In his role as chief astrologer he was responsible for detecting signs of bad government which were indicated by earthquakes. Zhang's device, which he called Hou Feng Di Dong Yi, was made of copper.
Sa kanyang papel bilang chief astrologer siya ay responsable para sa detecting palatandaan ng masamang pamahalaan na kung saan ay ipinahiwatig sa pamamagitan ng lindol. Zhang ng aparato, na kung saan siya tinatawag na Hou Feng Di Dong Yi, ay ginawa ng tanso.