Presidium models
Gemtest-P, PGT/CSE

Gemtest-duo, PDT-S7
(pre-2020); Gemtest-duo-II, PDT-II (from 2020)




This page tells you about calibration, when your machine might need calibrating, what is involved if you want to do it yourself, and how much we charge to do it for you. If, after reading this page, you decide to calibrate it yourself, please ask us for the instructions. This applies to two models:

- model gemtest-P, PGT-CSE (the first picture, above, the meter with the needle) - read this page to learn about calibration.

- model gemtest-duo PDT-S7 (the third picture, above, with the analogue meter and the LED display) and you bought it from us before 2020 - read this page to learn about calibration.

There are two new models which have a computerised calibration system, so you don't need to read this page:

- model gemtest-duo PDT-II, which is what you will have if you bought one after 1 January 2020 (exactly the same as the third picture, above, but marked "DUO TESTER II", with the analogue meter and the digital display) - there is no need to read this page.

- model gemtest-duo PGI, (the second picture, above), the handheld model with the LCD display - there is no need to read this page.


It's matching the reading to the measurement, i.e. the meter should point to 'ruby' if it's a ruby or 'glass' if it's glass. All measuring instruments go out of calibration given enough time or if they are bumped or dropped. This applies to any electronic measuring instrument (thermometer, weighing machine, gas detector...). They are not 'broken', they need recalibrating. You can calibrate an electronic gem tester yourself if you are very careful, or we can do it for you.


• when testing, wait for the tester to warm up (wait for the ‘ready’ light), place the probe firmly on the stone, the meter tends to go too high then move lower over the following few seconds. It’s the lower (settled) reading that counts. So don't worry if the reading in the first three to six seconds is too high, this does not mean it needs calibrating.
• this is a very sensitive thermal conductivity meter (like a diamond tester, but far more sensitive) so the machine, probe and stones must all be at room temperature.
• In factory these are calibrated at 22 degrees, the recommended temperature range is 18 to 27 degrees. If it's cool (approx. 15 to 18 degrees) the readings might be a little too high, calibrate the tester. If it's cold (approx. under 15 degrees) the tester might still work, but it might not, it is only designed to work at the recommended temperature range.
If it's hot (approx. 27 to 30 degrees) the readings might be a little too low, calibrate the tester. If it's very hot (approx. over 30 degrees) the tester might still work, but it might not, it is only designed to work at recommended temperature range.
• you can see, from the above, that it will not work outdoors in Northern Europe in the winter (e.g. at outdoor fairs and markets), recalibrating it will not help. Likewise, keep an eye on the temperature is your office is very cold first thing in the morning. Conversely, if there is a heat wave and the temperature is in the 30s, it will not work, recalibrating it will not help.

CALIBRATION OF THE THERMAL TESTER (the meter with the needle)

The manufacturers say that you should not attempt to calibrate it yourself. This is for two reasons. Firstly, very often it doesn't’t need calibrating (see above) and you will merely be calibrating it unnecessarily, and there is a high probability that you will get into a muddle and have to send it back to us in frustration. Secondly, calibrating it is very fiddly and you must be aware that if you damage the tester it cannot be repaired. WE know how to do this, so if in doubt, please contact us and we can calibrate it for you (there’s no charge if it’s under guarantee).

If you want to calibrate it yourself, here is some guidance:

• you must use a screwdriver of the correct size, 1mm to 1.5mm.
• you must use a screwdriver of the correct style. If you use a ‘cross’ type screwdriver, it must be a Frearson. Do not use a Philips or PoziDrive screwdriver, if you are not careful you could damage the screw head. If you can’t get a Frearson it is better to use a flat-head screwdriver of about 1.5mm. It must fit the slot neatly, not too thick so that you force it, not too narrow so that it damages the slot - explanation.
• you will need to shine a torch into the hole, see exactly the angle at which the screw head lies, then hold the screwdriver very precisely on the screw THEN you need to hold the probe on the calibration disc and at the same time turn the screw by a tiny fraction of a millimeter to adjust the meter. Since this requires two pairs of hands, it is best to for two people to do this. You will still have to make several adjustments until it is just right.

WARNING: if you turn the screw all the way round, as if trying to ‘screw it’ you will break it, please do not return the tester to us, you have broken it; if you use the wrong screwdriver and damage the screw head, you won’t be able to adjust it, please do not return it to us, you have broken it, the damage is not covered by the guarantee. If in doubt, please stop now, send the unit and the probe back to us, we will do it for you (there’s no charge if it’s under guarantee).

NOTE: if you use a new probe, it has to be calibrated, otherwise the new probe won't work.


We have the instructions to calibrate this too, we have never had to do this, we could try. The procedure is similar to that described above, but instead of using the built-in calibration disc you must use a 0.2 carat sample of GGG (Gadolinium gallium garnet).


We can send you the calibration instructions, just ask. But please do read all of this page first, you do need to be confident that you can do it without breaking the machine.

It will need calibrating whenever you buy a new probe. When you buy a probe you will be given a choice of two prices, with or without calibration.

For calibration only (i.e. you are not buying a new probe) these are our charges (2019), providing you originally bought the tester from us, these prices include VAT:

- for the GEMTEST-P (with the analogue meter) £6.20

- for the GEMTEST-DUO: for the analogue meter £6.20; for the digital meter £9.00.
(if you are buying a probe, the probe only operates the analogue meter so there is only the £6.20 to pay)

Return postage is £10.00 for Special Delivery or £6.50 for Recorded Delivery

If it is under guarantee and needs calibrating, there is no charge.

QUICKTEST, Watford, WD18 8PH, Tel. 01923 220206, email info(at)