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Total tolerance

The total tolerance is the combined manufacturing tolerance and measurement uncertainty for measurements that are taken to provide proof of a guarantee.
The acceptance test codes in DIN EN ISO 9906 contain tolerance factors that make it easier to apply the combined manufacturing tolerance and measurement uncertainty that always have to be taken into account during the acceptance test for the individual operating parameters. These tolerance factors for the flow rate (± tQ), pump head (± tH) and pump efficiency (± tη) must be applied to the guaranteed point. See Fig. 1 Total tolerance

VariableSymbolClass 1 %Class 2 %
Volume flow ratetQ± 4,5± 8
Pump discharge headtH± 3± 5
Pump efficiencyη-3-5

Fig. 1 Total tolerance: Tolerance factors

The head and flow rate guarantee is fulfilled if the H/Q curve determined from the measurements intersects or at least touches the vertical and horizontal bar of the tolerance cross. See Fig. 2 Total tolerance

Total tolerance: Evidence of compliance with guaranteed values of the flow rate, head and efficiency Fig. 2 Total tolerance: Evidence of compliance with guaranteed values of the flow rate, head and efficiency

The efficiency is derived from the point where the measured H/Q curve intersects the straight line running through the agreed operating point QG, HG and the zero point of the H/Q axis as well as from the point where a vertical is intersected by the η(Q) curve calculated from the measurements. The guarantee condition for efficiency is deemed to be within the tolerance limits if the value of this quantity is greater than or at least equal to the value for ηG ∙ (1 − tη) at this intersection point.
In the new version of DIN EN ISO 9906, the classes for assessing a pump measurement and the measurement results have been extended from 2 to 5. This also results in changes to the relevant total tolerance. See Fig. 3 Total tolerance


Total tolerance: Accuracy classes for acceptance testing of pumps with appropriate tolerance ranges Fig. 3 Total tolerance: Accuracy classes for acceptance testing of pumps with appropriate tolerance ranges. The η values do not result from an exact calculation using the other maximum values of the pertinent column. They are, in fact, based on experience gained in practical applications.

In accordance with this extended classification system, diagrams are available to illustrate the conditions for fulfilling the guarantee. See Fig. 4 Total tolerance

Total tolerance: Permissible tolerances for classes 1B, 2 and 3 Fig. 4 Total tolerance: Permissible tolerances for classes 1B, 2 and 3

Total tolerance