The SGS Spark Gap Setter is a device used to determine the output voltage of the PST100 and ST-AC100 Spark Testers.
The output voltage of the PST100 and ST-AC 100 can be roughly set by dividing the test voltage by 1700, the answer is in mm.
e.g. 20,000 /1700 = 11.76mm.
The test probe should be held at the calculated gap from a piece of earthed metal and the output voltage increased until the spark jumps the gap.