1. During operation, when the steam drum pressure changes, which will cause severe changes in saturated water and saturated steam, thus causing error in differential output. 

2. The compensating pipe of balancing container is designed and calculated according to that the water level is at zero level, but when the boiler is operating with deviation from zero water level, the measurement error will be caused. 

3. When the steam drum pressure drops suddenly, because of the condensate in positive pressure chamber possibly to be evaporated, the error of the instrument indication may be caused to occur.