Millimeters is not a useful unit of collected water! Granted millilitres wouldn't be much better without time as well.
I would assume time is on the scale of per day; that seems, at least to me, the only time unit they wouldn't have specified here. Of course, in an article this bloggy it's not a given that the reasonable interpretation of such vague units is the interpretation intended--but I think that there is a reasonable interpretation.
It might be a bit too technical, but anything less really can't give you a sense of scale.
Water scarcity is a particularly severe challenge in arid and desert climates. While a substantial amount of water is present in the form of vapour in the atmosphere, harvesting this water by state-of-the-art dewing technology can be extremely energy intensive and impractical, particularly when the relative humidity (RH) is low (i.e., below ~40% RH). In contrast, atmospheric water generators that utilise sorbents enable capture of vapour at low RH conditions and can be driven by the abundant source of solar-thermal energy with higher efficiency. Here, we demonstrate an air-cooled sorbent-based atmospheric water harvesting device using the metal−organic framework (MOF)-801 [Zr6O4(OH)4(fumarate)6] operating in an exceptionally arid climate (10–40% RH) and sub-zero dew points (Tempe, Arizona, USA) with a thermal efficiency (solar input to water conversion) of ~14%. We predict that this device delivered over 0.25 L of water per kg of MOF for a single daily cycle.
If you live in a desert, you're not trying to measure the water. You're trying to drink it.
The test device was powered solely by sunlight, and although it was a small proof-of-concept device, if scaled up its output would be equivalent to more than a quarter-liter of water per day per kilogram of MOF, the researchers say. With an optimal material choice, output can be as high as three times that of the current version, says Kim.
http://news.mit.edu/2018/field-tests-device-harvests-water-d...
Edit: And the underlying paper is here: https://www.nature.com/articles/s41467-018-03162-7
I wasn't sure about the numbers either, so I looked it up. The levels were higher than I would have guessed, generally ranging from 30%-50% relative humidity.
https://www.currentresults.com/Weather/Arizona/humidity-annu...
1 milliliter = 1 cm^3
237 ml = 1 cup
EDIT: Nevermind regarding the unit confusion. I'm still learning how to read.
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> The test device was powered solely by sunlight, and although it was a small proof-of-concept device, if scaled up its output would be equivalent to more than a quarter-liter of water per day per kilogram of MOF, the researchers say.
...
> The next step, Wang says, is to work on scaling up the system and boosting its efficiency. “We hope to have a system that’s able to produce liters of water.” These small, initial test systems were only designed to produce a few milliliters, to prove the concept worked in real-world conditions
[Edited]
It could be any, if the device depends on area. The more area you use, the more water you get. Weather report often uses litres per square metres. One meter high per square metre is a thousand litres, so one mm would be a litre.
Edit: in the article I read: These small, initial test systems were only designed to produce a few milliliter.
So it's much less :-)