Electronic testers need have their electronics checked (calibrated) against known samples.
For model AGT1 you need 14ct
For model AGT3 you need 14ct and 18ct
For model KEE you need 18ct, but also 22ct if you want to test high-carat accurately.
You don't have to buy our samples!
But if you use your own, check that each has a full British hallmark to guarantee the carat.
The sample you use for calibration must be yellow gold, and must be exactly 14ct, 18ct or 22ct otherwise all your tests will give wrong readings.
The weights vary from sample to sample, generally between 0.4g and 0.5g.