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I certainly do not want to speak for Phil, I asked him the same question more than a year ago so I hope I get this right.
One possible problem is the stain consistency. IOW, for a temp/concentration combination are we absolutely sure that the resulting stain always results in the same density? This can be very important for the development calculations. A half stop error might mean the difference between having great highlights with detail and/or having them blown out outside of what the printing method of choice can handle.
Other problem is the sampling light. How blue should the densitometer light be? For example, in pt/pd the best wave length to expose is about 326 nm, if my densitometer is reading at 346 nm then I might be reading lower densities than what my paper is "seeing". Since the stain is proportional, once again I am in danger of getting blown highlights since both my paper and my sampling method are not "seeing" the same thing. This applies to silver as well.
I hope I recounted Phil's points correctly, they are valid concerns one should be aware of. Having said that I have applied the BTZS methodology to Pyrocat HD successfully with excellent results, maybe I just got lucky and all the variables just fell in. The only thing I can tell you is to try and see if it works for you, heck, compared to Zone System testing the BTZS testing is a matter of a couple of hours instead of days and endless sheets of film, give it a shot and see how it works for you.
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