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Gustavo, evening..... Most folks don't use a water pan in an UDS.... the steam can put the fire out, and the temps won't get over 225 deg F / 118 deg C...... Try it without and taste the results... if you want to add a little moisture, a "soup can" size container will add a little moisture... If you have a lower shelf or heat deflector plate, that would be the place for a water container....
I will build my UDS with a section to fit a water pan, perhaps a few inches above the charcoal, because on high altitude waters boil at 195 F degrees, I had read about UDS smoking going well on moisture but my city it's kind of dry, usually we have low humidity.
I will try to see what happens with and without water pan and I will post my experience on high altitude smoking.
It will be an interesting experience and I am excited to see what happens on high altitude smoking with low humidity and less oxygen on in the air.