There are many different international standards for the measurement of Graphic Arts density. The American National Standards Institute (ANSI), the International Standards Organization (ISO) and others all define and describe conflicting but accepted methods. X-Rite's instruments are user configurable to allow them to conform to all currently published and accepted standards. If your measured data is not what you expect, be sure to check that your instrument is configured according to the method that you have adopted.
In the United States, Graphic Arts density is most commonly measured according to the "Status T" standard. Throughout much of Europe, "Status E" is preferred. Printers will passionately defend one method as better than the other. In addition, some users prefer to subtract the paper values from the ink while others measure paper directly as a component of the final color of the product. Finally there are instruments that are equipped with various optical filters (UV, polarization) etc., that will measure density differently from other devices that lack these physical components.
In short - - know how you are configured! In the United States the most common settings are:
See image below -
X-Rite densitometers will also allow you to enter a "Reference" and then use that entry as a zero point. For example you might enter a "1.00" as a reference and then measure an item whose (absolute) density is "1.20'. In this case, the densitometer would subtract "1.00" and you would have a measured value of "0.20". Not at all what you expected. Avoid reference mode unless you are clear in your intent to use this feature - great way to balance a color across a press sheet. When you are in the Density menu you should see "Density" and "Options" at the top of the screen, not "Den-Ref" as shown below:
You are now reading Absolute Density values, not subtracting paper or reference readings. To automatically set to the default (Auto, Absolute, Status T) you can also do a reset to the instrument.