Methods and apparatuses for indicating the presence of a threshold directional differential pressure between separated adjacent spaces. An inclined conduit contains at least one movable element that indicates whether the pressure difference between the two spaces is at least as high as a threshold pressure difference. The apparatus may include an on-board pitch indicator and a roll indicator which are used together to calibrate the apparatus and its installation. The apparatus may provide only one or more discrete number of pressure difference set points and be non-adjustable once installed. The inclined conduit may be non-rotatable relative to the baseplate, and the baseplate may be rotatable to change threshold pressure difference set points. The apparatus may provide a tamper-resistant indication of whether a threshold pressure differential is present.