Thanks for the explanation. And yes, people who don't use Dichlor in the tub AND enter the tub when there is Free Chlorine would likely have swimsuits degrade more quickly. These days with titanium heat exchangers, I think there's less risk of corrosion, but if you've got an older copper heater then the risk may be higher but I don't know how high -- I just know it's higher by a factor of around 20 (since the oxidizing power without the CYA is about that much higher).
The 20 ppm CYA with 4 ppm FC is similar to 40 ppm CYA with 8 ppm FC and is similar to 0.2 ppm FC with no CYA. All of these combinations (at a pH of 7.5) have the same amount of disinfecting chlorine (hypochlorous acid). Remember that this recommendation of mine was based on CT values of 30-50 for Pseudomonas aeruginosa, the bacteria that causes hot tub itch, as I described in
this post so you can see the source links I used -- not a lot of data for that bacteria -- there's much more data for E.coli and common heterotrophic bacteria.
As for the CYA buffering effect on chlorine, even buffering against pH changes, look at
this post where I show graphs of the traditional industry chlorine chart without CYA and the true chart with CYA where you can see how CYA not only holds chlorine "in reserve" but also buffers the small amount of disinfecting chlorine (hypochlorous acid) against pH changes.
I understand what the bacteriologist has measured and in fact most bacteria will be killed a lot faster than 20 minutes. The CT values (chlorine concentration in ppm times time in minutes) for 99% (2-log) kill is 0.08 for most bacteria so even chlorine with CYA at standard levels in pools will kill 99% in about a minute or two. It's the 99.99% (4-log) kill rate for hot tub itch with the CT of 30-50 that implies taking 20 minutes if you've got around 4 ppm FC with no CYA. That's probably what he was talking about. My calculation of 4 ppm with 20 ppm CYA was to have a kill rate at 50% that was faster than the reproduction rate of 15 minutes, but this calculation was on the edge and assumed the CT numbers were lower at the higher temperatures in the hot tub (as is typical with CT values). His not using CYA certainly would kill most anything except Cryptosporidium.
The only issue I have with his approach is when the chlorine residual drops low near zero. If that happens too long before you get into the tub, then there's time for bacteria to grow (and it's true that such bacteria would have to "drop in" from somewhere, such as from the edges of the tub). That's one of the risks. The other is the fecal-to-oral route I talked about. If there's no chlorine in the tub when you or others get in, then fecal bacteria could splash into your mouth (I know it's gross, but we're talking about why chlorine residuals are important), not just skin bacteria which are fairly innocuous. With all the jets and bubbling, you don't have to dunk your head to get exposure. The primary reason for residual chlorine when people use pools or spas is to kill this fecal bacteria before it can reach your mouth, nose, eyes and ears, whether it's your own bacteria or someone else's.
Quite frankly, the risks I'm talking about above are low risks. It's just that you never know when you'll be the unlucky one where a bacterial population causes a problem for you because you've got some sort of cut or sore and the hot tub itch then becomes a problem, etc. It's just that with residual chlorine before and during your soaking will virtually guarantee that there won't be bacteria nor other pathogens except for the real hearty ones of Giardia and Cryptosporidium cysts (which are protozoan cysts).
The standard cutoffs for being bacteriologically unsafe are a heterotrophic plate count (HPC) of > 500, Non-Fecal Coliforms or Fecal Coliforms of more than 1, and Non-Coliforms > 200. But these counts refer to specific sample sizes, plate sizes, etc. and not total bacterial populations in water. Basically, the goal is to not have any measurable number of coliforms (bacteria that live in warm-blooded animal intestines).
Anyway, you noted that you always have some residual dose of chlorine so you are right that you are probably fairly safe. By the way, water normally is CLEAR even with large bacterial populations. You can't use cloudiness as an indicator. It's large algae clumps that make water dull, then cloudy and then green. Bacteria usually aren't visible unless they form large colonies as on agar gels, but in water you usually can't tell it's bacteriologically unsafe.
Richard