scope:

Measured values obtained by flow measurement contain measuring errors.

Measuring errors can be due to:

• inaccurate determination of the scale by means of calculation (according to VDI/VDE 3513 Part l), or by means of empirical calibration

• inaccurate execution or mounting of the scale, or changes in scale caused by subsequent influences.

Furthermore, as explained in VDI/VDE 3513:1971, the following characteristics of variable-area flowmeters have an impact on the magnitude of the measuring error:

• The restoring force for the pointer (float) is constant throughout the measuring range.

• The characteristic curves of variable-area meters are nonlinear (broadening towards lower limit of scale).

• Friction forces impeding the mobility of the pointer (float) can become effective.

• Unsteadiness of the float may result in reading errors.

The above-mentioned characteristics lead to a characteristic dependence of the measuring errors on the measured values. These errors are distributed nonlinearly over the measuring range. The influence of errors is greater in the lower measuring range than it is in the upper measuring range. For this reason, the lower measuring range is considered separately when determining the maximum permissible errors.

This guideline applies to variable-area flowmeters of any design. An electrical output of the measuring system shall be considered separately in the documentation. The manufacturer shall state whether any separate measuring errors must be taken into account for the output or whether the measuring errors due to the electrical output are included in the values given for the system.