SCP-XXXX is to be kept in a 6m x 6m x 4m glass containment cell, which must be held at a consistent 80% humidity and temperature of no less than twenty(20) degrees Celsius.
Why 6x6x4? Would containment fail if it were 7x7x5 or 5x5x3? Dimensions shouldn't be provided unless absolutely necessary.
The whole "number (#)" thing is a Series I relic. We don't really use it anymore.
"is to be" is more tonally appropriate than "must be".
No water, in any phase, can be allowed to exit the glass chamber.
Nitpicking here, but "of any phase" sounds better than "in any phase".
The "can be" here kinda implies it's physically impossible for water to exit the chamber. Use "is to be".
Equipment necessary to maintain humidity are contained within.
Setting aside the weird sentence structure here (your vowel is separated from its noun by the majority of the sentence), the statement itself isn't necessary. We can assume equipment necessary to maintain SCProcs is included in containment - it's similar to how we can be assumed living entities are given food and water.
Only D-Class personnel are allowed entry into the chamber, for the purpose of maintaining said equipment or for experimentation.
Why aren't researchers allowed in? I get that they're more valuable to the Foundation than D-Class, but they're also trained to know how to act responsibly in potentially dangerous situations. In fact, I'd say it's probably safer to have a trained professional dealing with the thing than a D-Class, not to mention how hard it would be for researchers to study the anomaly without direct contact.
This primary glass cell must be contained with a secondary containment chamber measuring 10m x 10m 10m, in which the humidity must be at, or as close to 0% as possible.
"within", not "with"
More unnecessary measurements - just say the second chamber must be larger.
You need a comma between "close to" and "0%".
Any change in the humidity of this Secondary containment cell must be immediately reported to the site director.
"Secondary" shouldn't be capitalized.
What about this situation is so dangerous that the site director has to be notified? Why can't security and containment specialists on-site deal with the potential breach? Unless the entire site is in imminent mortal danger, I don't really think top personnel need to be involved.
SCP-XXXX appears as a humanoid silhouette, and is extremely hostile to all mammals, both living and deceased, but given the choice, strongly prefers live prey.
Run-on, try condensing and splitting: "SCP-XXXX appears as a humanoid silhouette. The entity demonstrates predatory behaviour towards all mammals, living or deceased, though prefers live prey given the opportunity."
Living humans are also targeted before other live mammals.
Awkward wording - try: "Humans are its preferred prey."
When any mammal, including humans comes into contact with SCP-XXXX, they are drained of all moisture over the course of 5-58 seconds.
You just said the thing likes humans, you don't need to say humans are included.
This leaves its subjects severely desiccated.
That's implicit in draining a living being of all moisture.
