According to the Uniform Mechanical Code, when a refrigerating system operates at a pressure below atmospheric, it shall be tested at a partial vacuum equal to the pressure of a water column of mercury not less than _______ inches in height.

According to the Uniform Mechanical Code, when a refrigerating system operates at a pressure below atmospheric, it shall be tested at a partial vacuum equal to the pressure of a water column of mercury not less than _______ inches in height.