How do you measure drifts of the order of 1ppm/year?

[Kiriakos GR]

This is a very nice question that mostly hobbyists will ask when they come close to some sort of measuring secondary standard.

How do you measure drifts of the order of 1ppm/year?

First of a few glossary explanations.
Your new and made by high precision electronic part, this is called at it best as secondary standard.
True measuring standards are stored and locked in vaults.

ppm this translates to part per million. 

One secondary standard with 1ppm change per year this translate to:
1V
Displayed as 1.000.000 at the screen of a multimeter with one million counts resolution.
This will change to  0.999.999  or  1.000.001 

Therefore if you did not invest buying one ... such high counts multimeter or voltmeter, there is no chance so you to measure such low ppm differences.

Now lets assume that you do own one such high counts measuring device, and this is was not calibrated (inspected) in past two months, them again it is not possible the measurement that you will do, this to be considered as error free (zero uncertainty).
Measuring equipment they also drift internally,  and such meters they should pass calibration inspection more frequently (three times per year).   
 
In the world of handheld multimeter, with resolution of  30.000 or 50.000 counts there is no chance to see small ppm changes.

At 50.000 counts display, when change of ppm this is large enough to form a number of ppt ( part per thousands) then it could be measurable by a handheld multimeter.   
 
For safe measurements at the range of 1ppm, even one million counts DMM this is not enough.
DC Volt measurements or resistor measurements at over one million display resolution this is a huge challenge and a job for dedicated measuring equipment.   
This could be an ultra high resolution resistors meter, or alternatively one high resolution DC volt meter.


Now it is time for me to break down the cost of such equipment in relation to thousands of dollar or Euro, for you.

I am aware of three highest counts multimeter in the market.
Retail prices of 10.000E up to 12.000E

Dedicated DC volt meter at such high counts I assume retail price of 3.500E
Dedicated resistors meter also at such high counts this it does have also 3.500E retail price.

Now think of law of ohm and make a new calculation.
R + V + I = 3 x 3.500 Euro = 10.500 Euro  ....  here is your total to pay for getting an high precision + high resolution multimeter, at your home.   

Prices it could or not include VAT