Gahṭikā-yantra, (also Ghaṭikā vessel, or clepsydra) was a water instrument used in time measurement in early Indian astronomy. The ghatikayantra was based around the unit of time known as a ghaṭikā, (also ghaṭī or nāḍikā). 
Ãryabhaṭa also describes it in his larger description of making a sort of clock-mechanism. In one ghaṭi, water would flow out of a cylinder and fill up the ghatika vessel. The cylinder would have ghati marks on it corresponding to aṅgula. Mercury is then poured into a guord, which floats on the column of water in the cylinder, and pulls on a string which will allow measurement of nāḍis elapsed. 
Lalla described it three different ways. One, as a vessel "according to one's liking" that can be adjusted to fill in one ghati. He describes it also as half of a kalaśa (spherical water vessel), with dimensions & descriptions of the bottom hole. Lastly he says it is a vessel with a finger hole in the bottom that timedly sinks.
Lalla and Śrīpati describe calculation adjustments that must be made in reading the graduations. Subbarayappa and Sarma point out this is to account for the fact that there are not exactly 60 ghaṭīs in a day.