Citation II S2/Bravo (C550)
Air Traffic Control Fully Certified
Situational Awareness
ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance
Weather; Human Factors
A C550 was descending north east bound from FL210 down to FL180 because I had a B350 level at 170 north east bound as well. The B350 had pilot discretion descent to 110 issued but wanted to remain at 170. As the C550 was descending through FL194; I realized I had low altimeters all around me (I believe 29.86) and I instructed the C550 to amend altitude and maintain FL190. He advised he had just went though FL190 and asked should he go back up. I told him it wasn't necessary since he seemed to have leveled off. I then decided to descend and turn the B350 to avoid any closer conflict. I descended the C550 to 160 and turned him 20 degrees left. I am not sure if I ever broke altitude separation but lateral separation was around 4 miles. I should have had some visual indicator to remind myself FL180 was unusable and non-assignable.
Enroute Controller described a loss of separation event when he/she failed to note the fact that the surrounding low altimeter settings precluded the use of certain flight levels.