.g., for a 10 to 100 µL
pipette at 100 µL the relative inaccuracy is ±2.0%. However, at 10 µL the
relative inaccuracy is ±20.0%.
Notes:
a. Inaccuracy is expressed as the deviation of the mean across all
samples from the set point volume. Inaccuracy can be expressed in
either absolute units such as microliters, or relative units such as
percent. Absolute imprecision is expressed as the standard
deviation (STD) across all samples. Relative imprecision is
expressed as the coefficient of variation (CV) across all samples.
b. For pipettes with nominal volumes between those provided in this
table, absolute inaccuracy tolerance limits are equal to ±2.0% of the
pipette’s nominal volume and the tolerance limit for absolute
imprecision is 1% of the pipette’s nominal volume.
c. Relative error tolerance limits at other volume settings can be
calculated by dividing the absolute tolerance limit (see Table 1) by
the set point volume. Multiply the result by 100 to convert it to a
percentage.
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