That would be fine, as long as it stays on the whole time, since it will increase back pressure and reduce flow rate over when it is not there.
Yes. It should definitely stay on all the time. Don't touch it! That's the rule for any instrument that is a permanent part of a test. In the July 18 test data, I can see in the data where the people moved some instruments and made some changes to compare the instruments. (There is no harm in doing that, as long as you put everything back.)
I often go back the Wright brothers, who made one of the first wind tunnels and did precision measurements of air flow and its the effects on various airfoils, with different chambers and so on. They compiled 3-digit precision data on lift and drag, measured with a balance scale. They graphed it and analyzed it to a fare-thee-well, in several notebooks with advanced engineering mathematics that is way over my head. They did various things similar to what Mizuno has done, such as a smoke test to check for wind speed and a laminar flow, which is what they needed, and to see the effect of the air passing over the airfoils. Mizuno uses incense; they used cigars. Here is an example of their data and a graph they made, from NASA:
The wind tunnel:
Anyway, the part about "don't touch anything!" reminds of a comment they made. They finally had to mark the position of all of the furniture in the room with chalk, and they had to mark the positions they would stand, and keep everything in the same location. Moving furniture or people had a measurable effect on the wind tunnel results! That is remarkable sensitivity. Without that kind of accurate and precise data, they could never have designed the airplane. It was engineering from start to finish, coupled with extreme bravery and superb athletic abilities.
Mizuno has put up an elaborate set of plastic sheets and buffers on the windows and around the calorimeter to keep conditions somewhat uniform. He checks everything manually and on the computer. I am not there, so I can only check the computer data. Here is a neutron test in an underground lab, also in a plastic tent to reduce noise, I think:
At places like MIT and SRI they have superb environmental controls that keep lab conditions very stable. That's a big help.