Generally speaking, calibration wedges are not unique or specific to any particular product.
Any step wedge will work with any densitometer.
It's always possible that a manufacturer will want a particular product to be calibrated at specific points, usually to correct for non-linearities in the response. But the level of error achieved using a general step wedge would be pretty small.
The 31-step Stouffer wedge that the OP mentioned would work fine, even if it's the non-calibrated variety.
How can you calibrate a densitometer from a non-calibrated step density? Seems that in order for a determination to be made that the unit is reading accurately and within it's tolerance, then it has to read a density of a true and known value. For my X-Rite for example, the step density on the calibration tablet that is used for calibration is 1.97-----if the reading is within +/- 0.02 density units, then I can consider it properly calibrated. If not, then I can adjust the densitometer to within that tolerance.
It's a question of terminology and accuracy.
The "non-calibrated" Stouffer step wedges are provided with nominal values only.
The "calibrated" wedges have each value measured individually, with the actual values written on the storage sleeve.
Both types are produced to the same level of accuracy. It's just a question of how closely the true value is presented.
As with any calibration process, accuracy is a sliding value. You use whatever level is required for your purposes.
I guess I see it this way. If you handed me your uncalibrated step tablet and said, here, calibrate your densitometer with this, I would then ask, but how do I know that the reading I get is true and accurate if I don't know before hand what the reading is supposed to be? And to that you would then say.................
The response of a densitometer is highly dependent upon the optical path. The film itself is part of that path and scattering within the film will materially affect density measurements.
That is why absolute density calibration is done under a restrictive set of conditions (both illumination and collection angles are 180 degrees, scattering in the step wedge material is defined). Under these conditions, 0.90 D is exactly 0.10 D less dense than 1.00 D in the wedge and 0 D means that no light is coming through.
That will not be true under your measurement conditions and your densitometer will not be completely linear in response, both because of instrument and reading conditions. However, it's the best that we can do in practical application and by calibrating your instrument to the step wedge you get an approximation of true density with the main difference being that it is on a sliding scale. There is no absolute zero.
Going any further than this gets complicated. For example, precision densitometers use an internal calibration standard to provide a fixed reference point, usually on every scan. You are unlikely to have this so you won't know if your instrument is varying from moment to moment (with line or battery voltage for eg). However, you can characterize the precision of your measurements by making repeated readings of ROD. Do you get the same ROD value every time you measure a step on the wedge (precision) and is the amount of variation from measurement to measurement constant at all intensity values (linearity)? Essentially, you are measuring signal to noise across the relevant density range.
Or maybe you just use the thing.
You know the value within the nominal accuracy of the wedge, which value is given in the product specification. For example, an uncalibrated wedge may have an accuracy spec of ±3% while its calibrated version is spec'd at ±2%, or similar.
A calibrated wedge is individually measured and the value of each step is written next to the nominal value, so it's accuracy is tighter.
You don't actually "calibrate" a densitometer, other than setting one upper endpoint, perhaps 2.5 or 3.0 or some such.
All other values are derived from that one setting.
You use the various steps to check the linearity of the instrument, which you can't change.
This is a transmission densitometer.
0 D means that there is no density in the light path, just air, so all light gets through.
One common usage for the zero setting control on the instrument is to select between absolute and relative density
measurement modes. In photography, we usually want relative densities, so set "zero" as looking through an empty
area of the negative, like the non-imaging edge area.