An apparatus is provided for indicating the presence of pressure within a collection chamber of a device such as a chest drainage device. The apparatus includes a bladder configured to be positioned within the collection chamber or within another chamber in fluid flow communication with the collection chamber. The bladder has a concavity formed therein and configured to contract or expand in response to pressure in the collection chamber. The apparatus also includes surfaces associated with the bladder that are configured to move with respect to one another upon contraction or expansion of the concavity of the bladder, thereby indicating pressure within the collection chamber.