In order to have a rotary motion vibration table approved for and maintain ISTA Certified Lab status, the lab must submit a phase relationship test, which is in the form of circles drawn by the table.
The circles are "drawn" to verify proper operation of a typical rotary motion vibration tester. This test is used to check the phase relationship of the primary and secondary drive shafts by verifying that each corner of the vibration platform moves simultaneously with the other corners.
The cirlces are ddrawn by attaching a pen or pencil to each corner of the table, parallel to the drive shafts, with the points extending beyond the edge. The machine is then operated at approximately 250 CPM and a blank piece of paper is brought into contact with the pen or pencil point such that the table motion is traced. This is repeated at each corner.
The best way to bring the paper in contact with the pen/pencil is to have the paper attached to a solid and stable surface (a clipboard or a large piece of corrugated, attached then to large box or hand truck) that can be brought smoothly and evenly up to the point of the pen or pencil.
If all four corners produce uniform 1" circles (with variation of no more than 1/16"), then the machine is considered in phase. Oval tracings indicate that the machine needs adjustment, which must be done before certification will be approved. Adjustment can be done in-house or by the equipment manufacturer or outside calibration service provider, and then verified with a new phase relationship test.
Source: ISTA Staff