The compensation vessels prevent the level difference which occurs with pressure changes in the pressure lines and which falsifies the measurement.
According to DIN 19211, the temperature in the compensation vessel must be assumed to be 50 K less than the steam temperature in the pipe when calculating the wall thicknesses. This is because the temperature in the compensation vessel during operation can only rise up to the saturated steam temperature.
A material acceptance test certificate A to EN 10204-3.1 is available for the materials from which the compensation vessels are made.