Based on the book by Ansel Adams, The Negative, Little, Brown and Company, 1981, Ch 4.
See Table 2, page 60 for a full description of each zone and Figure 4-4, page 54 for a B&W visual.
Spot reading taken in the Zone V image below as per page 51, NEF 12 bit RAW developed in
Lightroom 1.3. Nikon D300 with Tokina 50-135mm f/2.8 AT-X Pro DX

The first set of images were taken inside under controlled lighting allowing ISO 200.
We can note 11-12 Zones or steps of DR starting at Zone 0 - 2 and extending into zone IX.

For the second set of images (gray concrete plaque) conditions were optimal given overcast sky with even lighting.
However neither Zone 0 or I went to black as anticipated and, likewise,
detail extended past Zone VIII into Zone IX. The result is 10-11 zones of DR at ISO 400!

The third second set of images (tan hot pad) were taken inside under controlled lighting at ISO 1600. Zone 0 did go to black.
Zone I had more detail than expected. The highlights did not extend past Zone VIII. This is a DR of 7-8 zones of tt ISO 1600.
Thus the ISO increase of 2 stops from 400 to 1600 is offset by a DR decrease of 2-3 stops. These results can be
compared to the first test at ISO 200 where the anticiapted 3 stops of DR is seen over ISO 1600, or one stop over ISO 400.

Read 'A Digital Zone System?' by Chuck Gardner here http://super.nova.org/DPR/ZoneSystem/DigitalZoneSystem.pdf