Patriot, any Tymco, operated correctly, will pick up loose dust, without expelling it into the air. The key is though..... "operated correctly"
There is so many more ways they can have an inbalanced system, where it is not operating at peak. When out of balance, you have to open your leaf-bleader to compensate for inbalances (lest you "blow your flaps out").
Of course, a Tymco will not pick up heavy wet grindings, or brick-sized rocks like a broom can, but WILL pick up dust much more efficiently than a broom. To do this, you must also not use ground water, lest you merely "paint" the dust on to the ground, only to have the sun dry your water later, and never really picking up the dust.
Done correctly, with no leaks in the system, your circulating airflow, in a perfect-world, would be nearly 100% of your air going round and round in circle, only allowing for the space occupied by the incoming debri. But when you think of it, you are forcing that air, at high velocity, through those narrow tubes and chambers, right? So the air will try to find the "path of least resistance". Thus any time you hit an uneven spot in the ground (like, if your skid goes over a pebble or whatever), the air is going to try to blow-out. It will also try to blow out of any little rip or tear, gap in the flaps, weak weather-stripping, etc.... You litterally have to go over the whole sweeper with a fine-tooth comb. looking for any "blow-by" or "suction" (listen for the whistling sound) and plug up any such holes.
Next, instead of depending on the in-tank sprayer system, simply fill your hopper with about 1/4 of water. That will slosh around as you move & rock, creating a vapor with the incoming high-velocity air flow. That vapor will help arrest dust particles entering the hopper, making them heavier, and thus fall down, instead of being drawn back through the fan. The "cleaner" that air is when going back through the recirculating system, the better the odds are that escaping air is not dusty.
You need to have your leaf bleeder all the way closed, to attain maximum dust escape. Because afterall, the escaping air in the leaf bleeder is never 100% dust free. But if you close up the leaf bleeder, and see that the flaps are trying to "blow out", that means you have an imblance in the system. You simply must have a leak (whether its on the vacuum side, or the blow side, of the sytem), that is causing this. Either that, or your flap material is too flimsy to resist the blow out. It has to be just the right sturdiness to prevent blow-out, but flimsy enough to let objects eventually make their way under, during the forward movement of the sweeper.
As an example, years ago, my company accepted a job sweeping chip gravel, from bunch of streets bordering millionare's homes. The chip company had spread the chip, and done their own post sweep with broom sweepers. Unfortunately, the grade of chip was poor -grade, so there had been a lot of dust mixed in with the gravel. The days it sat between sweeps had allowed cars to drive over the loose chip, grinding it up even more, creating a lot of granulated dust. When the broom sweeper came through, naturally he used his ground water sprayers, so that he wouldn't look like a cyclone coming down the road
But each time, when the sun would dry his water, the dusty streets would return. So they call back the broom sweeper again, and the cycle kept repeating. The broom would pick up any remaining chip ok, but never quite got the dust up. By now, the thick layer of dust looked like ash or baby powder! It was getting on cars, inside expensive homes, etc.. The contractor became desparate, as he was facing multiple complaints. They considered washing the streets down with a water-truck, but were forbidden by the water-quality sewer people. So they canned the broom sweeper and called us. I looked at the task, and could see that .... in places ..... there was dust several inches thick by now. The streets were literally grey, rather than salt-&-pepper chip color by now, on account of all the smeary dust residue that was not coming up.
For this job, we used a Tymco 600. We went over it with a fine-tooth comb before we started, using a cotton-ball over every single potential leak spot. We looked for anywhere that the cotton ball attempted to blow away, or get sucked in, to any potential crack. After sealing up any thing, joints, connections, gaps, etc... We put our bathtub of water in the hopper, and closed the leaf-bleeder door. We did end up using very specifically aimed exterior water nozzles, aimed only at spots by the skids, where ....... invariably .... air was bound to escape through the skids at each bump. Also we aimed a sprayer at the gutter broom, which of course will have no way (as opposed to the vacuum head) of controlling its own dust. But we did not spray any water at the width of the sweeping head, so as to maintain a dry sweep.
The net result was that only a very slight pig-pen action around the head once in a while. It was manageable and below-the-complaint / noticeable level. With each pass, we left the streets dustless and salt-&-pepper colored. It certainly wasn't easy. We had to keep a close eye peeled for the slightest escape of dust, and cheat here and there and open the bleeder a crack, when the flaps and system were being stubborn. As long as you have a lot of water sloshing in the tank, you can get by with a smidgen open on the leaf-bleeder. That, as you know, improves your cross-flow by causing more "suck" than "blow". But that has to be balanced with the escaping dust problem, so it was a constant balancing act.
so yes, if done correctly, the tymco's can pick up dust without airborne dust. This is very important for other types of sweeping applications like ..... sealcoat prep, where the contractor wants the dust up (not only rocks and large debri), but doesn't want you to use groundwater (for fear of making the lot a painted brown mess).