This is the unit that earned Trans-Cal its reputation for reliability. TCI still receives units for initial repair that have been in service since that original production run in 1971. Like all TCI designs, this unit offers unprecedented reliability and a rock-solid design. This is an electro-mechanical instrument. An optical shaft angle encoder linked to a mechanical aneroid. Offering low current consumption and a high MTBF the D120 is still in production 38 years later. Offered in 20,000 ft. 30,000 ft., and 35,000 feet operating ranges, the D120-P2-T is a tried and true instrument.