Testers & scales calibrated before despatch

Gem & precious metal testers, since 1986

Calibrating Presidium gem testers

How to calibrate presidium gem testers and which probes to use. Including links to videos, instruction manuals and how to buy.






This article is about Presidium testers. If you are new to gem testers, read all about gem testing and the many different types of tester, click here.

There is a very clear guide illustrating the various models of Presidium electronic gem testers. It lists the various manufacturer's upgrades to the models, which replacement probe each one takes (they're not all the same!) and how easy they are to calibrate, click here to download (pdf, 4 pages) - it will answer the two questions:

Q1. Which probe should I order? (they don't all take the same probe).

When you buy a probe the tester will need calibrating, you can do this yourself or we can do it for you.

Q2. What is involved in calibration? - should I do it myself or get you to do it?

If you don't need to know about probes or calibration, the remainder of this page is for instruction manuals, videos etc.
or go to the bottom for an explanation of "calibration"

Replacement probe
See our illustrated guide (pdf, 4 pages)

Quicktest ref. gemtest-duo

Replacement probe:
For the version, PDT or the newer version, PDTII

Not sure which you have?
See our illustrated guide (pdf, 4 pages)




Calibration is the matching of a reading (e.g. on a meter or a digital display) to an actual measurement. For instance, the reading on a thermometer should match up with degrees, the reading on a weighing machine should match up with grams. Similarly, on a gem tester, the meter should match up with thermal conductivity, so that if a stone is ruby the needle point to 'ruby' or if it's glass the needle points to 'glass'.

All electronic measuring instruments go out of calibration given enough time or if they are bumped or dropped. This applies to everything (thermometers, weighing machines, barometers ETC.).

When a gem tester goes ‘out of calibration’ it will still give lower readings on low-conductivity stones and higher readings on high-conductivity stones, but the reading won't match the stones on the scale. The tester is not "broken", it needs calibrating.

Most electronic gem testers work on a "thermal conductivity" - they are especially sensitive to temperature. They are calibrated in the factory at a room temperature of 22°C. From about 19°C downwards the readings gradually become too high, from about 26°C upwards, the readings gradually become too low. If you are using your tester indoors and have heating in the winter and air conditioning in the summer, temperature will not be a factor. If not, you may wish to calibrate it in the winter if the temperature is generally below 18°C, and again in the summer if the temperature gets above 26°C.*

You will also need to calibrate it if you get a new probe.

All Presidium models from about late 2020 are easy to calibrate, you can do it yourself. Earlier models are very fiddly to calibrate, please our illustrated guide (pdf, 4 pages) then decide if you want to do it yourself or send it back to us.


  • Read all about gem testing and the many different types of tester, click here.