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Altitude performance is one of the key considerations in the head media interface design for hard disk drives (HDD). With conventional head media interface without thermal fly-height control (TFC), it is necessary to budget in the air bearing fly-height loss due to altitude effect. In order to reduce the sensitivity to altitude, some other performance need to be sacrificed. With TFC technology, there is an alternative approach to achieve superior altitude performance without trading off other performance. A pressure sensor is installed to the HDD to monitor the ambient pressure. When the pressure changes, the pressure sensor reports the pressure to the channel, and the TFC heater power is adjusted accordingly to maintain a constant fly-height at all altitudes. The pressure sensor has been implemented in the HDD successfully. The objective of this paper is to characterize the pressure sensor performance in the HDD application. The head-media spacing (HMS) was monitored with pressure sensor compensation turned off and on when exposed to different pressure to simulate altitude effects. The HMS was measured with harmonic sensor based on Wallace's spacing equation. It was demonstrated that the pressure sensor is effective in maintaining constant spacing at wide range of altitudes.