How to calliberate an Aneroid Barometer?

The marine barometer’s accuracy is subject to its calibration. This is normally done by calling on board the Port authorities , who come on board with their PRECISION BAROMETER which provides the barometric readings to the highest accuracy. These readings of the two barometers they compare and obtain the index error for the ship’s barometer and then using the set screw provided at the back of the barometer, the needle is adjusted. They put this index error in a card and keep it along with the ship’s barometer.
Now by applying the index correction and height correction one can obtain the atmospheric pressure to the highest accuracy.