Most of the faults of differential pressure transmitter are zero drift and blocking of connecting pipe, therefore in site it is rare to verify the scale point by point, but to check its zero and variation tendency, the specific methods are as follows: 

1.Zero check: close the stop-valve of positive pressure and negative pressure. Open the balance valve, at this moment, the current of electric differential transmitter shall be 4mA.

2.Variation tendency check: After zero, each valve recovers to its original status. Open the blow-down valve of negative pressure. At this moment, the output of the differential pressure shall be the maximum, namely the electric differential pressure transmitter is more than 20mA. If only the blow-down valve of the positive pressure chamber is opened, then the output is the minimum, namely the electric differential pressure transmitter is 4mA.

3.When the blow-down valve is opened, less or none of the medium to be measured is drained, showing that the connecting pipe is blocked, so it shall be dredged.