How does the flat crystal detector test the sealing face flatness of mechanical seals

The equipment of flat crystal detector is often used in the flatness test of mechanical seals. A flat crystal detector is an instrument made of Pyrex glass, fused crystal, or optical glass with a refractive coefficient of 1.561, with a transparent cylinder with a straight working end surface. Its diameters can be 60mm, 80 mm， 100 mm， 150 mm， 200 mm， 250mm. Because it is difficult to make, it is not so big. 

The accuracy of flat crystal detector is 0.03 µ m (grade 1) and 0.1 µ m (grade 2) ：if  the planarity of the workpiece is very high,and its deviation does not exceed 0.03um and 0.1um, the grade 1 flat crystal is usually used to detect the planarity of the mechanical seal’s sealing faces，so that the diameter of the flat crystal is greater than the outer diameter of the measured surface.If the outer diameter of the sealing ring of the mechanical seal exceeds 250mm or the sealing ring is made of non-metal material（which means poor reflection），they can be tested by this way：with the red Dan on the tested surface, we grind on the surface of the parts of the mechanical seal, if the contact trace of the ring is continuous instead of being interrupted, & the continuous contact area is greater than 80% of the total area，then the flatness of the sealing faces of the mechanical seal is qualified.