Hello! Recently, I decided to try developing TMax 400 in TMax RS at 1+9 dilution at 75 degrees rather than 1+3 at room temperature. For both setups I used continuous agitation with a Unicolor drum on a reversing motor base. Rather than going through extensive home testing, I used the BTZS kit. They sent five pre-exposed sheets of a calibrated step wedge which I developed and returned.
The rated film speed of TMax 400 was about 500 with the BTZS test. This ISO agrees with my own testing when I tested film speed using TMax 400 and 1+3 dilution of TMax RS. I measured the film density with a spotmeter. Though not terribly exact, there was a distinct increase in density between an ISO of 330 and 400, but minimal between ISO of 400 and 500. I called the ISO 400 since that agreed with Kodak.
Since I've read that most folks rate TMax 400 at an ISO of 320, I'm curious about these results that show TMax has an ISO of 500 with my setup. Have folks found TMax 400 faster than stated instead of slower? Looks like I should rate the film at ISO 500 for my setup instead of 400. Best regards.